Cellular Control Unit 1 Communication, Homeostasis and Energy.

Cellular Control

Unit 1Communication, Homeostasis and Energy

Meiosis

Module 1: Cellular Control

Learning outcomes

describe, with the aid of diagrams and photographs, the behaviour of chromosomes during

meiosis, the associated behaviour of the nuclear

envelope, cell membrane and centrioles.

(Names of the main stages are expected, but not the subdivisions of prophase);

Reproduction and variationAsexual reproduction

Single organism divides by mitosis New organism is genetically identical to

the parentSexual reproduction

Meiosis produces haploid gametes Which fuse at fertilisation to form a

diploid zygote This produces genetic variation amongst

offspring

Human Life Cycle

Adult46

HaploidSperm

23

HaploidEgg23

Diploid Zygote

46

fertilisation

Meiosis

Mitosis

Self assessment questions The fruit fly Drosophila melangaster has

eight chromosomes in its body cells. How many chromosomes will there be in a Drosophila sperm?

The symbol n is used to indicate the number of chromosomes in one set – the haploid number of chromosomes. For example in humans n = 23, in a horse n = 32. How many chromosomes are there in a gamete

of a horse? What is the diploid number of chromosomes

(2n) of a horse?

Meiosis

Meiosis is a reduction division Resulting daughter cells have half the original

number of chromosomes Daughter cells are haploid Can be used for sexual reproduction Source of genetic variation

Meiosis has two divisions meiosis I and meiosis II

Each division has 4 stages Prophase, metaphase, anaphase, telophase

Meiosis

You can view an animation of Meiosis at http://www.cellsalive.com/meiosis.htm

Meiosis I

Prophase IChromatin condensesHomologous pairs form a bivalentNucleolus disappearsSpindle forms

Metaphase IBivalents line up on equator of cell

Anaphase IHomologous chromosome in each bivalent are pulled to opposite poles

Telophase ITwo new nuclear envelopes formCell divides by cytokinesis

Early Prophase 1

Late Prophase 1

Metaphase 1

Anaphase 1

Telophase 1

Cytokinesis 1

Meiosis II

Prophase IINucleolus disappearsChromosomes condenseSpindle forms

Metaphase IIChromosomes arrange themselves on equatorAttach by centromere to spindle fibres

Anaphase IICentromeres divideChromatids pulled apart to opposite poles

Telophase IInuclear envelopes reform around haploid nucleiCell divides by cytokinesis

Prophase II

Metaphase II

Anaphase II

Telophase II

Cytokinesis II

Learning outcomes

explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles

Key words

AlleleLocusCrossing overMaternal chromosomePaternal chromosome

Alleles, locus and homologous chromosomes

Meiosis and variation

Meiosis enables sexual reproduction to occur by the production of haploid gametes.

Sexual reproduction increases genetic variation

Genetic variation increases the chances of evolution through natural selection

Meiosis and Variation

Crossing over – prophase I Independent assortment of chromosomes

– metaphase I Random assortment of chromatids –

metaphase II Random fertilisation Chromosome mutations

Number of chromosomes▪ Non-disjunction - polysomy or polyploidy

Structure of chromosomes▪ Inversion, deletion, translocation

Crossing over

During metaphase I

During metaphase I

No crossing over

Crossing over – new combinations of alleles

Independent Assortment

Learning Outcomes

explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive;

explain the terms linkage and crossing-over;

Glossary

Gene Locus Allele Genotype Phenotype

Heterozygous Homozygous Monohybrid cross Dominant allele Recessive allele

Genetics

Genetics is the study of inheritanceAllele

different varieties of the same geneLocus

position of a gene on a chromosome

Genetics

Dominant An allele whose effect is expressed in

the phenotype if one copy presentRecessive

An allele which only expresses as a homozygote

Co-dominant Both alleles have an effect on the

phenotype

Genotype genetic constitution of the organism

Phenotype appearance of character resulting from

inherited information

Homozygous Individual is true breeding Possesses two alleles of a gene e.g. RR

or rrHeterozygous

Two different alleles for a gene e.g. Rr

Monohybrid inheritance

Mendel’s First Law principle of segregation

“The alleles of a gene exist in pairs but when gametes are formed, the

members of each pair pass into different gametes, thus each gamete

contains only one of each allele.”

Inheritance of height in pea plants

Follow out the following cross to the F2 generation Homozygous tall pea plant with a homozygous

dwarf pea plant Write out the genotypic and phenotypic ratios

from the F2 generation

gene Allelerelationshi

pSymbol

Height of pea

plants

Tall Dominant Tdwar

frecessive t

Inheritance of height in pea plants Laying out the cross

P phenotype P genotype Gametes F1 genotype F1 phenotype F1 self-fertilised Gametes Random fertilisation F2 genotypic ratio F2 phenotypic ratio

Pupil Activity

Answer the questions on monohybrid inheritance Remember to write out each cross in

full.

Cystic Fibrosis

Cystic Fibrosis is caused by a mutation to a gene on one of the autosomes.

Mutation Changes the shape of the transmembrane

chloride ion channels (CFTR protein) The CFTR gene is found on Chromosome 7 The faulty gene is recessive

Genetic Cross conventions Use symbols to represent two alleles Alleles of the same gene should be

given the same letter Capital letter represents the dominant

allele Small letter represents the recessive

allele Choose letters where the capital and

small letter look different The examiner needs to be in no doubt

about what you have written

Inheritance of cystic fibrosis Three possible genotypes

FF unaffected Ff unaffected ff cystic fibrosis

Remember gametes can only contain one allele for the CFTR gene

At fertilisation, any gamete from the father can fertilise any gamete from the mother This can be shown in a genetic diagram

Genetic diagram showing the chances of a heterozygous man and a heterozygous woman having a child with cystic fibrosis.

Phenotype ratio of offspring Genotype ratio 1FF:2Ff:1ff

Phenotype ratio 3 unaffected:1cystic fibrosis

Can also be expressed as 25% chance of the child having cystic fibrosis Probability of 0.25 that a child will inherit the

disease Probability that 1 in 4 that a child from these

parents will have this disease.

Learning Outcome

Use genetic diagrams to solve problems involving sex-linkage and codominance.

Sex-Linkage

Sex-linked genes are genes whose loci are on the X or Y chromosomes

The sex chromosomes are not homologous, as many genes present on the X are not present on the Y.

Examples Haemophilia Fragile X syndrome Red green colour blindness

Sex Chromosomes

Factor VIII and Haemophilia

Haemophilia is caused by a recessive allele of a gene that codes for a faulty version of the protein factor VIII XH normal allele Xh haemophilia allele

possible genotypes and phenotypes

Inheritance of Haemophilia

Pedigree for a sex linked recessive disease

Codominance

Codominance describes a pair of alleles, neither of which is dominant over the other.

This means both have an effect on the phenotype when present together in the genotype

Codominance example

Flower colour in plants CR red Cw white

Genotypes CRCR red flowers CRCW pink flowers CWCW white flowers

Write out a genetic cross between a pure breeding red plant and a pure breeding white plant.

Carry out the cross to the F2 generation. Write out the genotype

and phenotype ratio for the F2 generation

Revision Question

Coat colour in Galloway cattle is controlled by a gene with two alleles. The CR allele produces red hairs and therefore a red coat colour. The Cw allele produces white hairs.

A farmer crossed a true-breeding, red-coated cow with a true-breeding white-coated bull. The calf produced had roan coat colouring (made up of an equal number of red and white hairs).

Explain the result and draw a genetic diagram to predict the outcome of crossing two roan coloured animals.

Inheritance of A, B, AB and O blood groups

Human blood groups give an example of codominance and multiple alleles There are 3 alleles present▪ IA

▪ IB

▪ Io

IA and IB are codominant Io is recessive

Remember each human will only have two alleles

Blood Groups

Genotype Phenotype

IAIA Blood Group A

IA Io Blood Group A

IAIB Blood Group AB

IBIB Blood Group B

IB Io Blood Group B

Io Io Blood Group o

Inheritance of blood groups

Carry out genetic crosses for the following examples Two parents have blood groups A and B,

the father is IAIo and the mother is IBIo

Father has blood group AB and the mother has blood group O

Mother is homozygous blood group A and the father is heterozygous B.

Learning Outcome

Describe the interactions between loci (epistasis).

Predict phenotypic ratios in problems involving epistasis.

Dihybrid Inheritance

Monohybrid cross Inheritance of one gene

Dihybrid cross Inheritance of two genes

Example – dihybrid cross

Tomato plants Stem colour

A purple stem a green stem Leaf shape

D cut leaves d potato leaves

NOTE In the heterozygote AaDd due to independent

assortment in meiosis there are 4 possible gamete combinationsAD Ad aD ad

Crosses

Cross a heterozygous plant with a plant with a green stem and potato leaves

Cross two heterozygous tomato plants

Dihybrid Inheritance

A woman with cystic fibrosis has blood group A (genotype IAIo). Her partner does not have cystic fibrosis and is not a carrier for it. He has blood group O.

Write down the genotypes of these two people.

With the help of a full and correctly laid out genetic diagram, determine the possible genotypes and phenotypes of any children that they may have.

Autosomal linkage

Each Chromosome carries a large number of linked genes

If two genes are on the same chromosome then independent assortment can not take place.

The genes are transmitted together and are said to be linked.

Linked Genes

Where linked genes are involved the offspring of a dihybrid cross will result in a 3:1 ratio instead of the 9:3:3:1 ratio.

Example: In peas, the genes for plant height and

seed colour are on the same chromosome (i.e. linked)

Learning Outcome

Describe the interactions between loci (epistasis).

Predict phenotypic ratios in problems involving epistasis.

Flower colour in sweet pea Flower colour

Colourless precursor of a pigment C Gene that controls conversion of this pigment

to purple P Both dominant alleles need to be present for

the purple colour to develop Cross

Cross two white flowered plants with the genotypes CCpp and ccPP

Follow this cross through to the F2 generation

Interactions of unlinked genes

A single character maybe influenced by two or more unlinked genes.

E.g. determination of comb shape in domestic poultry Dominant allele P pea comb Dominant allele R rose comb Two dominant alleles walnut comb No dominant alleles single comb

Genetic Crosses

Carry out a genetic cross between a true-breeding pea comb and a true breeding rose comb

Follow this cross through to the F2 generation

Inheritance of coat colour in mice

Wild mice have a coat colour that is referred to as “agouti”. Agouti (A) is dominant to black (a) C is a dominant gene required for coat

colour to develop A homozygous recessive cc means that

no pigment can be formed and the individual is albino

Inheritance of coat colour in mice

Carry out a cross between a pure-breeding black mouse (aaCC) and an albino (AAcc)

Follow this cross through to the F2 generation.

Epistasis

This is the interaction of different gene loci so that one gene locus masks or suppresses the expression of another gene locus.

Genes can Work antagonistically resulting in

masking Work complementary

Epistasis ratios

9 : 3 : 4 ratio Suggests recessive epistasis

9 : 7 ratio Suggests epistasis by complementary

action12 : 3 : 1 ratio or 13 : 3 ratio

Suggests dominant epistasis

Predicting phenotypic ratiosRead through pages 132 and 133 of

your textbook Answer questions 1 – 7

Complete the stretch and challenge question on “eye colour in humans”

Read through and complete the worksheet provided for you on epistasis

Learning outcome

Use the chi-squared (χ2) test to test the significance of the difference between observed and expected results.

χ2 (chi-squared) test

Allows us to compare observed and expected results and decide if there is a significant difference between them.

χ2 (chi-squared) test

Where Σ = the sum of O = observed value E = expected value

χ2 (chi-squared) test

Compare the χ2 value to a table of probabilities The probability that the differences between

our expected and observed values are due to chance.

If the χ2 value represents a probability of 0.05 or larger, the differences are not significant

If the χ2 value represents a probability of less than 0.05, it is likely that the results are not due to chance and there is a significant difference.

Degrees of freedom

The degrees of freedom takes into account the number of comparisons made. Degrees of freedom

= number of classes of data - 1

Table of χ2 values

Degrees of freedom

Probability greater than

0.1 0.05 0.01 0.001

1 2.71 3.84 6.64 10.83

2 4.60 5.99 9.21 13.82

3 6.25 7.82 11.34 16.27

4 7.78 9.49 13.28 18.46Critical value95% certain that the results are

not due to chance

Table of χ2 values

Degrees of freedom

Probability greater than

0.1 0.05 0.01 0.001

1 2.71 3.84 6.64 10.83

2 4.60 5.99 9.21 13.82

3 6.25 7.82 11.34 16.27

4 7.78 9.49 13.28 18.46Accept null hypothesisThere is no significant difference, results

have occurred due to chance

Table of χ2 values

Degrees of freedom

Probability greater than

0.1 0.05 0.01 0.001

1 2.71 3.84 6.64 10.83

2 4.60 5.99 9.21 13.82

3 6.25 7.82 11.34 16.27

4 7.78 9.49 13.28 18.46Reject null hypothesis: accept experimental hypothesis

Difference is significant, not due to chance

Mammal question

χ2 value = 51.8Degrees of freedom = 3Critical value (p=0.05) = 7.82

Reject the null hypothesisThere is a significant difference

between observed and expected results

Suggestions? The two genes are linked

Variation

What did you learn at AS level?

Learning Outcomes

Define the term variation.Discuss the fact that variation occurs

within as well as between species.Describe the differences between

continuous and discontinuous variation, using examples of a range of characteristics found in plants, animals and microorganisms.

Explain both genetic and environmental causes of variation.

Variation

Variation is the differences that exist between individual organisms. Interspecific variation (between species)▪ Differences that are used to assign

individuals to different species Intraspecific variation (within a species)▪ Individuals of the same species show variation

Variation can be inherited or influenced by the environment.

Types of variation

There are two main types of variation Continuous variation Discontinuous variation

There are two main causes of variation Genetic variation Environmental variation

Continuous variation

Existence of a range of types between two extremes

Most individuals are close to a mean value Low numbers of individuals at the extremes Both genes and the environment interact in

controlling the features Examples

Height in humans Length of leaves on a bay tree Length of stalk of a toad stool

Continuous variation

Use a tally chart and plot results in a histogram

Discontinuous variation

2 or more distinct categories with no intermediate values

Examples Earlobes attached or unattached Blood groups A, B, AB or o Bacteria flagella or no flagella Flowers colour of petals

Genetically determined The environment has little or no effect on

discontinuous variation

Discontinuous variation

Causes of variation

Genetic Variation Genes inherited from parents provide

information used to define our characteristics

Environmental Variation Gives differences in phenotype (appearance)

but not passed on by parents to offspring Examples▪ Skin colour tans with exposure to sunlight▪ Plant height determined by where the seed lands

Variation

What you need to know for A2!!

Learning outcomes

Describe the differences between continuous and discontinuous variation.

Explain the basis of continuous and discontinuous variation by reference to the number of genes which influence the variation.

Explain that both genotype and environment contribute to phenotypic variation.

Explain why variation is essential in selection.

variation

Variation can be: Discontinuous▪ Each organism falls into one of a few clear-cut

categories, no intermediate values▪ Qualitative differences between phenotypes

Continuous▪ No definite categories▪ A continuous range of values between two

extremes▪ Quantitative differences between phenotypes

Genes and variation

Discontinuous (qualitative) variation Monogenic inheritance Different alleles at same gene locus Different gene loci have different effects Epistasis, codominance, dominance and

recessive patterns of inheritance

Genes and Variation

Continuous (quantitative) variation Polygenic inheritance Two or more genes Each gene has an additive effect Unlinked genes

Polygenic Inheritance

Example –length of corn cobs Three genes – A/a, B/b and C/c Each dominant allele adds 2 cm length Each recessive allele adds 1 cm length

So AABBCC = 12 cm long aabbcc = 6 cm long

Hmmm!! How long would AaBBCc be? How long would aaBbCc be?

Genotype, environment and phenotype

The environment can affect the expression of the genotype

examples AABBCC has the genetic potential to

produce cobs 12cm long▪ This could be affected by▪ Lack of water, light or minerals

Obesity in humans▪ Affected by diet and exercise

Genotype, environment and phenotype

The environment influences the expression of polygenic traits more than monogenic traits.

Learning Outcomes

Use the Hardy–Weinberg principle to calculate allele frequencies in populations.

Population genetics

What is a population? Group of individuals of the same species

that can interbreed Populations are dynamic

The set of genetic information carried by a population is the gene pool.

Allele Frequency

To measure the frequency of an allele you need to know Mechanism of inheritance of that trait How many different alleles of the gene

for that trait are in the population

Hardy-Weinberg principle

The Hardy-Weinberg principle is a fundamental concept of population genetics

It makes the following assumptions Population is very large Random mating No selective advantage No mutation, migration or genetic drift

The equations

p frequency of the dominant allele q frequency of the recessive allele

The frequency of the allele will be in the range 0 – 1. 0 – no one has the allele 0.5 – half the population has the allele 1 – only allele for that gene in the

population

Ok – the equations

Equation 1p + q = 1

Equation 2p2 + 2pq + q2 = 1

Where▪ p2 frequency of genotype DD▪ 2pq frequency of genotype Dd▪ q2 frequency of genotype dd

Calculating the frequency of cystic fibrosis in the population 1 in 3300 babies are born with cystic

fibrosis All babies with cystic fibrosis have

genotype nn Calculate q2

Calculate q Calculate p Calculate frequency of genotype Nn If we have 30,000 people in our population

how many will be carriers of the cystic fibrosis allele

Question

Phenylketonuria, PKU, is a genetic disease caused by a recessive allele. About one in 15 000 people in a population are born with PKU.

Use the hardy-Weinberg equations to calculate the frequency of the PKU allele in the population.

State the meaning of the symbols that you use, and show all your working.

The Answer

Calculate q2 = 1 / 15000 = 0.000067Calculate q = 0.0082

Another question

Explain why the Hardy-Weinberg principle does not need to be used to calculate the frequency of codominant alleles.

Pupil Activity

Answer the Hardy-Weinberg practice question.

You have 10 minutes Starting NOW!!

The Answers

q2 = 0.52 / q = 0.72 p = 1 – 0.72 = 0.28 p + q = 1 p2 + 2pq + q2 =

1

Answer = 2pq / use of appropriate numbers;

Answer = 40%;

The other answers

Any three from: Small founder population / common ancestor; Genetic isolation / small gene pool / no

immigration / no migration / in-breeding; High probability of mating with person having

H-allele; Reproduction occurs before symptoms of

disease are apparent; Genetic argument – Hh x hh = 50% / Hh x Hh =

75% affected offspring; No survival / selective disadvantage;

Learning Outcomes

Explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection.

Explain how genetic drift can cause large changes in small populations.

Variation and Natural Selection

The set of alleles in a population is it’s gene pool

Each individual can have any combination of alleles in the gene pool producing variation Some individuals more likely to survive They reproduce and pass genes on to offspring Advantageous alleles become more frequent in

the population

Environmental Resistance

Environmental factors that limit the growth of a population offer environmental resistance

These factors can be biotic or abiotic

Selection pressures

An environmental factor that “selects” for some members of a population over others

Confers an advantage onto certain individuals

Stabilising Selection

If the environment stays stableThe same alleles will be selected for

in successive generationsNothing changes, this is called

stabilising selection

Stabilising Selection

Stabilising Selection

Directional Selection

Change in the environment resulting in a change in the selection pressures on the population

Previously disadvantageous alleles maybe selected for

Change in the genetically determined characteristics of subsequent generations of the species

A.k.a. evolution

Directional Selection

Directional Selection

Genetic Drift

A change in the gene pool and characteristics within the population.

This change has occurred by chance rather than as the result of natural selection.

Genetic Drift and Islands

Genetic drift is thought to happen relatively frequently in populations on islands. Small populations Geographically separated from other

members of their speciesEvidence

Many isolated islands have their own endemic species of plants and animals

Genetic Drift

Reduces genetic variationReduce the ability of the population

to survive in a new environmentMay contribute to the extinction of a

population or speciesCould lead to the production of a

new species

Genetic Drift – Frog Hoppers The colours of the

common frog-hopper are determined by seven different alleles of a single gene.

The range of colours and their frequencies, on different islands in the Isles of Scilly, are very variable,

There are different selection pressures on the different islands

Genetic Drift – Frog Hoppers

The answers

Learning Outcomes

Explain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms.

Speciation

Speciation is the formation of a new species.

Species Group of organisms, with similar

morphology and physiology, which can interbreed with one another to produce fertile offspring.

Speciation

In the production of a new species, some individuals must Becomes morphologically or

physiologically different from members of the original species

No longer be able to breed with the members of the original species to produce fertile offspring.

Isolation

Splitting apart of a “splinter group”Geographical isolation

Organisms are separated by a physical barrier

Reproductive isolation Two groups have become so different

that they are no longer able to interbreed

They are now a different species

Isolating Mechanisms

Large populations may be split into sub-groups by Geographic barriers Ecological barriers Temporal barriers Reproductive barriers

Geographical Barriers (AS recap)

Geographical barrier separates two populations of a species

Two groups evolve along different lines Different selection pressures Genetic drift

If barrier breaks down and two populations come together again, they may have changed so much that they can no longer interbreed

They are now two different species

Isolating Mechanisms

Speciation occurs when organisms live in the same place

The barriers which can prevent two closely related species from interbreeding include Ecological Temporal Reproductive

Ecological Barriers

Ecological barriers exist where two species live in the same area at the same time, but rarely meet.

Example Two different species of crayfish,

Orconectes virilis and orconectes immunis, both live in freshwater habitats in North America

Meet the Crayfish

Orconectes virilis Not good at digging, can’t

survive summer drying Lives in streams and lake

margins Orconectes immunis

Lives in ponds and swamps,

Can easily burrow into the mud when the pond dries up

In streams and lake margins O. virillis is more aggressive and will drive O. immunis out of crevices where it tries to shelter

Temporal Barriers

Two species live in the same place, and may even share the same habitat

Do not interbreed as they are active at different times of the day, or reproduce at different times of year

Example – flowering shrubs in Western Australia

Meet the shrubs

Banksia attenuata flowers in the summer

Banksia menziesii flowers in the winter

They can not interbreed

Reproductive barriers

Even if species share the same habitat and are reproductively active at the same time, they may not be able to interbreed Different courtship behaviours Mechanical problems with mating Gamete incompatibility Zygote inviability Hybrid sterility

Meet the Mallards

Different courtship behaviours A male mallard duck will

only mate with a female who displays the correct courtship behaviour

Although the pintail female looks similar to the Mallard female, her courtship behaviour will only attract a pintail male.

Learning Outcomes

Explain the significance of the various concepts of the species, with reference to the biological species concept and the phylogenetic (cladistic/evolutionary) species concept.

The Species Concept

In AS biology you defined a species as “a group of organisms, with similar

morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species”

The two species concept

Group of organisms Capable of

interbreeding Capable of producing

fertile offspring Reproductively

isolated from other groups

This is the Biological Species concept

Group of organisms showing similarities in characteristics Morphological Physiological biochemical Ecological Behavioural

This is the phylogenetic species concept

Biological Species concept

Group of organisms that can interbreed and produce fertile offspring.

Clear cut definitionLimitation

Can only be used for organisms that reproduce sexually

Phylogenetic species concept

Also known as the Evolutionary species concept Cladistic species concept

Different morphology between the two groups and certain that they evolved from a common ancestor

Not rigorous but allows decisions to be made

Comparing the genetics

Closely related organisms have similar molecular structures for DNA, RNA and proteins.

Biologists can compare specific base sequences (haplotypes) The number of differences caused by

base substitutions can be expressed as the % divergence

Cladistics

Clade Group of organisms with similar

haplotypes In cladistic classification systems is

assumes that the taxa are monophyletic, this means that it includes an ancestral organism and all it’s descendents.

Cladistic classification

Focuses on evolutionPlaces importance on using

molecular analysisUses DNA and RNA sequencingUses computer programmesMakes no distinction between extinct

and still existing species

Learning Outcomes

Compare and contrast natural selection and artificial selection.

Describe how artificial selection has been used to produce the modern dairy cow and to produce bread wheat (Triticum aestivum).

Selection

Natural Selection Mechanism for evolution Organisms best adapted to their

environments more likely to survive to reproductive age

Favourable characteristics are passed on Produces organisms that are well

adapted to their environment

Artificial Selection

Humans select the favourable characteristics

Humans allow those organisms to breed

Produces populations that show one characteristic to an extreme Other characteristics retained may be

disadvantageous

Artificial Selection and the modern dairy cow

Breeds of cows with higher milk production have been artificially selected for Milk yield from each cow is measured and

recorded Test progeny of bulls Elite cows given hormones to produce many

eggs Eggs fertilised in vitro Embryos implanted into surrogate mothers

A few elite cows produce more offspring than they would naturally

Disadvantage to high milk yields

Health costs for artificially selected cows is higher due to Mastitis Ketosis and milk fever Lameness Respiratory problems

Artificial selection and bread wheat (Triticum aestivum)Polyploidy

Nuclei contain more than one diploid set of chromosomes

Wild species of wheat have a diploid number (2n) of 14

Modern bread wheat is hexaploid (6n), It has 42 chromosomes in the nucleus of every cell

Getting from the ancestors to modern bread wheat

Wild einkorn

AUAU

2n = 14

Wild GrassBB

2n = 14

EinkornAUAU

2n = 14x

Domestication and artificial selection

Wild GrassBB

2n = 14

EinkornAUAU

2n = 14x

Sterile hybrid P

AuB

Wild GrassDD

2n = 14

Emmer WheatAUAUBB4n = 28

x

Mutation that double chromosome number

Wild GrassDD

2n = 14

Emmer WheatAUAUBB4n = 28

x

Sterile hybrid Q

AuBDMutation that double chromosome numberCommon Wheat

AUAUBBDD6n = 42

Continuing selection in wheat

Breeders are continuing to try and improve wheat varieties Resistance to fungal infections High protein content Straw stiffness Resistance to lodging Increased yield