Desiree S Baby Skin Color Questions

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
0
1
2
3
4
5
6
Phenotype: skin color

Phenotypic ratio #offspring/64

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
Possible genotype
Monsieur Aubigny

Madame Aubigny

Armand

Desiree

Desiree’s mother

Desiree’s father

Desiree’s baby

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.