Name _________________________________________________________ Period___________
“The Case of Desiree’s Baby”
Skin Color, Genetics, &
Biotechnology
Part I – A Mendelian approach
"Desiree's Baby" is a very moving story that shows us the
horror of racism, but is it scientifically accurate?
Questions
1. According to the story, Madame Valamonde and Armand noticed the baby's
dark skin several weeks before Desiree. Can you offer a possible explanation for
this gradual increase in pigmentation over the course of three months?
2. Assuming the inheritance of skin color in "Desiree's Baby" follows the simple
dominant/recessive pattern described by Mendel, is dark pigmentation best
explained as an example of a dominant or recessive allele?
3. Does skin pigmentation follow simple dominance rules? If not, what kind of
intermediate inheritance pattern would you assign it? Why?
4. Assign suitable symbols for each allele and use the information in the story to
develop possible genotypes for Desiree, Armand, and the baby.
Armand’s genotype
Desiree’s genotype
Baby’s genotype
5. Consider the skin pigmentation of biracial individuals, such as Halle Berry,
Lenny Kravits, Mariah Carey, or "The Rock." Can these phenotypes (skin colors)
be explained by the same Mendelian model?
Part II – Skin Color is a Polygenic Trait
Differences in skin color are largely due to differences in the amount of melanin,
a dark pigment produced by skin cells. There are different variations of melanin,
eumelanin and phaeomelanin, and many enzymes interact in the formation
and movement of skin pigments.
In an early model (and it still works; although the underlying sequence of
enzymatic activity is much more complex) skin coloration was thought to have
been controlled by 2-4 genes.
We shall assume that 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)
The offspring of this cross exhibit seven shades of skin color based on the number
of dark skin alleles in each genotype.
For a further explanation of working with this chart, take a look at
http://waynesword.palomar.edu/lmexer5.htm
Questions
6. Each parent produces eight different types of gametes, which can combine
in 64 different ways.
Complete the following table:
# of dark skin alleles
Phenotype: skin color
Phenotypic ratio #offspring/64
0
1
2
3
4
5
6
7. How many of these 64 possible offspring have skin that is darker than their
parents?
8. Each of these three genes has two alleles. Are the dark skin alleles dominant
to the light skin alleles?
9. There are more than 7 shades of skin color. Can you offer an explanation?
10. 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.
Individual
Monsieur Aubigny
Madame Aubigny
Armand
Desiree
Desiree’s mother
Desiree’s father
Desiree’s baby
Possible genotype
Part III – Types of Polygenic Inheritance
Polygenic inheritance involves two or more _________________ influencing the
________________. This means that the possible number of genotypes influencing
a single trait is significantly increased. Most human characteristics are thought to
be _______________________
Polygenic inheritance comes in either discontinuous or continuous forms.
Discontinuous variation
This is where individuals fall into a number of distinct classes or categories, and is
based on features that cannot be measured across a complete range. You
either have the characteristic or you don't. Blood group is a good example: you
are either one blood group or another - you can't be in between.
Discontinuous variation is controlled by multiple alleles of a single gene
Continuous variation
In continuous variation there is a complete range of measurements from one
extreme to the other, which when plotted shows a normal pattern of distribution.
Examples of continuous variation include:




Height
Weight
Hand span
Milk yield in cows
11. Is skin color a continuous or a discontinuous trait?
Part IV – Ethics and Biotechnology
The techniques used to screen for severe genetic disorders are now being used
to develop methods to make “designer babies”.
Please watch the newscast found here
http://www.geneticsandsociety.org/article.php?id=4573
Read the article on genetics selection of skin
http://www.geneticsandsociety.org/article.php?id=3611
Write a brief analysis of the implications of this new technology.
Modified from http://waynesword.palomar.edu/lmexer5.htm and National Center for Case Study
Teaching in Science.