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The
Genetics and Evolution of Skin Color: The Case of Desiree's Baby
Developed by Patricia Schneider.
Copyright 2003 National
Center for Case Study Teaching in Science.
Used with permission.
HANDOUT #2
Skin Color is a Polygenetic (Multiple Gene) Trait
Differences in skin color are largely due to differences
in the amount of melanin, a dark pigment, produced by skin cells.
At least three genes control the synthesis of melanin. Each
gene has two forms: an allele for high melanin production
or dark skin (A,B,C) and an allele for low melanin production
or light skin (a,b,c). Each dark skin allele (A,B,C) in
the genotype adds a small but equal amount of pigment to the skin.
There are seven different shades of skin color ranging from very
dark (AABBCC) to very light (aabbcc). Most individuals produce
a medium amount of melanin and are of intermediate skin color
(AaBbCc).
The following Punnett square shows the possible offspring from
a cross between two individuals of intermediate skin color.
AaBbCc X AaBbCc
(each square shows the number of dark skin alleles
in the genotype)
|
Gametes |
ABC |
ABc |
AbC |
Abc |
aBC |
aBc |
abC |
abc |
|
ABC |
6
AABBCC |
5
AABBCc |
5
AABbCC |
4
AABbCc |
5
AaBBCC |
4
AaBBCc |
4
AaBbCC |
3
AaBbCc |
|
ABc |
5
AABBCc |
4
AABBcc |
4
AABbCc |
3
AABbcc |
4
AaBBCc |
3
AaBBcc |
3
AaBbCc |
2
AaBbcc |
|
AbC |
5
AABbCC |
4
AABbCc |
4
AAbbCC |
3
AAbbCc |
4
AaBbCC |
3
AaBbCc |
3
AabbCC |
2
AabbCc |
|
Abc |
4
AABbCc |
3
AABbcc |
3
AAbbCc |
2
AAbbcc |
3
AaBbCc |
2
AaBbcc |
2
AabbCc |
1
Aabbcc |
|
aBC |
5
AaBBCC |
4
AaBBCc |
4
AaBbCC |
3
AaBbCc |
4
aaBBCC |
3
aaBBCc |
3
aaBbCC |
2
aaBbCc |
|
aBc |
4
AaBBCc |
3
AaBBcc |
3
AaBbCc |
2
AaBbcc |
3
aaBBCc |
2
aaBBcc |
2
aaBbCc |
1
aaBbcc |
|
abC |
4
AaBbCC |
3
AaBbCc |
3
AabbCC |
2
AabbCc |
3
aaBbCC |
2
aaBbCc |
2
aabbCC |
1
aabbCc |
|
abc |
3
AaBbCc |
2
AaBbcc |
2
AabbCc |
1
Aabbcc |
2
aaBbCc |
1
aaBbcc |
1
aabbCc |
0
aabbcc |
The offspring of this cross exhibit
seven shades of skin color based on the number of dark skin alleles
in each genotype.
Questions
- Each parent produces eight different types of
gametes, which can combine in 64 different ways. Complete
the following table:
- How many of these 64 possible offspring have
skin that is darker than their parents?
- Each of these three genes has two alleles.
Are the dark skin alleles dominant to the light skin alleles?
- There are more than 7 shades of skin color.
Can you offer an explanation?
- Use this model of skin color inheritance and
the information provided in the short story to develop possible
genotypes for each of the following characters: Monsieur and
Madame Aubigny, Armand Aubigny, Desiree's mother and father,
Desiree, and Desiree's baby.
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