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The Genetics of Color-Blindness. Dr. Rick Hershberger • http://www.rickhershberger.com. 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?. Anatomy of an Eyeball. - PowerPoint PPT Presentation
Citation preview
1
The Genetics of Color-Blindness
The Genetics of Color-Blindness
Dr. Rick Hershberger • http://www.rickhershberger.com
2
The Genetics of Color-Blindness 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?
4
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
6
The Genetics of Color-Blindness How Color-Blind People See Things
What people with normal color vision see.
What a red-green color-blind person
sees.
7
The Genetics of Color-Blindness 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.
8
The Genetics of Color-Blindness 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)
10
The Genetics of Color-Blindness 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.
12
The Genetics of Color-Blindness 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.
13
The Genetics of Color-Blindness 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?
14
The Genetics of Color-Blindness
Punnett Squares for X-linked Traits:Why Color-Blindness is More Common in Males
XRXR
girlXRYboy
XRXr
girlXrYboy
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
girlXRYboy
XrXr
girlXrYboy
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.
15
The Genetics of Color-Blindness 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
16
The Genetics of Color-Blindness
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
17
The Genetics of Color-Blindness 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.
18
The Genetics of Color-Blindness
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?
19
The Genetics of Color-Blindness
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
20
The Genetics of Color-Blindness
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: 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
21
The Genetics of Color-Blindness
The Answers
What happens if Gretchen marries a man who has normal vision?
22
The Genetics of Color-Blindness
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
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”
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.
Using Prof. H’s Step #1:
Because he is color-blind, he has the mutant
Xr allele.
23
The Genetics of Color-Blindness
non-carrier
1
1 1
XXXRX
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
XRXR
genotypes
probabilities
non-carrier
Using Prof. H’s Step #1:
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
Using Prof. H’s Step #1:
Because she is NOT color-blind, she must
have at least one dominant normal XR
allele.
Using Prof. H’s Step #3:
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.
24
The Genetics of Color-Blindness
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
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.
Using Prof. H’s Step #1:
Because she is NOT color-blind, she must
have at least one dominant normal XR
allele.
XrY XRXR
Using Prof. H’s Step #1:
Because Gretchen is a female, she has
two Xs.
25
The Genetics of Color-Blindness
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
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
Using Prof. H’s Step #1:
Because the “Son-in-Law” is a male, he has a
Y.
Using Prof. H’s Step #1:
Because he is NOT color-blind, he must have a normal XR allele.
26
The Genetics of Color-Blindness
1 1 1 1 1XYXY XX XX XX
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
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.
Using Prof. H’s Step #3:
Daughters get Dad’sX chromosome, so all daughters will inherit a normal XR allele and
have normal color vision.
Using Prof. H’s Step #3:
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.
27
The Genetics of Color-Blindness
XYXY XRX XRX 1XrYXRY XX
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
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.
28
The Genetics of Color-Blindness
XYXY XRX XRX 1XrYXRY XX
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
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.
29
The Genetics of Color-Blindness
XYXY XRX XRX 1XrYXRY XX
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
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
30
The Genetics of Color-Blindness
XYXY XRX XRX 1XrYXRY XX
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
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”
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
31
The Genetics of Color-Blindness
XYXY XRX XRX 1XrYXRY XX
non-carrier
1 1
ANSWER: Here’s what happens if Gretchen marries a man who has normal vision?
carrier
Gretchen
Pam
Rick
Possible Son-in-Law
The “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.
32
The Genetics of Color-Blindness
The Answers
What happens if Gretchen marries a man who is red-green color-blind?
33
The Genetics of Color-Blindness
XYXY XRX XRX 1XrYXRY 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
The “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.
34
The Genetics of Color-Blindness
How will Gretchen’s choice of husband affect whether her children will be color-blind?
35
The Genetics of Color-Blindness 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 sons will be color-blind,
• half of their daughters will be color-blind,
• half of their daughters will be carriers.
Normal Son-in-Law
girl boy
girl boy
2
GretchenYXR
XR
Xr
XRYXRXR
XrYXRXr
Gretchen
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.