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

  1. Each parent produces eight different types of gametes, which can combine in 64 different ways.  Complete the following table:


  2. How many of these 64 possible offspring have skin that is darker than their parents?
  3. Each of these three genes has two alleles.  Are the dark skin alleles dominant to the light skin alleles?
  4. There are more than 7 shades of skin color.  Can you offer an explanation?
  5. 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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