Chapter 6: Genetics 18 Things You Should Know about Genetics.

Chapter 6:Genetics

18 Things You Should Know about Genetics

When told to, put the PTC paper on your tongue

PTC, or phenylthiourea, is an organic compound having the unusual property of either tasting very bitter, or being virtually tasteless, depending on the genetic makeup of the taster.

• The ability to taste PTC is a dominant genetic trait.

T = Taster t = non-taster– If you can taste, you are either TT or Tt.– Those who can not taste are tt

• About 70% of people can taste PTC, varying from a low of 58% for Aborginal people of Australia to 98% for Native Americans

• Ever wonder why people resemble their parents &/or siblings?

• How do farmers select the best plants or animals for breeding purposes?

• How’d you like that PTC paper?

• Why did some people in your class/family taste it and others didn’t?

How do we inherit our traits?

TraitsTraits are distinguishing characteristics which we inherit from our parents and can pass on to our children.

– PTC tasting ability ______________– Tongue rolling ability ______________– Hand preference _______________– Eye color _______________– Hair color _______________– Dimples _______________– Freckles _______________

What are you for these traits?

Free earlobe

Widows peak

Hitchhikers thumb

Mid – digit hair

Dimples

Chin Dimple

Straight pinkie finger

2nd toe larger than 1st

Genetics

Studies the Studies the transmission of transmission of traits or traits or characteristics characteristics from 1 from 1 generation to the generation to the nextnext

Gregor MendelGregor Mendel(1822 – 1884)

The Father of Genetics • Central European monk (Now the

Czech Republic) discovered the basic underlying principles of heredity.

• Mathematician who studied statistic

• Work completed “Experiments on Plant Hybridization” in 1865 but his work was not discovered until 1900.

Bill Nye

• He used pea plants and controlled how they were bred over many generations

• Saw that certain traits show up in offspring plants without any blending of parent characteristics.

Why all purple?

Why not pink?

P

Mendel's work1. Removed the male parts of the flower to prevent self-

pollination

2. Used the pollen from a Purple flower producing plant and pollinated only flowers from other purple flower producers

3. Observed generations for 2 years to be certain of PurebredsPurebreds (a genetically uniform line – pedigrees)

Purple X Purple White X White Got all Purples Got all whites

4. Crossed plants with contrasting traitsPurple X white white X

Purple

Mendel expected to get something between purple and white but instead got all purples!!

Did the white trait disappear forever?

Next, he took purple flowered plants from the First Generation (F1) offspring and crossed them. Would the white trait reappear?

To Mendel’s surprise, the white trait reappeared in the Second Generation (F2) but only 25% of the flowers were white.

75% of the flowers were purple.

Why?

Mendel didn’t just use the trait for flower color, he looked at other traits in pea plants and found a similar pattern.

Every time he crossed on purebred with the contrasting purebred, only one trait only one trait ever showed. ever showed.

Then when he crossed the F1 offspring, the hidden trait reappearedhidden trait reappeared.

Mendel’s conclusionsMendel’s conclusions1.1. Traits (genes) Traits (genes) are located on

chromosomes and sexually reproducing organisms get two copies of a trait, one from each parent

2. Each parent only donates one gene per trait in their gametes.

Gene from mom Gene from dad

Genes and chromosomesGenes and chromosomes

Chromosome

Genes

Eye color

Hair color

Skin color

Nose sizeNose shape

Eye shape

hair texture

Ear size

widow’s peak

Ear lobes

Law of SegregationLaw of Segregation

3. During gamete formation (Meiosis), the two genes are separated.50% of the gametes will contain one trait while the other 50% will contain the other trait

In the example above, you can see that there are two parent chromosomes. In the same location on both chromosomes, one chromosome has a T gene while the other has a t gene for say being tail or being short.

When the gametes are produced during Meiosis, the two genes will be separated, thus his Law of Segregation Law of Segregation

More about how Mendel figured out more than he realized

Mendel never said the term genesgenes, but he did call them factors.

These factors or genes, we now know are on the DNA and code for a particular protein.

Each gene has a specific location or locus on a chromosome

Mendel said that there are different forms of a particular factor such as eye color. There are 3 different factors that determine our eye color– Blue– Green– Brown

For hand preference, there are two factors for the hand preference trait– Left– Right

AllelesAlleles are the different forms of a trait/gene.

There can be two such as hand preference or many such a hair color, blood type and eye color

Allele forAllele for Allele forAllele for

Allele forAllele for

Next Principle is his Law of DominanceLaw of Dominance: – One factor “masks” the appearance of

another factor. It prevents it from showing

P = Purple flower color – Dominant allele

P = white flower color – recessive allele

• Dominant – trait that must show if present

• Recessive – trait that will only show if in the pure form

More genetic vocabulary to learn • Genotype: Shows the actual genetic

makeup. (Use symbols for genes)– PP or Pure dominant or homozygous (same

genes) dominant– Pp or Hybrid or heterozygous (mixed genes_– pp or Pure recessive or homozygous

recessive

Homozygous vs Heterozygous: Pure vs Hybrid (mixed)

• Phenotype: – Tells appearance (describes the trait)

Purple flower (if Pp or Pp) white flower (if pp)

– NO HYBRIDS HERE!!– Describe what they look like– For Eye color:– Blue– Green – Brown– For Hair color; Red, Blond or Brown

Vocabulary Review1. Genetics ___

2. Allele ___

3. Purebred ___

4. Law of Segregation _

5. Law of dominance __

6. Genotype ___

7. Phenotype ___

8. Dominant ___

9. Recessive ___

10.Genes ___

11.Heterozygous ___

a) Inheritable distinguishing characteristics – traits

b) When present, this allele will show

c) BB, Bb, or bb – Gene makeup

d) Brown eyes, left handed… - Physical appearance

e) Homozygous – both genes for a trait are the same

f) Study of how traits are passed on

g) Alternate form of a gene

h) Allele which only shows when it is alone

i) Two genes for a trait separate during meiosis

j) One factor masks the expression of the other

k) Hybrid – having two different alleles for a trait

F

G

E

I

J

C

D

B

H

A

K

Predict the likelihood of two individuals who are hybrids for having allergies (Having allergies is a dominant trait so we will use an “A” for the gene).

A = Allergies

a = No allergies

Parent Cross: ___ x ___

A aA

a

Aa Aa

AA

Aa

Aa

aa

Phenotype ratioPhenotype ratioWhat is the likelihood of these two parents having children with allergies to not having allergies?

3:1 or 75%:25%

Genotype ratioGenotype ratioWhat is the likelihood of having a child who is a purebred for allergies? Hybrid for allergies? Purebred for not having allergies?

1:2:1

25%50%

25%

Punnett SquarePunnett SquareIs a grid system for predicting all possible

genotypes resulting from a genetic cross

The axes will have the alleles found in the two parents which result from segregation of the alleles during meiosis

Trait: Hand Preference in humans. Being right handed is dominant to left hand preference.

R = Right Hand preference

r = Left Hand preference

Cross two individuals with one being pure or homozygous dominant and the other being a hybrid or heterozygous for being right handed

Parent Cross: ___ x ___

R R

R

r

RrRR

RR

Rr

RR

Rr

50%

0%

Possible Phenotypes____ Right handed____ Left handed

Possible Genotypes___ RR___ Rr___rr

50%

50%

50%

For the same trait, cross a person who is pure for being right handed with a lefty.

Parent Cross: ___ x ___

R R

r

r

rrRR

Rr

Rr

RR

Rr

100%

0%

Possible Phenotypes____ Right handed____ Left handed

Possible Genotypes___ RR___ Rr___rr

0%

100%

0%

Trait: Earlobe position: In humans, having free earlobes is dominant to having an attached earlobe

F – Free earlobes

f – Attached earlobes

Cross a two people who both are hybrids x for having free earlobes.

Parent Cross: ___ x ___

F f

F

f

FfFf

FF

ff

Ff

Ff

75%

25%

Possible Phenotypes___ Right handed____ Left handed

Possible Genotypes___ RR___ Rr___rr

25%

50%

25%

Back to Mendel’s workIn pea plants, being a tall plant is dominant

over being short plants.___ = Tall allele

___ = Short allele

Cross a Hybird tall with a homozgyous short

Parent cross: ___ x ___Possible Phenotypes___ Tall___ Short

Possible Genotypes___ TT___ Tt___ tt

1. In humans, having a chin dimple is dominant to not having dimples.a.Show the cross of two parents who are both hybrid for having dimples.b.Give the expected Phenotype ratio and genotype ratio for this cross.

____ = Dimple gene ____ = Non dimple gene

Parent cross ______ x ______

Phenotype ratio :

Genotype ratio :

2.For the same trait, cross a hybrid with an individual who does not have dimples

Parent cross ______ x ______

Phenotype ratio :

Genotype ratio :

Probability What is the probability of flipping a head? What is the probability of flipping a tail?

50:50 chance of getting heads or tail if you toss one coin

Coin Toss Activity:

Flip one coin 10x. Keep track of the # of Heads and Tails you get.

____ Heads ____ Tails

**Need large numbers to get accurate predictions**

What happens when you toss two coins?

• If you toss 2 coins 100 times, you should get: • 25 Heads/Heads• 50 Heads/Tails• 25 Tails/Tails• As the # of trials increases, the ratios predicted by the

laws of probability get closer• Actual outcomes get closer to calculated predictionsActual outcomes get closer to calculated predictions

Now flip two coins 100 times and keep track of Now flip two coins 100 times and keep track of the number of combo you get.the number of combo you get.

  H/H H/t t/t 1      

2      

3      

4      

5      

6      

7      

8      

9      

10      

Total/1000      

Actual Genotype Ratio

Actual Phenotype

Ratio      

Predicted Genotype Ratio

Predicted Phenotype ratio

1

  H/H H/t t/t 1      

2      

3      

4      

5      

6      

7      

8      

9      

10      

Total/1000      

Actual Genotype Ratio

Actual Phenotype

Ratio      

Predicted Genotype Ratio

Predicted Phenotype ratio

3

How could you tell if you were a hybrid right handed person or pure for the trait?

You would need to mate with someone who is a lefty.

• If any of your children are lefty, then you know you had to be a hybrid

• Test Cross-Test Cross-– A cross between an organism with an

unknown dominant genotype with an individual with the recessive phenotype

Is your black guinea pig pure for its coat color?In Guinea pigs: B = Black coat b = white coat

Cross the (BB) guinea pig with a pure recessive (bb)

Cross the (Bb) guinea pig with a pure recessive (bb)

If:   BB x bb If: Bb x bb

If any of the offspring are white coated, then we knew the black guinea pig was Bb

What happens when a red carnation is crossed with a white carnation?

We would expect to get Red carnations.

• WRONG!!!• We would end up getting all Pink

carnations.

• Neither the red allele nor the white allele are dominant to each other. Instead, their expression blends and we get hybrid pink carnations

How did this happen?

P1: Red carnations (rr) x White carnations (ww)

• F1: Pink carnations (rw)

F1: Pink carnation (rw) x Pink carnation (rw)

F2: 25% Red (rr) 50% Pink (rw) 25% White (ww)

1:2:1 phenotype ratio!!

1:2:1 genotype ratio!!

Incomplete DominanceHeterozygous individuals will show a phenotype of phenotype of

something in something in between between the other two phenotypes

A green betta (gg) crossed with a royal blue betta (bb) gives a steel blue betta (gb)

Incomplete Dominance

Hair color Eye colorHeightFace shapeWavy hair

Pitch of male’s voice (tt – tenor, tb baritones, bb – low bass)

Tay-Sachs Disease - • Inability to produce the enzyme hexosaminidase

A.  • Causes fluid pressure on brain then breakdown of

brain. Starts a ~ 6 months w/ death by 2 – 3 years.  

• Most common among the descendents of Eastern European Jews (Ashkenazi Jews).

• Tt individuals produce 40-60% of enzyme

Crosses:

1. Cross a Round faced person with an square faced person

2. Cross a Black with a white horse

3. Cross a Wavy haired person with a curly haired person

Codominance Condition in which both alleles of a gene are both alleles of a gene are

expressed or active expressed or active • Roan horses: Red is codominant with white.

Roans

Blood types in humansA is codominant with B resulting in AB bloodA type blood has the A proteins on their RBCsB type has the B proteins on its RBC

Both A and B are dominant to O A = B > o

As you can see, AB blood is a mixture of both A and B proteins.

Animation

PopulationPopulation    AA BB ABAB OO

US Whites   39.7 10.6 3.4 46.3

African Americans   26.5 20.1 4.3 49

African   25 19.7 3.7 51.7

Navaho Native Am   30.6 0.2 0 69.1

Ecuadorian Am   4 1.5 0.1 94.4

Japanese   38.4 21.9 9.7 30.1

Russians   34.6 24.2 7.2 34

French   45.6 8.3 3.3 42.7

Problems1. Figure out what blood type

two parents must be in order to have 4 children, each with a different blood type.

2. Who could be the father? A woman with O type blood has a child with B type blood

Man #1 is A type blood

Man #2 is AB blood

3. What could be your blood type?

Polygenic Inheritance • Some human traits are controlled by

more than one set of genes that determine the expression of the traits.

• Ie. Eye color , hair color, height, body weight, skin color, etc…

Light blue 0 dominate alleles

Blue 1 dominate allele

Blue-green 2 dominate alleles

hazel 3 dominate alleles

Light brown 4 dominate alleles

Brown 5 dominate alleles

Dark brown / black 6 dominate alleles

How is sex determined?

Below is a KaryotypeKaryotype (a picture of all the chromsomes in a cell) of the two different individuals.

Can you see a difference between the two.

Sex ChromosomesBoth have 22 pairs of AutosomesAutosomes (Body Chromosomes)

• Sex Chromosomes are the big X & little Y chromosome

• The female on the left has two X (XXXX) chromosomes while the male on the right has an X and a Y (XYXY)

Abnormal Karyotypes

Extra X Chromosome – 47XXYExtra 21 – Trisomy 21

Only has an X – 45X0 Extra 18 – Trisomy 18

Does the difference between the size of the X and Y chromosomes make a difference in the inheritance of

some traits?

It makes a difference if the trait is inherited only on the larger X chromosome.

• Since the Y is so much smaller, some traits are only inherited on the X chromosome

• We call these traits “Sex-linked Sex-linked traits”traits”

• If a mother gives her son the trait on the X chromosome, the father’s Y can’t save him from having it since the Y doesn’t have the normal gene

Unmatched area – no homologous genes present on the Y chromosome

Normal female Carrier female Female w/ trait Normal male Male w/trait

Colorblindness – Recessive gene carried on the X chromosome– Individuals can’t see certain colors– If you are looking at a traffic light, where is the

red light?

In humans, colorblindness is inherited on the X chromosome and is recessive. The gene for normal color vision is dominant.

Cross a female who is a carrier of the gene (XNXn) with a normal male (XNY)N = Normal color vision genen = Colorblind gene

• What is the probability of them having a son who has the colorblind gene (n) on his X chromosome and is therefore colorblind? ______%

• What is the probability of them having a colorblind daughter? _______%

50%

0%

What if the mother is a carrier for colorblindness and the father is colorblind? What about the children?

N = Normal color vision genen = Colorblind gene

Parent Cross: XNXn x XnY

• What is the probability of them having a son who has the colorblind gene (n) on his X chromosome and is therefore colorblind? ______%

• What is the probability of them having a colorblind daughter? _______%

50%

50%

How could we follow the colorblind trait through a family?

PedigreePedigree

Is a chart that can trace the phenotypes and genotypes in a family through generations

Squares are malesCircles are femalesIf colored in, they have the traitIf half colored, they are carriers.

Pedigree for AlbinismAlbinism is a recessive autosomal gene so it

isn’t sex linked

Dwarfism pedigreeDwarfism is dominant and autosomal