1 The Genetics of Color- Blindness Dr. Rick Hershberger .

1

The Genetics of Color-Blindness


Dr. Rick Hershberger • http://www.rickhershberger.com

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Outline

•How our Eyes See Colors•Defects in Human Color Vision•A Gene for Red-Green Color Blindness

•Inheritance•X-Linkage•Pedigree Analysis

- Testing my Daughter’s Prom Date?

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Anatomy of an Eyeball

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The Genetics of Color-Blindness The Retina Contains Two Types of

Light-Detecting Cells

• Rods – “See in shades of grey”- Cannot distinguish different wavelengths

(colors) of light.- More sensitive to low light. Used for

night-vision.• Cones – “See in colors”

- Three types of cones; differ in which photoreceptor protein (opsin) they make.- L-cones sense long-wavelength (red)

light- Make the long-wavelength opsin protein

- M-cones sense medium-wavelength (green) light

- Make the medium-wavelength opsin protein

- S-cones sense short-wavelength (blue) light

- Make the short-wavelength opsin protein

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Photoreceptor Proteins

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How Color-Blind People See Things

What people with normal color vision see.

What a red-green color-blind person

sees.

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Types of Color Vision Deficiencies

• Trichromacy (“three-color vision”)- Normal Color Vision

• Anomalous Trichromacy (“unusual three-color vision”)- See all three primary colors.- One color is seen weakly

- Protanomaly (L-cone defect) red-weak- Deuteranomaly (M-cone defect) green-weak- Tritanomaly (S-cone defect) blue-weak

• Dichromacy (“two-color vision”)- See only two of the three primary colors- One type of cone is totally absent or nonfunctional.

- Protanopia (L-cone absent)- Deuteranopia (M-cone absent)- Tritanopia (S-cone absent)

• Rod Monochromacy (no cones at all) (“no-color vision”)- Sees no colors, only shades of gray.

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How Color-Blind People See Things

Defect in L-cone (poor red

vision)

Normal

Defect in M-cone (poor

green vision)

Defect in S-cone (poor blue

vision)

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Human cells have 46 chromosomes,organized as 23 pairs.

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X and Y: Our Sex Chromosomes

•Our 23rd pair of chromosomes are our “sex chromosomes”, because they determine which sex we are.

•Females have two X chromosomes.

•Males have one X chromosome and one Y chromosome. - If you inherit a Y chromosome, you

become a male.- The SRY gene on the Y

chromosome controls your gender.

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The Genetics of Color-Blindness The X Chromosome and X-Linked

Traits

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Punnett Squares for X-linked Traits

XRXR

girlXRYboy

XRXr

girlXrYboy

YXR

Normal Jack

XR

Xr

CarrierJill

Color-blind boys get their

trait from their carrier

moms.

XRXr

girlXRYboy

XRXr

girlXRYboy

YXr

Color-Blind Jack

XR

XR

NormalJill

Color-blind dads make ALL of their daughters carriers!

“Carriers” exhibit the dominant trait (are

unaffected) but carry

the defective allele and can pass the trait

on to their children.

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Incidence of Color Vision Deficiencies

Classification Incidence (%) Incidence (%)

in Males in Females

Anomalous Trichromacy 6.3 0.37Protanomaly (L-cone defect) 1.3 0.02Deuteranomaly (M-cone defect) 5.0 0.35Tritanomaly (S-cone defect) 0.0001 0.0001

Dichromacy 2.4 0.03 Protanopia (L-cone absent) 1.3 0.02Deuteranopia (M-cone absent) 1.2 0.01Tritanopia (S-cone absent) 0.001 0.03

Rod Monochromacy (no cones) 0.00001 0.00001

Why are most kinds of color-blindness more common in men

than women?

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Punnett Squares for X-linked Traits:Why Color-Blindness is More Common in Males

XRXR

girl

XRYboy

XRXr

girl

XrYboy

YXR

Normal Jack

XR

Xr

CarrierJill

For a boy to be color-blind, he only needs to inherit ONE Xr allele, from his carrier mom.

XRXr

girl

XRYboy

XrXr

girl

XrYboy

YXr

Color-Blind Jack

XR

Xr

CarrierJill

For a girl to be color-blind, she must

inherit TWO Xr alleles, one from her color-blind dad and one

from her carrier mom.

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Pedigrees are Genetic Family Trees

males

females

normal

affected

dad mom

sondaughterson daughter

first born last born

Boys are square? Girls are round?

in order of birth

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For traits that are controlled by genes on the 22 pairs ofautosomes(non-sex chromosomes)

Genotypes and Phenotypes for Recessive Traits

A_AA or

Aaaamales

females

A_AA or

Aaaa

dominant

recessive

XAY XaYmales

females

XAX_

XAXA orXAXa

XaXa

dominant

recessiveFor traits that

are controlled by genes on theX chromosome(X-linked traits)

“Carriers” exhibit the dominant trait (are

unaffected) but carry

the defective allele and can pass the trait

on to their children!

carrier

carrier

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Professor Hershberger’s Rules for Interpreting Pedigrees

• Step 1: Match a genotype to each phenotype.- If the individual exhibits the recessive phenotype,

he/she is aa (or XaXa for an X-linked trait)- If the individual exhibits the dominant

phenotype, he/she is A_ (or XA_ for an X-linked trait).

• Step 2: Where possible, track alleles (genes) UP the pedigree, from child to parent.- Because children get one allele from each parent.

• Step 3: Where possible, track alleles (genes) DOWN the pedigree, from parent to child.- Because each parent gives one of his/her alleles

to each child.

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You are theGenetic Counselor.

Gretchen is a carrier for red-green color-blindness. How will Gretchen’s choice of husband affect whether her children will be color-blind?

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You are the Genetic Counselor!What if Gretchen marries a man who has normal vision?

non-carrier

carrier

Gretchen

Gretchen’s Children

Pam

Rick

girl boy

girl boy

22

Possible Son-in-Law

Gretchen33

33

2

2

1: Label the pedigree chart with the genotypes of Rick, Pam, Gretchen, the “son-in-law”, and Gretchen’s possible children.2: Enter Gretchen’s and her possible mate’s alleles into the Punnett Square above.3: Determine the possible genotypes of their children from the Punnett Square.4: Enter the probabilities for each of Gretchen’s possible children onto the pedigree chart.

The “Son-in Law”

1 1

1 1

1

4

1

4

1

4

1

4

genotypes

probabilities

non-carrier

1

4

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You are the Genetic Counselor!What if Gretchen marries a man who is color-blind?

non-carrier

carrier

Gretchen

Gretchen’s Children

Pam

Rick

girl boy

girl boy

22

Possible Son-in-Law

Gretchen33

33

2

2



1 1

1 1

1

4

1

4

1

4

1

4

genotypes

probabilities

non-carrier

1

4

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The Answers

What happens if Gretchen marries a man who has normal vision?

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non-carrier

1 1

1 1

Y

ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?

carrier

Gretchen

Pam

Rick

Possible Son-in-Law



XrY

genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

1

4

1

4

1

4

1

4

1

4

Using Prof. H’s Step #1:

Because Rick is a male, he has a

Y.


Because he is color-blind, he has the mutant

Xr allele.

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non-carrier

1

1 1

XXXRX


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



XRXR

genotypes

probabilities

non-carrier


Because Pam is a female, she has two

Xs.

girl boy

girl boy

22

Gretchen33

33

2

2

1

4

1

4

1

4

1

4

1

4

XrY


Because she is NOT color-blind, she must

have at least one dominant normal XR

allele.


Because Pam’s father and grandfather are not color-blind, and none of her brothers or nephews are, it’s

likely that the Xr allele does not appear in

her pedigree. We can assume she did not inherit the Xr allele and is thus NOT a

carrier.

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non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

1

4

1

4

1

4

1

4

1

4

XXXRXXRXr

Using Prof. H’s Step #2:To be a female, she had to inherit an X chromosome

from her father. Her father’s only X

chromosome carries the Xr allele. Therefore, she must have inherited her father’s

Xr allele, and is thus a carrier.


Because she is NOT color-blind, she must

have at least one dominant normal XR

allele.

XrY XRXR


Because Gretchen is a female, she has

two Xs.

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non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

1

4

1

4

1

4

1

4

1

4

XXXRXXRXr

XrY XRXR

YXRY


Because the “Son-in-Law” is a male, he has a

Y.


Because he is NOT color-blind, he must have a normal XR allele.

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1 1 1 1 1XYXY XX XX XX

non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

4 4 4 44

XXXRXXRXr

XrY XRXR

YXRY

XRX XRX XrXrUsing Prof. H’s Step

#1:Males are XY.

Females are XX.


Daughters get Dad’sX chromosome, so all daughters will inherit a normal XR allele and

have normal color vision.


Sons get Dad’sY chromosome.

0%

no

Using Prof. H’s Step #3:If Gretchen marries a man with normal color vision, they will NOT have any color-blind daughters, since all

daughters will inherit their dad’s normal XR

allele.

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XYXY XRX XRX1

XrYXRY XX

non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

4 4 4 44

XXXRXXRXr

XrY XRXR

YXRY

XRXR XRXr XrXr

0%

Using Prof. H’s Step #3:The probability that any

son will be color-blind will be determined by their

odds of inheriting the XR or Xr allele from Gretchen.

Using Prof. H’s Step #3:The probability that any

daughter will be a carrier will be determined by their odds of

inheriting the XR or Xr allele from Gretchen.

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XYXY XRX XRX1

XrYXRY XX

non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

4 4 4 44

XXXRXXRXr

XrY XRXR

YXRY

XRXR XRXr XrXr

0%

YXR

XR

Xr

A Punnett Square is used to calculate the

probabilities of various possible

offspring.

One parent’s alleles are used as row headings. These

represent the genotypes of the gametes formed by that

parent. In this case, these are Gretchen’s possible egg cells.

The other parent’s alleles are used as column headings. These represent the genotypes of the gametes formed by that parent. In this case, these are the Son-in-Law’s possible sperm cells.

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XYXY XRX XRX1

XrYXRY XX

non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

4 4 4 44

XXXRXXRXr

XrY XRXR

YXRY

XRXR XRXr XrXr

0%

YXR

XR

Xr

YXR

YXR

Carry the one parent’s alleles down within each column.

Carry the other parent’s alleles across within each

row.

XRYXRXR

XrYXRXr

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XYXY XRX XRX1

XrYXRY XX

non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law



genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

4 4 4 44

XXXRXXRXr

XrY XRXR

YXRY

XRXR XRXr XrXr

0%

YXR

XR

Xr

YXR

YXR

XRYXRXR

XrYXRXr

25% 25% 25%25%

If Gretchen marries a man with normal color-vision, each of their children will

have a 25% chance of being either• a male with normal color vision• a male with color-blindness• a female non-carrier• a female carrier

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XYXY XRX XRX1

XrYXRY XX

non-carrier

1 1


carrier

Gretchen

Pam

Rick

Possible Son-in-Law


genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

4 4 4 44

XXXRXXRXr

XrY XRXR

YXRY

XRXR XRXr XrXr

0%

YXR

XR

Xr

YXR

YXR

XRYXRXR

XrYXRXr

25% 25% 25%25%

If Gretchen marries a man with normal color-vision,

• half of their sons will be color-blind,

• none of their daughters will be color-blind,

• half of their daughters will be carriers.

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The Answers

What happens if Gretchen marries a man who is red-green color-blind?

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XYXY XRX XRX1

XrYXRY XX

non-carrier

1 1

ANSWER: Here’s what happens if Gretchen marries a man who is red-green color-blind?

carrier

Gretchen

Pam

Rick

Possible Son-in-Law


genotypes

probabilities

non-carrier

girl boy

girl boy

22

Gretchen33

33

2

2

4 4 4 44

XXXRXXRXr

XrY XRXR

YXrY

XRXR XRXr XrXr

25%

YXr

XR

Xr

YXR

YXR

XRYXRXr

XrYXrXr

25% 25% 25%0%

If Gretchen marries a man with red-green color-

blindness, • half of their sons will be

color-blind,• half of their daughters

will be color-blind,• the other half of their

daughters will be carriers.

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How will Gretchen’s choice of husband affect whether her children will be color-blind?

35


How will Gretchen’s choice of husband affect whether her children will be color-blind?

Color-Blind Son-in-Law

girl boy

girl boy

2

GretchenYXr

XR

Xr

XRYXRXr

XrYXrXr

If Gretchen marries a man with red-green color-blindness,


• half of their daughters will be color-blind,


Normal Son-in-Law

girl boy

girl boy

2

GretchenYXR

XR

Xr

XRYXRXR

XrYXRXr

Gretchen

If Gretchen marries a man with normal color-vision,


• none of their daughters will be color-blind,


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1 The Genetics of Color- Blindness Dr. Rick Hershberger .

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