Blueprint of Life

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HSC Biology Topic 2 Blueprint of Lifecopyright 2005-22009 keep it simple sciencewww.keepitsimplescience.com.au

keep it simple science

1

but first, an introduction...

HSC Biology Topic 2

BLUEPRINT of LIFEWhat is this topic about?To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. EVOLUTION OF LIFE

2. MENDELIAN GENETICS3. CHROMOSOMES & DNA STRUCTURE

4. MOLECULAR GENETICS & MUTATION5. REPRODUCTIVE TECHNOLOGIES &

GENETIC DIVERSITY

EvolutionIn earlier topics you were introduced to theconcept of Biological Evolution, and themechanism of Natural Selection. Now you willstudy in more detail the evidence for evolutionand Theory of Evolution.

GeneticsThe science of inheritance was discoveredabout 150 years ago by a monk living in anobscure monastery in central Europe. In thistopic you will study the simple patterns ofinheritance that he discovered.

Gregor Mendel (1822-1884)experimented with gardenpeas for 12 years to work outhow parents pass on theircharacteristics to offspring.

You will study his results andthe reasons for his successas an experimenter.

Chromosomes & DNAYou will next move on to study things thatDarwin & Mendel would have been delighted tohave known... the cellular and molecular basis ofGenetics and Evolution. You will see the linkbetween inheritance, cell division,chromosomes and the basis of it all... the DNAmolecule and the Genetic Code.

In the section onMolecular Genetics

you will learn how the DNAacts to control all the

characteristics of each organism(the phenotype) and how Mutations can occurto alter the code and produce newcharacteristics for evolution to work on.

In the final section you will look briefly at wheremodern Biology is heading into ReproductiveTechnologies and Genetic Engineering. (This controversial area can be studied morethoroughly in one of the later Option topics)

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Chromosomes




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CONCEPT DIAGRAM (Mind Map) OF TOPICSome students find that memorizing the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic,

come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this Mind Map to practise on.

Sex-llinkage &

Codominance

Meiosis &gamete

formation

DNAReplication

Mendelsexperiments

Divergent &

ConvergentEvolution Pedigrees

(Family trees)

The link toevolutionArtificial insemination

and pollination.Cloning

PunctuatedEquilibrium

Effects ongeneticdiversity

GeneticEngineering

THEBLUEPRINT

OF LIFE

Chromosomes &

DNA

MendelianGenetics

Molecular Genetics&

MutationsReproductiveTechnologies

& Genetic Diversity

Evolutionof Life

TheEVIDENCE

for Evolution

The THEORYof EVOLUTION

Chromosomes,genes & DNA

Mutation&

mutagens

Transcription &Translation

Dominant-RRecessiveinheritance.

Monohybrid crosses




3

The Evidence for EvolutionThere is overwhelming evidence that life onEarth has undergone a sequence of changesover the past 3.5 billion years or so. Here, briefly(K.I.S.S. Principle) are the main sources of thatevidence:

Palaeontology The Study of Fossils The fossil evidence is undoubtably our mostimportant, direct evidence showing that life onEarth has changed.

The fossils not only show that life was oncedifferent, but that the changes follow a patternor sequence... this is evolution.

How do we know this to be fact? Not only do wehave many fossils of extinct organisms, but wecan place them in time sequence to see thepatterns.

Initially, scientists could only place fossils inrelative time order by correlating thesequences, as suggested in the diagram above.

Now we can also place absolute times on manyfossils by Radiometric Dating... thedetermination of the actual age of things bymeasuring the residual radioactivity of certain,naturally-occurring radio-isotopes in the rocks.

Fossils of Transitional FormsOnly a tiny fraction of all the zillions oforganisms that have ever lived have ended upbeing fossilised. The fossil record is, therefore,incomplete and a rather patchy record ofevolution.

Despite this patchiness, there are somefossils that have given us a glimpse of thechange that occurred when one type of life wasevolving into another. Such fossils are calledTransitional Forms

Perhaps the most famous is Archaeopteryx.(Archae= ancient, pteryx= wing)

When this fossil was first discovered, itsskeleton was clearly that of a small dinosaur.Only later was it noticed that the faint outlinesaround the fossil were the impressions offeathers. This was a dinosaur-bird!

Transitional fossils are important evidence thateach new type of life that appears in the fossilrecord did in fact evolve from a previousancestor. Scientists have discoveredtransitional fossils showing:

reptiles evolving to become mammals. ferns evolving to become cone-bearing plants. lobe-finned fish evolving into amphibians.

...and many more, including fossils ofour own probable ancestors, who were veryape-like, but undoubtably human-like too!

1. EVOLUTION OF LIFE

Paatterns of ChaangeAncient Life Modern LifeSimple types More complex typesLess variety Greater varietyLess like More and more

present-dday life resembling modern life

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4Biogeography (how life is distributed)When Charles Darwin travelled around the worldon board HMS Beagle in the 1830s, it was thedistribution of different types of life in differentplaces that first gave him the idea for Evolution.

He noticed that each continent had its owncharacteristic biota, often showing similarfeatures although often not closely related toeach other. For example, each continent hascharacteristic grazing mammals:

All theseanimals arerelativelylarge, fast-running,with flatgrindingteeth, and all lead pretty much the same lifestyle.So why is there not just one type of large grazerin the world, living on every continent?

Evolution explains why. On each separatecontinent different animals have evolved to fillthe large grazing herbivore role, oftenbeginning that evolution from an ancestor quitedifferent to that on other continents.

Another aspect of Biogeography was seen byDarwin on a much smaller scale in the GalapagosIslands. He discovered that the islands werepopulated by many different species of smallbirds... finches. Although all were obviouslyrelated to each other, each separate island had itsown particular species.

Why?

The explanation is: the islands were first colonised by one type offinch which spread to all of the islands in the group. on each separate island conditions were different(eg different foods available) so each populationevolved in a different way, into a different species.

Comparative EmbryologyIf the embryos of various vertebrate animals arecompared, it is found that they are remarkablysimilar, even though they later develop intoquite different animals.

Why should they be so similar when they growup to be very different animals? Why should (forexample) a human embryo have a tail and gillarches like a fish?

Comparative AnatomyWhen the body structures of differentorganisms are compared, it is often obvious thatthey share common features, even though thosebody parts might be used in different ways.

The classic example of this is the PentadactylLimb structure of the vertebrate animals.

Each limb is used in very different ways byanimals that have very different lifestyles, yet allhave exactly the same basic arrangement ofbones. Why?

Once again, these are FACTS that are consistentwith, and easily explained by, the concept ofEVOLUTION.




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BIOGEOGRAPHY doesnt prove Evolution hasoccurred, but many facts about the distributionof organisms are best explained by Evolution

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Evolution explains this as the result of all theseanimals evolving from a common ancestor

which had an embryo like this.

Evolution explains this as the result of all theseanimals evolving from a common ancestor

which had this bone structure.

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Some Vertebrate Embryos...

...andwhat they develop into




5

BiochemistryAll the life-forms on Earth have remarkablesimilarities at the molecular level. All organismshave basically the same:

genetic code in their DNA. enzymes used for basic metabolic processes,

such as cellular respiration. structural chemicals in their basic cell

components, such as the phospholipids in cell membranes.

WHY SHOULD ALL ORGANISMS HAVE THE SAMESTRUCTURES AND CODES IN THEIR DNA?

Once again, this doesnt prove that evolutionoccurred, but it is consistent with the idea that

life on Earth has evolved from common ancestorswho had these biochemical features.

Selective Breeding proves that a species can be changed.

Humans can do it artificially,in the wild it happens by Natural Selection

New Technologies Change Our Ideas About Evolutionary Relationships

The new technology of analysing the sequence of DNA molecules is changing ourideas about the evolutionary relationships between living things.

Selective BreedingPeople wonder how one organism can justturn-into another by evolution. No individualanimal changes during its life-time. The changesoccur from one generation to another, as certainfeatures are selected in favour of others.

Humans have been doing it to plants & animalsfor centuries. Human farmers have always chosenwhich seeds to keep for next years crop, or whichbull to breed with the cows.

This has drastically changed all these plants andanimals. Modern wheat is nothing like the wildgrass we believe it was bred from. Cabbages andcauliflowers used to be the same thing, but havebeen changed by selective breeding.

All breeds of dogs aredescended from the wolf.

For example, the evolutionaryrelationship between LIZARDS,CROCODILES and BIRDS.

Traditionally crocodiles & lizards areclassified as Reptiles and considereda separate class to the Birds. It wasalways thought that the lizards andcrocodiles were more closely related toeach other than to the birds.

TRADITIONAL CLASSIFICATION

BIRD isDIFFERENT

Croc & Lizard in SAME GROUP

Based on bodystructure

This relationship has been overturnedby DNA analysis.

DNA sequencing shows that birds andcrocodiles are more closely related toeach other than either is to the lizards.

Stand by for more revelations asDNA analysis reveals more!

Croc & Bird in SAME GROUP

CLASSIFICATION IN THE FUTURE?

Based on DNAstudies, which

showevolutionary relatedness

LIZARD isDIFFERENT


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The Darwin-Wallace Theory of Evolution

You need to learn the details of this theory. Thisis best done as a series of numbered points:-

1. All organisms produce moreoffspring than can survive

Plants scatter thousands of seeds and maybeonly one ever makes it to plant maturity. Oystersproduce a million eggs at a time, but only 1 or 2survive. In all species, the breeding rate is farhigher than the survival rate.

2. In every species there is variationamong the individuals

Each animal or plant is slightly different to theother members of the species. The differencesmay be slight, and might not be easily visible,but theyre important!

If you put points 1 & 2 together, you can see thatsurvival is uncertain, AND the chances ofsurvival are not equal for each individual.

3. Nature selects which individuals survive

This is called Natural Selection, and it isexplained in more detail at right.

4. Those best suited to theirenvironment survive, reproduce, and pass on their characteristics

This is called Survival of the Fittest, and it willbe discussed in more detail later.

5. Over generations, the specieschanges...It Evolves

Each generation is slightly different than before,because only some, selected individuals havebred and passed on their characteristics.



Origin of a TheoryDarwin & WallaceIn the 1830s a young naturalist,Charles Darwin (1809-1882), voyagedaround the world on the naval surveyship HMS Beagle. He becameconvinced that living things hadchanged, and spent the next 20 yearsresearching, gathering evidence anddeveloping the how of a theory toexplain evolution.

Meanwhile, another Englishman, AlfredWallace (1823-1913) came up withexactly the same idea after his journeysin the 1840s and 1850s. We often giveDarwin most of the credit for this majorscientific theory, but Wallacescontribution should not be forgotten.

In 1859 they jointly published an outlineof their theory, and Darwin followed upwith his detailed book Origin ofSpecies about 2 years later.

CharlesDarwin

Natural Selection Natural Selection refers to the way thatthe conditions of nature constantly selectwho survives and who dies. If survivaldepends on speed to run from a predator,then the faster individuals survive and theslower ones get eaten. In a drought thoseplants with slightly thicker cuticle on theirleaves conserve water and survive, whileothers die.

How Natural Selection WorksA population of a species with a lot of variations

hairy long long light big squatneck legs colour ears body

Then, the climate changes... winters get colder

hairy survives Squat survives

Survivors breedover many generations

Many die in the harsh winters

The survivors pass on their characteristics.Soon most of the population are squat and hairy.No single animal changed, but the population has

changed because of which animals survived and reproduced.

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becommes a differrenntt species.

Natturral Selecttionn

Surrvival of tthe

Fittttestt


7

More on Natural SelectionAny characteristic might be a help to surviveunder different conditions... a stronger immune system survives diseases. being more inquisitive might find more food. more timid behaviour might avoid dangers. brighter coloured flowers might attract

more pollinators and ensure reproduction. larger body size might deter predators. smaller body size might allow more food

gathering on small branches of trees.

It may seem as if Evolution is an intelligentforce which directs organisms in a an appropriatedirection. In our fictitious population of animals,(previous page) the climate became colder and itwould seem that some evolutionary force causedan appropriate change in the population... theanimals became squat & hairy to conserve bodyheat better.

But of course squat and hairy were already inthe population. Their type simply became morecommon, and finally predominant, because theysurvived in greater numbers and reproduced topass on their characteristics to their offspring.

Evolution is not intelligent. Nature selects thesurvivors from the different types available.Survivors breed... if youre dead, you cantreproduce!

Effects of Changing ConditionsEnvironments can change in many differentways. For example, we know that ice-ages comeand go, and climates change as the continentsgradually drift to new locations. Any changein an environment will result in NaturalSelection picking out different characteristicsfor survival and the species will evolve in linewith the environmental change.

If the change is sudden and drastic, there will befewer survivors every generation and thespecies may decline into extinction!



A Change in the Physical Environment

A classic example of micro-evolution is thechange in the population of the English PepperMoth which has been studied and documentedover the past hundred years.

This moth always rests during the day on treetrunks, which in natural forests, are mostlycovered in light-patterned lichens. Under theseconditions the light peppery moths are themost common form, although occasional blackmoths occur.

During the coal-burning phase of the IndustrialRevolution many forests were damaged bypollution. The lichens were killed and treetrunks blackened with soot.

It was observed that the Pepper Moth populationchanged in the proportion of peppery to blacktypes.

Now that industrial pollution has been stopped,the moths have evolved back to being mostly ofthe lighter-coloured type.

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A Change in the Chemical Conditions of the EnvironmentOver many generations the non-resistant typeswere killed, and resistant types kept surviving andbreeding until almost the entire population wasresistant. DDT was no longer useful for killinginsects. (Just as well, because DDT causedecological damage by Biological Magnification.)

The DDT acted as a Chemical Selecting Agentresulting in the evolution of the insects by naturalselection and survival of the fittest.

Similar examples have been observed withbacteria becoming resistant to Penicillin and otherantibiotics.

Another example of micro-evolution wasobserved when DDT insecticide began to be usedagainst a variety of insects, such as disease-carrying mosquitoes or crop-eating pests.

Initially, the chemical was a huge success,destroying the insect populations. But thenNatural Selection did its thing...

Among the millions of insects in eachpopulation there was variation. A few individualshad a natural resistance to the DDT and theysurvived and reproduced and passed on theirresistance to their offspring.


8

The Importance of VariationIt is vital for the on-going survival of a speciesthat it has variation among the individuals of thepopulation.

When changes occur in the environment, aspecies with a lot of variation has more chanceto survive, because out of all the differenttypes there is a good chance that at leastsome will survive to breed and continue thespecies.

A species with little variation within it, mighthave no survivors from an environmentalchange, and become extinct.

What constitutes an environmental change? It could be...

a change of climate.

a new disease, predator or competitor.

a change in availability of a food.

...or any other biotic or abiotic factorof the environment.



Natural Selection and CompetitionIn a previous topic it was pointed out thatcompetition between 2 species for the sameresources usually results in a winner and aloser... the loser becoming extinct, at leastwithin the area of competition.

However, this is not the only possible outcome.If among the natural variations within eitherspecies there are some individuals who areperhaps less fussy about their food or nestingsites (what ever the resource might be) and whocan survive on slightly different foods, or utilisedifferent nesting sites, then they will have anadvantage to survive.

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Survivors breed and pass on theircharacteristics. Over generations the speciesevolves to be using a slightly different resource.Therefore, it is no longer in competition andboth species can survive... a Win-Winsituation.

This outcome is called Resource Partitioningand is commonly observed in nature. Forexample, the predatory big cats of the Africanplains avoid competition because each hasslightly different preferences among the variousprey animals. They also have different huntingtechniques, so they avoid direct competition.

More on Survival of the FittestSurvival does not just mean survival. Thebiggest toughest animal in the herd, who scarespredators away, gets to eat most of the food andlives to a ripe old age, is a complete failure if itdoes not reproduce!

In evolutionary terms Survival means tosurvive and reproduce.

Reproduction is the true measure of survival.

An animal which does not live long, butmanages to produce many, viable offspring is anevolutionary success!

Fittest refers to those individuals with acombination of characteristics best suited totheir environment. It doesnt just meanbiggest, fastest,strongest... thosebest suited to someenvironments mightbe the smallest,sneakiest, mostcautious types.

... or the mostchaarming aand

sociaallyaacceptaable


9

Convergent Evolution(Converge=come together)If totally different organisms live in thesame kind of environment and leadsimilar lifestyles they will be subject tothe same sorts of Selection Pressuresand evolve many of the same features,so they may come to resemble eachother even though not closely related atall.

A classic example is the shark (a fish),dolphin (a mammal) and the extinctichthyosaurus (a reptile).

All 3 animals are (were) fast-swimmingfish-hunters of the ocean. All evolvedthe same streamlined shape, powerfultail, dorsal fin and sharp teeth to suitthis lifestyle. The resemblance issuperficial, and they are very differentin the details of body structure andmetabolism. For example, the shark isa gill-breathing ectotherm, while thedolphin is a lung-breathing endotherm.

There are many other examplesof such convergence.



Directions of EvolutionIt is easy to imagine that evolution has a target that it aims for.

Looking back at fossil ancestors of a modern species, its easy to think that thoseancestors were deliberately evolving to become the modern, final species.Ancestors always look primitive compared to their modern descendants.

Of course, the modern species is NOT the final outcome at all. Evolution has no targets. It is an on-going process which continues to cause changes.

In 100 million years time, the fossils of todays creatures will seem primitive.

Divergent Evolution (Diverge = move apart)One of the aspects of evolution to beaware of is the importance of Isolation.

Rember Darwins finches on theGalapagos Islands?

From one ancestral species of finchover a dozen different species evolved,one on each of the islands in the group.Once a population of finches coloniseda new island they were isolated fromother populations. Movement of birdsbetween islands must have been a rareevent, and each population waseffectively cut off from otherpopulations.

On each island conditions weredifferent... different foods wereavailable, different conditions of watersupply, shelter, nesting sites, predatorsand so on. Natural selection causedeach population to evolve along adifferent path until they becamedifferent species... they had diverged!

Now, even if they mixed together again,they could not interbreed because theirmating rituals, sperm & egg cells andDNA have changed to becomeincompatible. They are forever separate.This is how we think all species havearisen from common ancestors overmillions of years.

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More


The most important, direct evidence forEvolution comes from a).......................................,the study of b)......................... This shows thatlife on Earth has changed from beingc).................. complex to become d).....................complex, and showing e)..................... variety tof)....................... variety and becoming more andmore like g)..........................................................over a period of h)................................ of years.

Initially scientists could only place fossils ini).......................... time order byj)............................... the fossils from one place toanother. The technique of k)...................................dating has allowed absolute ages to bemeasured.

An important type of fossil is al)............................ form, which is an in-between organism. A good example ism)................................... It appears to be an)........................... in the process of evolving intoa o)....................

pollution changing the background colours ofits environment. This led to a change in the ratioof p)........................................................................

Another example was due to a chemical changein the environment. When the insecticideq)....................... was widely used it acted as ar).......................... agent, causing mant pestspecies to evolve to become s)..............................

Competition for resources can result in onespecies losing and becoming t)............................However, another possible outcome is calledResource u)................................. which resultsif one species evolves to avoid the competitionby v).........................................................

Divergent Evolution is when one speciesw).......................... into ...........................................This happens when different populations of aspecies become x)................................ from eachother. Each population is subject to differenty).......................... pressure due to the differingenvironments.

z)....................... Evolution is when quiteunrelated species evolve to aa).............................each other. This can happen because they live inab).................... environments and so naturalselection favours evolution of similarac).............................. A good example is theshark and ad)........................... which have manycommon ae).................................. even thoughthey are not af)............................... to each other.

10



Worksheet 1 Evidence for Evolution `Name....................................

Worksheet 2 Theory of Evolution Name....................................

Biogeography is the study of p)............................................ It gives indirect evidence for evolution.

Comparative Embryology reveals that vertebrateembryos are all q)............................... to eachother, even though each animal grows up to ber)..................................... This is explained byevolution as being due to these animals allevolving from a s)................................................... Similarly, the study of Comparative Anatomyshows much the same. For example, theforelimbs of vertebrates have the samet)................................ even though each animaluses the limb u).................................................This 5-digit limb structure is calledv).....................................

Biochemistry reveals that all organisms havethe same genetic code in their w).........................and use the same x).................................... tocontrol their metabolic processes. This isevidence of y)..........................................................

Selective Breeding of domesticated plants andanimals shows that z)............................................

The Theory of Evolution was developedindependently by a).................... and ...................

It can be summarized in 5 main points:1. All organisms produce b)..................................2. Every species shows c)......................................3. d)..................................................... occurs tochoose the survivors.4. Those organisms e)................................. to theenvironment survive and f)..............................This is called g)...................................................5. Over generations, the species h).......................because the selected characteristics are beingpassed on in greater proportions.

Natural Selection refers to the way that theforces of nature choose who survives. Theseforces of nature could include disease,i)............................... and .......................................,and any factors in the environment. Survival ofthe Fittest refers to how the organismsj)........................................ to the environment willsurvive and k).....................

Variation within a population is vital so that if theenvironment changes, there are more likely tobe some individuals who will l).............................and m).......................................

Changes to actual populations have beenobserved. The n)......................... Moth of Englandwas observed to undergo a population changeover generations due to o).....................................

Multiple Choice1. The fossil record shows a pattern, from ancienttimes to recent times, of:A. more complexity to less complexity.B. greater diversity to less diversity.C. increasing complexity and diversity.D. no distinct pattern of change.

2. The famous archaeopteryx is a:A. common fossil, found in many rock layers.B. transitional form, showing a stage

of evolution.C. index fossil, useful for correlating between

locations.D. living fossil; an extremely primitive bird

alive today.

3. Two types of evidence which both suggestdescent from a common ancestor are:A. embryos of vertebrates & the pentadactyl limb.B. convergent evolution and biochemistry.C. DNA structure and the biogeography.D. natural selection and survival of the fittest.

4. Natural Selection refers toA. those organisms best suited to the

environment survive and reproduce.B. factors of the environment determining which

organisms survive and which do not.C. the changes to living things over generations.D. the way that not all organisms can survive

because too many offspring are produced.

5. The really important part of Survival of theFittest is:A. being very well adapted to the environment.B. living a long and healthy life.C. being chosen to survive.D. reproducing many offspring.

6. A species has a better chance of long-termsurvival during changes to the environment if:A. it reproduces asexually.B. most individuals are similarly adapted.C. the species has a lot of genetic variation.D. there are many large, strong individuals.

7. An example of microevolution that hasactually been observed to happen, is:A. the extinction of the dodo.B. divergence of Darwins finches on the

Galapagos Islands.C. convergence of the shark and dolphin.D. change in the population of Pepper Moths.

Longer Response QuestionsMark values given are suggestions only, and are togive you an idea of how detailed an answer isappropriate. Answer in space provided, or on reverse.

8. (4 marks) Explain the difference between relative datingof fossils and absolute dating, by outlininghow each is achieved and what each can tell youabout fossils.

9. (3 marks)Comparative Anatomy can give evidencesupporting the idea of evolution. a) Outline an example of comparative anatomythat gives such evidence.

b) State how this example is evidence ofevolution.

10. ( 3marks)Briefly discuss, using an example, howadvances in technology have changedscientific thinking about evolutionaryrelationships.

11. (5 marks)Outline the Darwin-Wallace Theory of Evolution.

12. (4 marks)Differentiate between convergent anddivergent evolution, giving examples of each.

11




Worksheet 3 Test Questions Section 1 Name....................................




12

In Charles Darwins lifetime there were 2 parts ofhis Theory of Evolution that no-one couldexplain:Where Does Variation in a Species Come From?

(That will be explained later in this topic)How Are Characteristics Passed On toOffspring? That will be explained right now!

Gregor Mendels ExperimentsMendel was the Abbot of a monastery in whattoday is the Czech Republic. He was trained asa teacher and was not a professional scientist,but became interested in discovering howinheritance works. The monks grew most oftheir own food, so Mendel worked hisinvestigations into the day-to-day vegetablegardening by choosing to experiment withgarden peas.

First he bred his pea plants over severalgenerations to select plants that were purebreeding for certain contrastingcharacteristics, such as...

Each type was pure breeding, meaning that ifthey were self-pollinated they always producedoffspring of exactly the same type as themselves.

Then he cross-pollinated 2 contrasting types toobtain hybrid (cross-breed) offspring. Theresult was that all the offspring showed thecharacteristic of one parent and none took afterthe other. For example, when TALL plants werecrossed with DWARF:

Next, he self-pollinated the plants of the F1.

Mendel didnt do this with one or two plants, butwith hundreds. His 2nd generation totalledthousands of plants, not just a few.

He got the same result with flower colours, seedshapes, seed pod colours, and so on. In everycase the first generation always took after oneparent completely, and the second generationalways appeared (in their thousands) in a ratioof very close to 3:1.

2. MENDELIAN GENETICS

TALL DWARF PURPLE WHITEFLOWER FLOWER

X

F22 (2nd Generation) Offspringoccurred in a ratio of 3:1

X XX iiss sshhoorrtt-hhaanndd ffoorrccrroosssseedd wwiitthhF11 (1st Generation) Offspring were ALL TALL

Mendel explained this result by suggesting thatthe factor (GENE) which caused Tallness was

DOMINANT to the gene for Dwarfism. Dwarf is RECESSIVE to Tall.

SSEELLFFPPOOLLLLIINNAATTEEDD

SSEELLFFTTAALLLL FF1 ppllaanntt

Reasons for Mendels Successas an Experimenter

Gregor Mendel had discovered the basicmechanism of inheritance. Scientistsbefore him had studied inheritance, buthad failed to discover the simple patterns.Why was he successful?

He chose simple, easily-identifiedcharacteristics which occurred in just 2forms. e.g. height was either tall or dwarf.

He began with pure-breeding parentplants.

He bred large numbers of plants sothat the numbers of offspring werestatistically significant.

(If hed bred just a few offspring thenrandom chance could have producedmisleading results.)


13

The diagram (above, right) explains why Mendel observed a ratio of about 3:1 in the plants of his F2 offspring.

His experimental ratios were approximately 3:1, but not exactly 3:1. This isbecause the actual combinations of gametes at fertilisation occur at random.

He bred large numbers of plants and so his actual ratio was very close to theoretical.

Reasons Why Mendels Work Was IgnoredMendel published his results in 1865 in a German Science journal and was totally ignored. Why?

He was not a recognised scientist, and had no contact with the scientific establishment of histime. His work was read by only a small circle of people, who failed to see its significance.

His work was written only in German, while all the important science of that time was appearing in English or French, in well-known journals in England, France & USA.

It was not until well after Mendels death that in 1900 his work was discovered by scientists,and his great contribution was recognised. He is now known as the Father of Genetics.



These are Phenotypes...descriptions of the outward

appearance

These areGenotypesthe actual

genespresent

OOnnllyy 11ggeennee iissppaasssseedd

iinnttooggaammeetteess

GGAAMMEETTEESS

PPoossssiibblleeGGAAMMEETTEESS

PARENT PLANTS Tall x DwarfTT tt

Tall x TallTt Tt

T

TtTall

t

FFeerrttiilliissaattiioonnAll the F11offspringreceive thiscombination

of genes

MMeennddeellss PPuurree-BBrreeeeddiinnggppaarreenntt ppllaannttss wweerree

hhoommoozzyyggoouuss ffoorr eeaacchhttrraaiitt...... 22 ggeenneess tthhee ssaammee

SSeellff-ppoolllliinnaattiinnggtthhee FF1 ppllaannttss iiss

tthhee ssaammee aassccrroossssiinngg wwiitthh

tthhee ssaammeeggeennoottyyppee

ALL THE F11OFFSPRING ARE

TALL, BUTHETEROZYGOUS

T Tt t

TT Tt Tt ttTALL TALL TALL DWARF

Ratio of Phenotypes 3 Tall : 1 Dwarf

Next, the F11 plants were self-pollinated to produce the F22

ppoossssiibblleeffeerrttiilliissaattiioonnss

Mendels Explanation of Results(Using Modern Terminology)

Each characteristic is produced by factors (wenow call them genes) carried by the plants. Forexample, there is a gene for tallness of stem,and a corresponding gene for dwarf stem. Thereis a gene for purple flower and another for whiteflowers, and so on for other characteristics.

The genes which control opposite forms ofthe same characteristic are called alleles, orallelic genes. Genes for tall and dwarf arealleles. Genes for purple flower and whiteflower are alleles.

One of the alleles is Dominant over the other,which is said to be Recessive. We usually useletters of the alphabet to designate this:e.g. Tall ( T ) is dominant to Dwarf ( t )

Purple flower ( P ) is dominant to white ( p )

Each plant carries 2 genes for a characteristic.The 2 genes may be the same as each other(homozygous) or different to each other(heterozygous).

Example: for the height characteristic, thepossibilities are:

TT = homozygous, Tall plantTt = heterozygous, Tall plant (T dominant)tt = homozygous, Dwarf plant

Although each plant carries 2 genes for eachcharacteristic, only one gene is passed into thegametes (pollen or ovules). Each parent passeson one gene, so the offspring gets one fromeach parent and gets back to having 2 genes forthe characteristic.

Which one of the 2 genes for each characteristicis passed on is completely at random.


14

Monohybrid Crossesmono = 1 (referring to just one characteristic)hybrid = cross-breed

It has been found that there are manycharacteristics, in all sexually-reproducingorganisms, which are inherited just the wayMendel discovered... these are cases of Mendelian Genetics andyou need to be able to predict the outcomes ofsimple crosses.

Study the following examples to help you workthrough the next worksheet.

Sample Problem 1In mice, black coat (B) is dominant to albino (b).Predict the outcome of mating a heterozygousblack mouse with an albino.

Solution

Parents are

Sample Problem 2In drosophila fruit flies, the allele for long wings(L) is dominant to the allele for short wings ( l ).A pure breeding long winged fly was crossedwith a short winged fly. Their offspring wereallowed to mate among themselves to produce asecond generation. There were 240 flies in theF2. Predict how many of each phenotype wouldoccur.

SolutionTo work out the F1, a punnett square is not reallyneeded.

Parents genotypes: LL x l l

Gametes possible: L only and l only

All the F1 flies must be Genotype: Ll

Phenotype: 100% Long winged

Then, for the F2:

If the F2 comprises 240 flies, we should expect close to180 long wing flies and 60 short wing flies.

However, this is a statistical prediction only, andwe should not be surprised if the actual numberswere (say) 190 to 50, just by random chance.



Bb bb

B & b b & b

b b

B

b bb

Bb

bb

Bb

x

ggaammeetteess

PPhheennoottyyppeess ooff OOffffsspprriinnggBBllaacckk :: AAllbbiinnoo

11 :: 115500%% :: 5500%%

You will soon come to realise that only 3 outcomes are possible:

100% : zero 50% : 50%75% : 25%

The Punnett Square

The working out of a cross by a diagram canbe a bit messy and confusing. A scientist calledPunnett invented a simpler method which youmust learn to use.

The Punnett Square working (right) shows thesame cross... the formation of the F2 plants inMendels experiment.

L l L l

L & l L & l

L lL

l L l

LL

l l

L l

x

ggaammeetteess

PPhheennoottyyppeess ooff OOffffsspprriinnggLLoonngg WWiinngg :: SShhoorrtt WWiinngg

33 :: 117755%% :: 2255%%

GGeennoottyyppeess ooffppaarreennttss

LLiisstt ooff aallllppoossssiibblleeggaammeetteess

PPuunnnneetttt SSqquuaarreettaabbllee sshhoowwss aallll

tthhee ppoossssiibblleeooffffsspprriinnggggeennoottyyppeess

TThhee pphheennoottyyppeessooff ooffffsspprriinngg aarree

wwrriitttteenn aass aa rraattiioooorr ppeerrcceennttaaggee

Tt Tt

T & t T & t

T t

T

t Tt

TT

tt

Tt

x

ggaammeetteess

ppaarreennttggeennoottyyppeess

tthhee ttaabbllee sshhoowwss aalllltthhee ppoossssiibbllee

ggeennoottyyppeess ooff tthheeooffffsspprriinngg

pphheennoottyyppeess ooffooffffsspprriinngg sshhoouulldd bbeesshhoowwnn aass aa rraattiioo oorr

ppeerrcceennttaaggee

PPhheennoottyyppeess ooff OOffffsspprriinnggTTaallll :: DDwwaarrff

33 :: 117755%% :: 2255%%




Pedigrees (Family Trees)Another skill you must learn is how to read,interpret and construct a pedigree diagram.

This is a diagram showing the inheritance of atrait through a family. It is used particularly withhuman families to trace some characteristicover a number of generations.

Symbols Used in Pedigree Diagrams

Malewith trait being studied

Female with trait

Horizontal connections are marriage lines.Vertical lines lead to children of that couple.Each generation is numbered by RomanNumerals.Individuals may be numbered for identification.

ExampleIn humans, some people can roll their tonguewhile others cannot. This is passed on bysimple Mendelian Inheritance. Here is apedigree of a family showing how this trait waspassed on.

15

Interpretation of this PedigreeIn Generation I, individuals 1 & 2 were bothtongue-rollers. They had 4 children, a daughterand 3 sons. Most of the kids can roll theirtongues, but son 4 cannot.

This means the inability to tongue roll must berecessive.(Whenever a child shows a trait different to bothparents, the childs phenotype must berecessive.)

Therefore, tongue-rolling ability must bedominant.

We can now assign symbols... tongue-rolling = R non-rolling = r...and work out most peoples genotypes:

1 2 3 4 5 6 7 8 9Rr Rr ? rr ? Rr rr rr rr

(Individuals 3 & 5 might be either RR or Rr. ... more information needed to be sure which)

Questions & AnswersHow can we be sure that parents 1 & 2 are bothRr (heterozygous)?A: Since they produced son 4 who is a non-roller (must be genotype rr) both 1 and 2 mustbe carrying the recessive gene. Therefore, bothmust be Rr.

Can we be sure that son 6 is Rr and NOT RR?A: He married a non-roller (rr) and both thechildren in generation III are non-rollers.Therefore, son 6 must have passed on arecessive gene to his children. He must beheterozygous (Rr) to do this.

If 6 and 7 had another child, could it be atongue roller? Whats the chance?A: Yes. The cross is Rr x rr.If you work out a punnett sqare for this, you willsee that the expected outcome is 50% rollersand 50% non-rollers. The chance for the nextchild is 50% either way, and is NOT affected bythe fact they have already had 2 non-rollerchildren.

Male without trait

Female without trait

11

55

22

33 44 66 77

88 99

I

II

III

GGeenneerraattiioonnss

Uses of Pedigree DiagramsPedigree diagrams are used to study human inheritancebecause it is not moral or ethical to carry out breeding

experiments on people to discover which traits aredominant/recessive.

Some human disorders are inherited. Examples arehaemophilia (in which blood will not clot properly) and

colour-blindness (inability to distinguish certain colours).Health professionals can study affected families by

compiling a pedigree chart, then advise people about therisks to future children. This allows people to make

informed decisions about family planning.

Sarah can...Nathan cant




Gregor a)........................ was a monk whoexperimented with b)................ plants anddiscovered the basics of how characteristics arec)................................. He started with plants thatwere d).................-breeding for 2 opposing traits,and crossed them. For example, he crossedpure-breeding tall-stem plants withe)................................, f).....................-stem plants.In the first generation, or g)........., the offspringplants were 100% h)................. When these wereallowed to i).....................-pollinate, the F2 (whichmeans j)........... .............................) showed a ratioof k)....................... of tall to dwarf plants.

He explained these results as follows:Each trait is determined by factors we now calll)................. The genes which control theopposing traits of a characteristic are calledm)......................., or n).............................. genes.For each characteristic, an organism always haso)........... genes, which may be p)..........................(homozygous), or may be different (calledq)...................................) In sexual reproduction,the r)...................... (eg sperm/eggs) contain only1 of the genes for each trait. When the gametesunite at s).........................., one gene from eacht)............. are brought together in each offspring.

16

Worksheet 4 Mendelian GeneticsFill in the blank spaces Name....................................

Worksheet 5 Mendelian Genetics Problems Name....................................

One of the alleles is dominant to the other, whichis called u).......................... The v).........................gene will always show its effect, but therecessive one can only be expressed in the casewhere it is w).................-zygous. The appearanceof an organism due to its genes is called thex)..............................., while the genotype is they).....................................................................

Mendel was successful, where others had failed,because:1. he chose z)................................. characteristics2. he began with aa)......................... parent plants3. he bred ab)................... numbers of plants sothe numbers of offspring would beac)........................................ significant.

However, Mendels achievements were notrecognised because he was notad).............................................. and because hisfindings were published in an obscure journalwritten in ae)................................. and did notcome to the attention of the scientificcommunity until after his death.

1. In pea plants, green seed pods (G) is a dominanttrait over yellow seed pods (g)a) What is the phenotype of a plant, if thegenotype is:i) GG? ii) gg? iii) Gg?

b) What is the genotype of a plant with seedpods that are:

i) green, and it is pure-breeding?

ii) heterozygous?

iii) yellow?

c) Use a punnett square to predict the outcomeof each of the following crosses. In each case,state the expected phenotypes of the offspringas a percentage. (Answer on reverse)

i) Gg x Gg

ii) Gg x GG

iii) gg x GG

iv) Gg x gg

2. In rats, black fur (B) is dominant to brown (b). a) If a pure-breeding black rat mated with a pure-breeding brown rat, what would be the

i) genotypes of the offspring?

ii) phenotypes of the offspring?

b) One of the offspring from the cross describedin part (a) was crossed with a brown rat.

i) Use a punnett square to predict the outcome.

ii) In fact, they produced 7 babies; 5 black & 2brown. Is this result surprising? Explain youranswer.

In humans, some people have little fingers thatare straight, while others have curved littlefingers. This characteristic is inherited bysimple Mendelian inheritance. Study thepedigree diagram, then answer the questionswhich follow.Shaded shapes represent curved little fingers.

a) Is the curved little fingers trait dominant orrecessive? Explain your answer referring tospecific individuals above.

b) Assign the letters S and s appropriately tothe 2 alleles operating in this pedigree.

c) Extra information: individuals 2 & 4 arehomozygous.Using the symbols chosen, work out thegenotypes of everyone in the pedigree, as far asis possible.

d) Couple 1 & 2 had children who all havestraight fingers. Was there any chance theymight have had a child with curved little fingers?Explain your answer.

e) Person 5 later married a girl with curved littlefingers. Use a punnett square to predict thefinger shapes of their children. (on reverse)

f) In fact, person 5 and his wife had 2 beautifullittle girls both with straight fingers. Is thispossible? Is your prediction wrong?

17




Worksheet 6 Pedigrees 11

55

22 33 44

66 77 88 99

11111100

I

II

III

Worksheet 7 Test Questions section 2 Name....................................Multiple Choice1. When Gregor Mendel crossed pure-breedingtall pea plants with pure-breeding dwarf plants,and bred them through 2 generations, he foundthat the F2 phenotypes were:A. 100% tall plantsB. 75% tall: 25% dwarf, approximately.C. 50% tall : 50% dwarf, approx.D. ratio of 3:1, dwarf : tall, approx.

2. The genotype of a pea plant described asheterozygous tall would be:A. Tt B. T C. TT D. tt

3. In mice, Black coat (B) is dominant to white(b). If a heterozygous mouse mated with a whitemouse, you would expectA. approximately 75% of the babies to be black.B. all the babies to be black.C. approximately 50% of the babies to be whiteD. all the babies to be white.

Longer Response Questions4. (6 marks)Explain the difference between each pair ofwords:a) Dominant & Recessive genes.b) Homozygous & Heterozygous.c) Genotype & Phenotype.

5. (5 marks)Outline the reasons why:a) Mendels experiments were successful indiscovering the fundamentals of howinheritance works.

5. (cont)b) Mendels discoveries were ignored by otherscientists for many years.

6. (5 marks)In pea plants, green seed pods(G) is dominant toyellow pods (g). A pure breeding green pod plant was crossedwith a pure-breeding yellow pod plant. a) Explain why all the offspring will have greenpods.

b) One of these offspring plants was latercrossed with a yellow pod plant. Showing allworking, predict the phenotypes (as ratio orpercentage) of the offspring from this secondcross.

7. (8 marks)a) Use the following information to construct apedigree diagram.

Inheritance of red-hair in the Meggs family.Fred and Mary both have blonde-brown hair.They have 4 children, a daughter and 3 sons.Their daughter and one son are red-heads, theother 2 sons are similar to their parents. The red-headed son married a red-headed girl and theyhave a son and a daughter.

b) State whether red-headedness is adominant or recessive trait, giving reason(s).

c) Predict the hair colour of Fred & Marysgrandchildren. Explain your answer.




2 Daughter cells IIddeennttiiccaall ttoo eeaacchh ootthheerr aanndd ttoo

tthhee ppaarreenntt cceellll

4 Gamete cells NNOOTT iiddeennttiiccaall ttoo eeaacchh ootthheerr,,

nnoorr ttoo ppaarreenntt cceellll

18

ChromosomesMendels discoveries were re-discovered bymainstream science in 1900. At about the sametime, new techniques in using stains tohighlight specific parts of the cell had led to thediscovery of chromosomes. The combination ofMendelian Genetics + Chromosomes was thenext big breakthrough. Things to know...

Chromosomes are thread-like structures in thenucleus of cells. They only become visible (to alight microscope) during cell division.

Chromosomes have genes along their length. There may be 1000s of genes on onechromosome.

Chromosomes occur in pairs. Chromosomesin a pair are called homologous. Homologouschromosomes correspond with each other bycarrying allelic genes in correspondinglocations.

Study this diagram to getthe idea.

Consider 3 sets of genes.Genotype is AaBbCc.(heterozygous for eachcharacteristic)

This is how the genescould be located on somechromosomes.

Notice that homologouspairs correspond with each other, but are NOTidentical.

Before cell division, each chromosome isduplicated.

Study the diagram.

The original and its exactcopy remain attached toeach other, as a doublechromosome.

Each part of a doublechromosome is called aChromatid.

The chromatids in adouble chromosome areidentical... (look at thegenes in the diagram)but the homologous partners are notidentical... merely corresponding.

Mitosis & MeiosisYou should already be familiar with thedifference between these processes of celldivision in terms of their outcomes.

Now look more carefully at what happens to thechromosomes during each process.

3. CHROMOSOMES & DNA STRUCTURE

HHoommoollooggoouuss ppaaiirr

AAnnootthheerr HHoommoollooggoouuss ppaaiirr

HHoommoollooggoouuss ppaaiirr ooffDDoouubbllee cchhrroommoossoommeess

Indentical Chromatids ineach chromosome

ORIGINAL BODYCELL

with 4chromosomes(2 homologous

pairs)

TThhiiss cceellll iiss DDIIPPLLOOIIDD((aabbbbrreevv.. 22nn))

((cchhrroommoossoommeess iinn ppaaiirrss))

IInn tthhiiss ccaassee,,22nn == 44

DDIIPPLLOOIIDD CCEELLLLSS22nn == 44

HHAAPPLLOOIIDD CCEELLLLSSnn == 22

Mitosis Meiosis

IInn BBOOTTHHpprroocceesssseess,, tthheecchhrroommoossoommeess

aarree ffiirrssttdduupplliiccaatteedd,, ttooffoorrmm ddoouubbllee

cchhrroommoossoommeess

HHoommoollooggoouussppaaiirrss hhaavvee

sseeppaarraatteedd,, aannddcceellll ddiivviiddeess iinn 22..

NNooww cceellllss ddiivviiddeeaaggaaiinn,, aanndd

sseeppaarraattee tthheecchhrroommaattiiddss..

CChhrroommaattiiddsssseeppaarraattee

CCeellll DDiivviiddeessiinn 22

HHaapp

llooiidd

mmeeaa

nnss hh

aallff tt

hheecchh

rroomm

oossoomm

ee nnuu

mmbbee

rr..TThh

ee cchh

rroomm

oossoomm

eess aa

rree nn

oott iinn

ppaaii

rrss..

AAaa

BBbb

CC cc

AA AA

BB BB

aa aa

bb bb

CC CC cc cc

CChhrroommoossoommeesslliinnee uupp iinn aassiinnggllee lliinnee

CChhrroommoossoommeesslliinnee uupp iinn tthheeiirrhhoommoollooggoouuss

ppaaiirrss

19

Chromosomes & Mendels GenesIn 1902, two scientists independently noticed asimilarity between the way that chromosomesbehaved in meiosis, compared to how Mendelsgenes were inherited.

Walter Sutton (USA) and Theodor Boveri(Germany) had both been studying meiosisusing new staining techniques which madechromosomes more visible. Both were aware ofMendels discoveries, which had been re-discovered just 2 years before.

The obvious similaries made it clear that the genesmust be located on the chromosomes in the cellnucleus.

Chromosomes Contain DNAChemical analysis reveals that chromosomesare made of Deoxyribonucleic Acid (DNA forshort) wrapped in proteins.

We now know that DNA is a double helix-shapedmolecule which carries a chemical code... it is agene.

Each chromosomecontains 1000s of DNA

molecules (wrapped inprotein) each one specifying

one of the organisms manyhereditary traits.

The Structure of DNALike many biological molecules, DNA is apolymer, made of many smaller units which arejoined in long chains. The basic unit of DNA is aNucleotide. (named for nucleus)

The 4 different bases are usually known just bythe first letter of each name:

A AdenineC CytosineG GuanineT Thymine

DNA molecules are composed of 2 strands ofnucleotides (one running upside-downcompared to the other) which are joined by thebonding between base molecules.

Finally, the entire molecule is corkscrewed intoa double helix, rather like a spiral staircase or ladder.




Chromosomes Mendels During Meiosis Genes

Begin in homologous Two genes for eachpairs characteristic

Pairs separate The 2 genes separatein meiosis in gamete formation

Gametes have only 1 Gametes have only 1of each chromosome of each gene pairpair (haploid)

At fertilisation, each At fertilisation, eachgamete supplies one gamete supplies onechromosome, so the gene, so the offspringzygote gets back to gets back to havinghaving paired chromo- two genes persomes (Diploid) characteristic

A NUCLEOTIDE

PPhhoosspphhaatteeggrroouupp

SSuuggaarr((DDeeooxxyyrriibboossee))

Base moleculeThere are 4 different

bases, so 4 nucleotidesare in DNA

OOnnee NNuucclleeoottiiddee

Two Strands of Nucleotide Chains

BBaasseess AA-TT bboonnddeedd

BBaasseess CC-GG bboonnddeedd

A

A

G

G

T

T

C

C

Notice that the onlybase combinations are

A-TT and C-GG

SSuuggaarrss && pphhoosspphhaatteessaarree tthhee ssiiddee rraaiillss

BBaasseess aarree tthheesstteeppss ooff aa

llaaddddeerr

PPaarrtt ooff aa DDNNAAmmoolleeccuullee

Chromosomes


20

Morgan produced hundreds of flies in theexperiment so, like Mendel, his results werestatistically significant.

The Explanation:The genes for eye colour are carried on the Xchromosome.The dominant (Red) gene can be designated asXR. The recessive (White) gene is Xr The male y chromosome does NOT carry oneof these alleles at all.

The possible female (XX) genotypes &phenotypes are:

XRXR Red eye female (homozygous)XRXr Red eye female (heterozygous)XrXr White eye female

The possible male (Xy) genotypes & phenotypesare:

XRy Red eye maleXry White eye male

Note that females get 2 genes, but males onlyget one because their y chromosome lacksthis allele totally. A male cannot beheterozygous for this trait and cannot have ahidden recessive gene.

WHEN DOING PUNNETT SQUARES WITH SEX-LINKAGE, YOU MUST TRACK THE

X AND y CHROMOSOMES...

see next page.



Non-Mendelian InheritanceGregor Mendel discovered the basics of Genetics, but it was found early in the 20th century

that genes dont always work in that simple Mendelian fashion.

In a normal human body cell there are 46chromosomes arranged in 23 homologouspairs. Of these, 22 pairs are called autosomesand are the same size and shape in males as infemales.

The 23rd pair are the sex chromosomes:

Females have a matching pair of chromosomesthat areknown as X chromosomes. A female is described as XX

Males have one Xchromosome, and one smaller y chromosome.

Males are described as beingXy.

What Determines Sex?In humans, and in many other species, sex is determined by a special

pair of chromosomes... the sex chromosomes.How these chromosomes are passed on tochildren can be shown using the Punnett Squarediagram to track, not genes, but the sexchromosomes:

Notice thatfemales can only pass on an X chromosome in their eggs.

Males sperm cells can eithercarry an X or a y.

Which type ofsperm fertilisesthe eggdetermines the sex of the baby.

Xy XX

X & y X & X

X X

X

y Xy

XX

Xy

XX

x

ggaammeetteess

PPhheennoottyyppeess ooff OOffffsspprriinnggGGiirrllss :: BBooyyss

11 :: 115500%% :: 5500%%

GGiirrllss

BBooyyss

FFaatthheerr MMootthheerr

Thomas Morgan began experimenting withDrosophila fruit flies and quickly found theywere ideal for genetics experiments.

In 1910, in an experiment involving flies withdifferent eye colours, Morgan realised that theway this characteristic was being inheriteddepended on the sex of the fly... males andfemales were inheriting eye colour differently.

Sex-Linkage InheritanceThe common and normal eye colour in the fliesis red. Morgan discovered a male fly with whiteeyes. He set out to do a Mendel-type breedingexperiment:

PARENTS

F1 generation

All have red eyes. Morgan concluded (correctly)Red is DOMINANT,White RECESSIVE

F2 generation

Females 100% Males 50%: 50%red eyed.

Whats going on?

X

White-eeyed Red-eeyedMale Female

F11 flies were allowedto mate amongthemselves


21

Morganss Sex-Linkage Experiment (continued...)

Here is the experiment explained by PunnettSquare.



Xrry XRRXRR

Xrr & y XRR & XRR

XRR XRR

Xrr

y XRRy

XRRXrr

XRRy

XRRXrr

x

ggaammeetteess

PPhheennoottyyppeess ooff OOffffsspprriinngg ((bbootthh sseexxeess))RReedd::WWhhiittee

110000%% :: 00

The F11 are allred-eeyed.Note thatfemales are allheterozygous.

Then, the F1 generation were allowed to breedamong themselves. Notice that the F1 femalesare all heterozygous red eyed ( XRXr ) and themales are all red eyed ( XRy ).

XRRy XRRXrr

XRR & y XRR & Xrr

XRR Xrr

XRR

y XRRy

XRRXRR

Xrry

XRRXrr

x

ggaammeetteess

PPhheennoottyyppeess ooff OOffffsspprriinnggFFEEMMAALLEESS MMAALLEESSRReedd :: WWhhiittee RReedd :: WWhhiittee

110000%% :: 00 5500%% :: 5500%%

The F22 arered : white3 : 1

just likeMendelsresults, butthe sexes aredifferent.

F2F1

There are a number of genes in humans that are sex-linked. The best-known are 2 genetic disorders:

Red-Green Colour Blindness, is where a person cannot distinguish between certain colours. Haemophilia, is a condition in which the blood does not clot properly, putting the person at constant

risk of internal bleeding. It was always a fatal condition, but in modern times people withhaemophilia are treated with clotting factors from donated blood.

Both conditions are sex-linked, inherited as recessive genes on the X chromosome. They occur much more often in males than females.

Inheritance by Co-DominanceAnother example of Non-Mendelian Inheritance is known as Co-Dominance. This is a fairlycommon situation in which the 2 alleles for a characteristic do not show a Dominant-Recessive

pattern, but when both genes are present (heterzygous) they are both expressed, and may result in an in-between phenotype.

Example: In Shorthorn cattle, there is gene (R)that causes the hair of the animals coat to bered (actually a rusty-brown colour). To be red, the animal must be homozygous for this gene. Genotype RR

The other allelic gene (W) causes the coat to be white, if the animal is homozygous.

Genotype WWIf an animal is heterozygous, with both alleles present, neither gene dominates the other. Both genes are expressed, producing mottled patterns of red and white hair which is called roan.

Genotype RW

Note that it is best to use 2 different CAPITAL letters as symbols in this case, since neither gene isrecessive.

F2Phenotypes: RED : ROAN : WHITE

1 : 2 : 125% : 50% : 25%

Try to verify for yourself the F2 result by using aPunnett Square.

Here is what happens in a Mendel-type cross:

RR WWXPARENTS

gametes: R only W only

F1 100% Roan

RW RW RW RW

FF1 bbrreeeedd aammoonnggtthheemmsseellvveessgametes: R or W

RR RW RW WW




22

Crossing-OverDuring meiosis homologous chromosomes alsoswap pieces of chromatid with each other,further mixing up the possible genecombinations:

Variation Caused by Sexual Reproduction

The simple fact that sexual reproductioninvolves TWO parents, creates a lot of variation.The offspring receives genes from 2 differentindividuals, thereby getting a new mix oftraits.

SummaryMeiosis creates variations in the wayhomologous chromosomes separate,AND in the process of Crossing-Over.

Further variation comes fromcombining genes from 2 parents.

GAMETE FORMATION WITHOUT CROSSING-OOVER

WITH CROSSING-OOVER

Possible genecombinationsin gametes

or

Possible genecombinationsin gametes

MEIOSIS

MEIOSIS

AB

aBAB

ab

abAb

Genetics, Sex and EvolutionDuring his lifetime, Charles Darwin freelyadmitted that there were 2 big gaps in hiscontroversial Theory of Evolution...

How are characteristics inherited?When the fittest survive and breed, how dothey pass on their survival traits to theiroffspring?

Where does variation come from?Natural Selection needs differences betweenindividuals to choose the survivors. Why isthere variation anyway?

Variation Caused by MeiosisThe process of meiosis to produce the spermand egg cells increases variation, even beforefertilisation occurs.

Study the diagram of meiosis on page 18.Remember that homologous chromosomes areNOT identical.

Each pair of homologous chromosomes line upand separate at random, and independently ofall other pairs, so the number of differentpossible gametes is very large. In humans, with23 pairs of chromosomes, it is possible formeiosis to produce about 8 million differentcombinations of chromosomes in the gametesof each person!

The Science of Genetics can now explain that

Variation comes from MEIOSIS andSexual Reproduction

These chromosomes have exchanged pieces of chromatid with each other.

This has mixed up the combinations of genes A, a, B and b.

AA AA

BB BB

aa aa

bb

AA

BB BB

aa

bb bb

bb

RReemmeemmbbeerr,, eeaacchh ggaammeettee ggeettss jjuusstt oonnee ooff tthheessee 44 cchhrroommaattiiddss

AAaa

The Effect of EnvironmentIts not just an organisms genes that produce its phenotype; the environment has an effect as well.For example, consider some of Mendels peaplants with different genes for stem height.

Now, imagine planting them (as baby seedlings)in very poor soil, so that normal growth was notpossible.

The point is that the genes may control what theorganism COULD grow up to be, but theenvironment may influence this, possibly alteringthe final appearance (phenotype).

Genotype TT

Phenotype TALL

Genotype tt

Phenotype DWARF

IN GOOD SOIL

Genotype TT

Phenotype DWARF

Genotype ttPhenotype DWARF

IN POOR SOIL

In Hydrangea plants, if cuttings are taken from asingle individual (the cuttings would begenetically identical) and grown in differentsoils, the flowers on each cutting can bedifferent colours. If the soil is slightly acidic theflowers will be blue, but in slightly basic soiltheyll be pink.

Identical twins have inherited exactly the samegenes. In the USA about 40 years ago, a famousstudy was done on identical twins who had beenadopted into separate families and raised indifferent environments. The study found quitelarge differences between the twins in intelligence,personality, and even appearance. Presumablythese differences were due to different foods,education, etc.




23

Worksheet 8 Chromosomes & DNA Name....................................Fill in the blank spaces.

Chromosomes are thread-likestructures in the a).................... of a cell.They are only visible duringb).......................................

Chromosomes come in pairs, referred toas c).............................. The members ofa c)............................... pair are notidentical, but correspond with eachother because they carryd).................................... genes incorresponding locations. Before a celldivision, each chromosome ise)............................ to make a doublechromosome. The 2 parts are calledf)................................ and they areg)................................. to each other.

In Mitosis, the chromosomes line uph).......................... and thei)............................. separate, so that thedaughter cells are j).......................... toeach other, and to the k).......................cell.

In Meiosis, the chromosomes line upl)................................... and the firstdivision separates them).................................. pairs. Then thecells divide again to form n).............(number) gametes, each witho)....................... the original number ofchromosomes.

Body cells with pairs of chromosomesare called p)........................... whilegametes are said to beq)............................... Human body cellshave a total of r).................... (number)chromosomes, while egg or sperm cellshave s)............. (number)

In 1902, two scientists t)..........................and .......................... noticed thesimilarities between how Mendelsgenes worked and how u)........................behave during v)......................... Fromthis observation it was clear thatw)...............................................................

Chromosomes have been analysedchemically and found to containx)............................... and .........................It is known that the DNA moleculescontain a chemical code which is ay)........................ Thus, eachchromosome contains many genes,each one encoded by a molecule ofz)....................

The DNA molecule is a aa)......................of repeating units calledab).............................. Each one is madeof 3 smaller parts; ac)....................... and............................... and .........................

There are 4 different bases, known bythe intial letters of their names; ad)......,......., ....... and ....... (letters) The DNAmolecule is a double- ae).....................shape, made of af)............... (number)strands resembling a twisted ladder. Therungs of the ladder are formed by 2bases bonding with each other. Thebases can only bond in combinationsag).............. and ...............

Sexual reproduction producesvariations in a population because:

Offspring receive ah)......................................................................

ai)................................... of homologouschromosomes in meiosis.

the process of aj)....................................................... in meiosis.

WHEN COMPLETED, WORKSHEETS BECOME SECTION SUMMARIES


24



1. Refer to Morgans experiment with fruit flies.p20a) Why are there 3 genotypes for female flies,but only 2 for males?

b) From which parent (mother or father) does awhite-eye male fly inherit his white eyes?Explain.

c) i) What is the genotype of a female, who is heterozygous?

ii) What is her phenotype?

iii) Explain why this genotype is often referredto as a carrier female.

d) Use a Punnett Square to predict the outcomeof each cross below. (You must track the X & ychromosomes. Offspring phenotypes shoulddescribe the sexes separately)

i) Xry crossed with XRXr

ii) XRy crossed with XrXr

2. In humans, a recessive gene (Xn) carried on the Xchromosome causes colour-blindness. Thenormal vision gene can be symbolized by XN.Use these symbols to write the genotype of:a) a homozygous normal-vision female.b) a normal-vision male.c) a colour-blind male.d) a colour-blind female.e) a carrier female (heterozygous)f) A man with normal vision married a womanwho is colour blind. Use a Punnett Square topredict the pattern of inheritance in theirchildren.

3. Refer to the information about Co-Dominance.

Use Punnett Squares to predict the phenotypesof calves born if:a) a roan bull mated with a red cow.

b) a white bull mated with a roan cow.

4. In a particular breed of chickens, the feathercolour is controlled by 2 alleles B and W.Genotype BB produces black feathers.Genotype WW produces white feathers. Theheterozygous genotype BW results in black &white speckled feathers.

Use a Punnett Square to predict the colours ofchicks from: (show working on back of sheet)

a) a speckled rooster and speckled hen.b) a black hen and a white rooster.c) a black rooster and a speckled hen.

5. Some plants have flower colours controlled asfollows: There are only 2 alleles involved, butthere are 3 phenotypes possible... red flowers,white flowers and pink flowers.a) Suggest how just 2 genes can result in 3different colours.

b) Suggest suitable symbols for the alleles.

c) Use these symbols to write the genotype for:i) red flowerii) pink floweriii) white flower

d) Use a Punnett Square to predict thephenotypes resulting from crossing a red-flowering plant with a white-flowering plant andbreeding through to the F2 in a Mendel-typeexperiment. (show working on back of sheet)

Worksheet 9 Sex-Linkage & Co-Dominance Problems Name...............................

Multiple Choice1. A pair of homologous chromosomes:A. are identical to each other.B. carry totally different genes C. will line up as a pair during mitosis.D. carry alleles in corresponding locations.

2. Meiosis produces:A. 4 haploid gamete cells.B. 2 diploid body cells.C. 4 identical gametes.D. 4 non-identical body cells.

3. A nucleotide contains:A. sugar, phosphate and 1 of 4 possible bases.B. a base, phosphate, & 1 of 4 possible sugars.C. amino acids in a polypeptide chain.D. base pairs A-T and C-G.

4. In human sperm cells you would expect to find:A. 46 chromosomes including an Xy pair.B. 46 chromosomes including either an X or a y.C. 23 chromosomes, including an X.D. 23 chromosomes, including either an X or a y.

5. If a recessive gene is sex-linked you wouldexpect:A. males and females to show the

phenotype equally.B. a heterozygous female will show

the recessive phenotype.C. males may inherit the trait only from

their mother.D. males will pass the recessive gene on

to their sons.

6. In a species of small African rodents it wasnoticed that 2 grey coloured animals produced alitter of babies that were about 25% white, 25%black and about 50% grey. It seems likely that:

A. grey is dominant to both black and white.B. co-dominant alleles are controlling

coat colour.C. a mutation occurred to produce new

colours in the babies.D. grey is a recessive gene, black and white

are co-dominant.

Longer Response QuestionsAnswer on reverse, if insufficient space.

7. (6 marks)a) Sketch one pair of homologouschromosomes, just before a cell was to undergoa cell division. If the organism is heterozygous for allelic genesQ and q, show on your diagram thepositions of all copies of these genes.

b) Draw simple sketches to show:i) how these 2 chromosomes would line up formitosis.

ii) the chromosomes in ONE of the cellsresulting from mitosis.

iii) how these 2 chromosomes would line up formeiosis.

iv) the chromosomes in ONE of the cellsresulting from meiosis.

8. (5 marks)In Drosophila fruit flies, the normal eye colour isred. A recessive, sex-linked gene causes whiteeyes.Using symbols XR, Xr and y, predict thephenotypes (separately for each sex) of thecross between a white-eye male and aheterozygous red-eye female. Show yourworking.

9. (5 marks)A certain species of African rodent wasdescribed in Multiple Choice Question 6. Using appropriate (declared) symbols, predictthe outcome of a mating between a grey andblack animal of this species.

10. (3 marks)a) Explain, with the help of simple diagrams ifyou wish, the process of crossing over withchromosomes.

b) Explain how this process contributes to thegenetic diversity of a species.

c) State one other process which increasesgenetic diversity in a species.

25




Worksheet 10 Test Questions section 3 Name....................................




26

How DNA Structure Was DiscoveredBy the middle of the 20th century it wassuspected that DNA was probably the geneticchemical and it was known that it containedsugar, phosphate and the 4 bases A,C,G and T.What no-one could understand was, if DNA wasthe genes, how could it:

Replicate (duplicate) itself for cell division? Control the phenotype of an organism?

It seemed likely that the key to this problem wasto find out the structure of the DNA molecule.The story of what happened is a classic exampleof how Science and scientists make progressusing collaboration and communication.

In 1953, English scientist Francis Crick hadbecome an expert at interpreting the shapes ofmolecules using the new technique of X-rayDiffraction.

Meanwhile, at another laboratory, MauriceWilkins (New Zealand) managed to prepare apure crystal of DNA, and Rosalind Franklin(England) was able to get an X-Ray Diffractionimage of it, but neither understood how to makeany sense of the pattern it produced.

Then a young American, James Watson, whowas interested in understanding the DNAmystery, visited the Wilkins-Franklin laboratory.With their collaboration, he took their data toCrick for his expert interpretation. Betweenthem, Watson and Crick made one of the mostnotable scientific breakthroughs in the historyof Biology... they figured out the base-pairing,double-helix structure of DNA and realisedimmediately how this structure could lend itselfto replication... an essential feature of a gene.

No one of these scientists could have madeprogress alone. Each had certain data, or skillsor expertise, but only by bringing it all togetherwas the great discovery possible. Successcame from different people communicating andunselfishly sharing their knowledge and talents.

DNA ReplicationSo how does the structure of the DNA moleculelend itself to replication?

The key is the way the complementary basesbond together in the double stranded structure.

This means that if you have ONE STRAND of aDNA molecule it is a mirror-image template forthe other. If you split a DNA molecule into 2separate strands, each strand can be used tobuild a new, complementary strand.

Thats exactly what happens to all the DNA in eachchromosome, before a cell division occurs.

4. MOLECULAR GENETICS & MUTATION

Pairs of Bases Sugar & phosphate chains

Double-sstranded Helix

sugar-pphosphate side railA G T C C A

BasesT C A G G T

The only combinations that will bond areA-TT and C-GG

G CT AC GC GA T

CTAAGCGATCG

GATTCGCTAGC

TCGCTAGC

AGCGATCG

A

A

AA

T

T

G

G

C

CC

C

GG

G

T

T

T

CA

GT

DNAREPLICATION Original, double-

stranded DNAMMoolleeccuullee iiss uunnttwwiisstteeddaanndd uunnzziippppeedd bbyyeennzzyymmeess SSppaarree nnuucclleeoottiiddeess

NNuucclleeoottiiddeessmmaattcchh uupp wwiitthhccoommpplleemmeennttaarryybbaassee oonn oorriiggiinnaall

ssttrraanndd..

EEnnzzyymmeessccoonnnneecctt tthheemm iinnppllaaccee,, ffoorrmmiinnggaa nneeww ssttrraanndd..

WWhheennccoommpplleetteedd,,

eeaacchh nneeww DDNNAAmmoolleeccuullee iissttwwiisst

Blueprint of Life

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