Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS



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.

Grade Levels: 9-14
Subject Matter: Biology, Physical Anthropology, Genetics
Time Allotment: 1-2 class sessions
Description: This lesson plan explores the genetics and evolution of skin color, using a short story by Kate Chopin called "Desiree's Baby" as a starting point.

Patricia Schneider teaches at Queensboro Community College in Bayside, NY. This lesson plan was developed for the National Center for Case Study Teaching in Science with support from The Pew Charitable Trusts as part of the Case Studies in Science Workshop held at the University at Buffalo, State University of New York, on June 10-14, 2002.

OVERVIEW

Students will explore the genetics of skin color and theories about its evolution by pondering the mysteries of Kate Chopin's short story "Desiree's Baby," which was first published in 1893. It is a story of race and gender in antebellum Louisiana. Desiree is deeply in love with her husband, Armand, and he is a loving husband and a proud father until he notices their infant's dark skin. Because Desiree was abandoned as a child, her ancestry is unknown. Armand concludes that she is not white and tells her to leave. His rejection drives Desiree to take her own life and that of the baby. In the last line of the story, Armand discovers that he is also of mixed ancestry.

Students read the story and then discuss a set of questions about genetics probing the puzzle of how Desiree's baby can have a darker skin color than either Desiree or the baby's father, Armand. Students then use an AaBbCc Punnett Square to explore polygenetic skin color inheritance. Finally, students will read assigned background articles and write an analytical essay scrutinizing various theories accounting for the evolution of different skin colors.

The case was developed for urban community college students in their first semester of general biology. The course curriculum is organized around the general theme of evolution. By the time the case is introduced, students have covered evolution, biochemistry, cell biology, and Mendelian genetics. The case is also appropriate for anthropology and biology courses for non-majors.


GLOSSARY

allele
melanin
polygenetic
genotype
phenotype
gamete
epistasis
Punnett square
Albinism
homozygote
heterozygote

MATERIALS

  • RACE - The Power of an Illusion Episode 1: The Difference Between Us (on video or DVD)
  • "Desiree's Baby" and other handouts (below)
  • "Skin Deep" article by Nina Jablonski and George Chaplin in the October 2002 issue of Scientific American

OBJECTIVES

Students finishing this case will be able to:

  • Explain polygenetic inheritance
  • Describe the inheritance of skin color
  • Discuss the "sunscreen" and "vitamin" hypotheses of skin color evolution
  • Write a short essay summarizing the key points in a popular science article

LESSON PLAN

The short story and the accompanying three parts of the case were designed to be presented using the progressive disclosure method (for more information on this method, also known as the Interrupted Case Method, see http://ublib.buffalo.edu/libraries/projects/cases/coots/coots_prologue.html)

Part I: A Mendelian Approach (20 minutes)

  1. Distribute copies of the short story Desiree's Baby (Handout #1) as a reading assignment.

  2. Have the students work in small groups to answer the questions at the end of the short story handout.

  3. Discuss the answers together as a class. Note that most classes quickly agree on answers to all the questions except for number 4. Many students feel that skin color must follow a simple dominant / recessive pattern, even though this could not explain the intermediate phenotype of "biracial" individuals.

Part II: Skin Color is a Polygenetic (Multiple Gene) Trait (20 minutes)

  1. Display the following skin color inheritance chart for the class by copying it onto the chalkboard or using a computer projector or overhead transparency.

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.

  1. Distribute Handout #2 and ask the students (working in small groups) to determine the number of offspring with each skin color. If time permits, have them use the number of offspring with each skin shade to plot a bell-shaped curve on the chalkboard.

  2. Discuss the results together as a class. Ask the students what they think someone with the aabbcc genotype might look like. Many students incorrectly assume that such an individual is an albino. Note that aabbcc individuals are not albinos; they have fair skin (like Northern Europeans) due to low melanin production while albinos have no pigmentation in their skin, hair, and eyes because they lack the enzyme tyrosinase needed for melanin production. Albinism is inherited as an autosomal recessive at another locus. Two recessive albinism alleles at the tyrosinase gene locus will prevent expression of the genes (A, B, and C) that govern the amount of melanin production. This gene interaction is called epistasis.

    Students sometimes raise the issue of Michael Jackson's skin color. He is said to suffer from viteligo, an autoimmune disorder that destroys melanin producing cells.

  3. Any class time that remains could be devoted to a discussion of other polygenetic traits such as height, body build, intelligence, and Type II diabetes. Note that gene expression is often influenced by the environment. Sunlight can darken skin just as nutrition can affect height. Also, be sure to note that since different traits are influenced by different genes, they are inherited independently one from another. Skin color is not an indication of any other trait, be it height, blood type, or intelligence.

Part III: Evolution of Skin Color

  1. Begin this part of the lesson by showing the following video, which explores the science of human variation:

    RACE - The Power of an Illusion
    , Episode 1: The Difference Between Us.

    If you only have time to screen the segment on skin color, begin the video at 23:41 (with Stephen Jay Gould saying: "My favorite trivia question in baseball...") and play through 28:03 (ends with Joseph Graves saying: "Oh, this is the place where we go from the light race to the dark race..."). If you are using the DVD, go to scene 8.

    (To further explore how race is socially constructed, see Episode 3: The House We Live In. Note that in this episode (DVD Scene 7), historian James Horton describes how southern states had different laws defining who is "black," so that some individuals could literally change race by crossing state boundaries.)

  2. Have students read "Skin Deep" by N. Jablonski and G. Chaplin in the October 2002 issue of Scientific American. NOTE: This article is available online for a fee. For alternative readings that can be found online at no cost, follow these links:

    a. "A New Light on Skin Color" - National Geographic Online Extra
    b. Transcript of radio interview with Nina Jablonski about her skin color research
    c. California Academy of Sciences article about Jablonski's work

  3. Give students one week to complete the writing assignment below:

    Using details on the evolution of skin color from the Jablonski/Chaplin article, write an essay in your own words that discusses both the "sunscreen" and "vitamin" hypotheses and answers the following question:

    DNA analysis indicates that we are all descended from a single ancestral group of Africans.  If we are all "out of Africa," why are there so many different skin colors?

    Ask your students to provide a thorough explanation addressed to students who missed the last few class sessions. The assignment should be typewritten and no more than one page in length.

FOLLOW-UP ACTIVITY

The Sorting People section of this Web site is an interactive game allowing students to experience the diversity of skin colors and other traits across "racial" groups. Click here to open a new window and access the game or point your students to www.pbs.org/race. Click on "Learn More" then "Sorting People" then "Explore Racial Traits."

ASSESSMENT

Students can be evaluated according to their class participation and their essay can be graded using a holistic scale (Bean, 1996). Individual learning can also be assessed on the next unit exam by questions pertaining to the genetic and evolutionary principles emphasized in the short story, the scientific articles, the written assignment, and class discussions.

ANSWER KEY

An answer key to the questions posed in this lesson is available to teachers via the National Center for Case Study Teaching in Science (NCCSTS) website. In order to access the answer key, you will first need to obtain a password from the NCCSTS. To do this, complete the application form at http://www.sciencecases.org/register.asp. Please allow 3-4 business days for a response.

BLOCKS OF ANALYSIS

Polygenetic Inheritance

One of the reasons why Mendel was so successful in working out the basic principles of heredity was that he studied simple traits. He chose traits in the garden pea, which appeared in two distinctly contrasting (either-or) forms, e.g., tall vs. short plants, yellow vs. green pods, and smooth vs. wrinkled seeds. A single gene controls each of these traits. Few characteristics follow this simple pattern of inheritance. Most traits result from the additive effect of many genes mediated by the environment. These polygenetic traits are characterized by small gradations in phenotype, known as continuous variation. Graphing the distribution of one of these traits produces a bell-shaped curve in which extreme values are much rarer than intermediate values. Environmental factors influence the expression of polygenetic traits - e.g., poor nutrition limits height, sun exposure darkens skin color.

Inheritance of Skin Color

Skin color is largely determined by the amount of melanin. Dark-skinned individuals produce more melanin than light-skinned individuals. At least three genes regulate the amount of melanin produced. Each gene has two forms: dark skin allele (A, B, and C) and light skin allele (a, b, and c). Neither allele is completely dominant to the other, and heterozygotes exhibit an intermediate phenotype (incomplete dominance). Each dark skin allele in the genotype adds pigment by increasing melanin production. There are seven different shades of skin color ranging from very light (aabbcc) to very dark (AABBC); most individuals have the intermediate skin color (AaBbCc). A cross between two individuals with intermediate skin color produces offspring with a range of phenotypes (bell-shaped curve).

Evolution of Human Skin Color

It appears that our earliest modern human ancestors (Homo sapien sapiens), who lived 100 - 150,000 years ago in eastern Africa, had dark skin to protect them against the deleterious effects of ultraviolet radiation. Many scientists used to believe that dark pigmentation evolved in Africa as a "sunscreen" to protect against skin cancer. However, this could not be the only selective pressure since most deaths from skin cancer occur only after reproductive age. According to the most recent theory, different skin colors evolved to ensure reproductive success by regulating the production of two critical vitamins.

Ultraviolet radiation (UV) catalyzes the synthesis of vitamin D, which is required for absorption of calcium and development of the skeleton. Vitamin D deficiency can lead to rickets, a crippling bone disease. But overexposure to UV radiation will break down vitamin B folate (folic acid), which is necessary for fetal neural development and fertility. Anthropologist Nina Jablonski theorizes that dark skin evolved near the equator. There, UV radiation penetration is high enough to stimulate vitamin D production while the dark skin protects against the breakdown of folate. Light skin evolved when early humans migrated to the high latitudes where UV radiation is much lower. The amount of melanin gradually decreased to facilitate vitamin D synthesis under low UV conditions. Today, as a result of recent migrations, many individuals do not live in the climate for which their skin is adapted. Dark-skinned people in high latitudes can get their vitamin D from sources like fish, while light-skinned people in the tropics can protect against folate breakdown by covering up with clothing.

RESOURCES

Allman, W.F. 2002. Eve Explained: How Ancient Humans Spread Across the Earth. http://dsc.discovery.com/convergence/realeve/feature/feature.html

Blake, E. 2000. Why Skin Comes in Colors. http://www.calacademy.org/calwild/winter2000/html/horizons.html

Campbell, N.A., and J.B. Reece. 2002. Biology (6th ed). San Francisco, CA: Benjamin Cummings.

Cummings, M.R. 2003. Human Heredity, Principles and Issues (6th ed). Pacific Grove, CA: Brooks/Cole-Thomson Learning.

Goodman, A.H. 2000. Why Genes Don't Count (for Racial Differences in Health). American Journal of Public Health, 90(11):1699-1702.

Holden, C. 1991. New Center to Study Therapies and Ethnicity. Science 251(4995):748.

Jablonski, N.J., and G. Chaplin. 2002. Skin Deep. Scientific American 278(4):74-81.

Jablonski, N.J., and G. Chaplin. 2000. The Evolution of Human Skin Coloration. Journal of Human Evolution 39(1):57-106.

Kirchweger, G. 2001. The Biology of...Skin Color. http://www.discover.com/issues/feb-01/departments/featbiology/

Online Mendelian Inheritance in Man. National Center for Biotechnolgy Information (NCBI). http://www.ncbi.nlm.nih.gov/omim/

MendelWeb http://mendelweb.org/home.html

Solomon, E.P., L.R. Berg, and D.W. Martin. 2002. Biology (6th ed). Brooks/Cole-Thomson Learning. Pacific Grove, CA.

Chopin, K. 1893. Desiree's Baby.
http://www.underthesun.cc/Classics/Chopin/desiree/
http://www.pbs.org/katechopin/library/desireesbaby.html
http://www.4literature.net/Kate_Chopin/Desiree_s_Baby/


REFERENCES

Bean, J.C. 1996. Engaging Ideas: The Professor's Guide to Integrating Writing, Critical Thinking, and Active Learning in the Classroom. San Francisco: Jossey-Bass.


RELATED STANDARDS

From Mid-Continent Research for Learning and Education at http://www.mcrel.org/


Life Sciences Standard 4 Level IV (Grades 9-12):

Knows ways in which genes (segments of DNA molecules) may be altered and combined to create genetic variation within a species (e.g., recombination of genetic material; mutations; errors in copying genetic material during cell division)

Knows that new heritable characteristics can only result from new combinations of existing genes or from mutations of genes in an organism's sex cells; other changes in an organism cannot be passed on

Knows that mutations and new gene combinations may have positive, negative, or no effects on the organism

Understands the concepts of Mendelian genetics (e.g., segregation, independent assortment, dominant and recessive traits, sex-linked traits)

Knows features of human genetics (e.g., most of the cells in a human contain two copies of each of 22 chromosomes; in addition, one pair of chromosomes determines sex [XX or XY]; transmission of genetic information to offspring occurs through egg and sperm cells that contain only one representative from each chromosome pair; dominant and recessive traits explain how variations that are hidden in one generation can be expressed in the next)

Life Sciences Standard 7 Level IV (Grade 9-12)

Understands the concept of natural selection (e.g., when an environment changes, some inherited characteristics become more or less advantageous or neutral, and chance alone can result in characteristics having no survival or reproductive value; this process results in organisms that are well suited for survival in particular environments)

Knows how natural selection and its evolutionary consequences provide a scientific explanation for the diversity and unity of past and present life forms on Earth (e.g., recurring patterns of relationship exist throughout the fossil record; molecular similarities exist among the diverse species of living organisms; the millions of different species living today appear to be related by descent from common ancestors)

<BACK TO TOP

 

   © 2003 California Newsreel. All rights reserved.