GENETICS 133 Understanding Variation in Skin Color Handout new update University of Wisconsin, Madison

UnderstandingVariation 3
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in Human Skin Color
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Activity
Student Handout
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INTRODUCTION
A look around the world showsthat people’s skin comes in many different shades from the lightest pink to
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darkest brown. All of this variation can’t be explained by differences in a single gene. Skin comes in such an array of
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colors because it is a polygenic trait. Polygenic traits are determined by a combined effect of more than one gene
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located at different loci (locations) throughout the genome and each with two or more alleles.
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PROCEDURE
Answer the questions in each of the sections that follow.
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PART 1: Using a model to understand skin color genetics
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Scientists often use models to explain or predict phenomena. In this section, you will explore a simple mathematical
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model to understand the relationship between the number of genes involved in a trait and the number of phenotypes
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generated from different combinations of alleles. The assumptions of the model are listed below:
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• Each gene involved in skin color variation has exactly two alleles. Allele 1 results in pigment; Allele 0 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3




results in no pigment. b3 b3 b3 b3




• Each gene affects skin color to the same degree. b3 b3 b3 b3 b3 b3 b3 b3




• Each allele acts in an additive fashion. Inother words, any one allele is not dominant over the other. b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3 b3




Using this model, if there is one gene affecting skin color (gene A) with two alleles (A1A0), there’d be four possible
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genotypes: A1A1, A1A0, A0A1, and A0A0. According to the model, A1A0 and A0A1 would result in identical phenotypes, so
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there are three possible phenotypes: two pigment alleles (A1A1), one pigment allele (A1A0 and A0A1), and zero
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pigment alleles (A0A0).
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The table on the following page shows all the genotypes possible when there are one, two, or three genes
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contributing to skin color in our model. Complete the table by determining the number of unique phenotypes for
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two genes. Use colored pens or pencils to circle the genotypes that correspond to each unique phenotype. Hint: Add
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up the number of pigment alleles (A1, B1, or C1) present in a genotype. Genotypes with the same number of pigment
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alleles will have the same phenotype.
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The Biology ofSkin Color
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, Activity
Understanding Variation in Human Skin Color b3 b3 b3 b3 b3
Student Handout b3




Table 1: Data from the model in three different scenarios.
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Scenario PossibleGenotypes b3 NumberofUnique 3
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Phenotypes b3




1:Onegene (A)
A1A1 A1A0 A0A1 A0A0 3
3
b 3
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A1A1B1B1 b 3 b 3


A1A1B0B1 b 3 b 3


A0A1B1B1 b 3 b 3


A0A1B0B1

2: Two genes
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A1A1B1B0 b 3 b 3


A1A1B0B0 b 3 b 3


A0A1B1B0 b 3 b 3


A0A1B0B0
(A, B) b3

A1A0B1B1 b 3 b 3


A1A0B0B1 b 3 b 3


A0A0B1B1 b 3 b 3


A0A0B0B1
A1A0B1B0 b 3 b 3


A1A0B0B0 b 3 b 3


A0A0B1B0 b 3 b 3


A0A0B0B0
A1A1B1B1C1C1 A1A1B1B1C0C1 A1A1B0B1C1C1 A1A1B0B1C0C1 7
A0A1B1B1C1C1 A0A1B1B1C0C1 A0A1B0B1C1C1 A0A1B0B1C0C1
A1A1B1B1C1C0 A1A1B1B1C0C0 A1A1B0B1C1C0 A1A1B0B1C0C0
A0A1B1B1C1C0 A0A1B1B1C0C0 A0A1B0B1C1C0 A0A1B0B1C0C0
A1A1B1B0C1C1 A1A1B1B0C0C1 A1A1B0B0C1C1 A1A1B0B0C0C1
A0A1B1B0C1C1 A0A1B1B0C0C1 A0A1B0B0C1C1 A0A1B0B0C0C1
A1A1B1B0C1C0 A1A1B1B0C0C0 A1A1B0B0C1C0 A1A1B0B0C0C0

3: Three genes
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A0A1B1B0C1C0 A0A1B1B0C0C0 A0A1B0B0C1C0 A0A1B0B0C0C0
(A, B, C) b3 b3

A1A0B1B1 C1C1 A1A0B1B1C0C1 A1A0B0B1C1C1 A1A0B0B1C0C1
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A0A0B1B1C1C1 A0A0B1B1C0C1 A0A0B0B1C1C1 A0A0B0B1C0C1
A1A0B1B1C1C0 A1A0B1B1C0C0 A1A0B0B1C1C0 A1A0B0B1C0C0
A0A0B1B1C1C0 A0A0B1B1C0C0 A0A0B0B1C1C0 A0A0B0B1C0C0
A1A0B1B0C1C1 A1A0B1B0C0C1 A1A0B0B0C1C1 A1A0B0B0C0C1
A0A0B1B0C1C1 A0A0B1B0C0C1 A0A0B0B0C1C1 A0A0B0B0C0C1
A1A0B1B0C1C0 A1A0B1B0C0C0 A1A0B0B0C1C0 A1A0B0B0C0C0
A0A0B1B0C1C0 A0A0B1B0C0C0 A0A0B0B0C1C0 A0A0B0B0C0C0


1. Describe the basic relationship between the number of unique skin color phenotypes generated in the
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model and the number of genes responsible.
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The number of unique skin color phenotypes increases when number of genes increases
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2. Look at the number of unique phenotypes for one gene and two genes. The number of unique phenotypes for
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three genes is seven. Develop a mathematical expression to summarize the number of phenotypes (P) that
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can form from N number of genes.
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P=2N+1



The Biology ofSkin Color
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