How Do Genes Determine Skin Color 2o769

How Do Genes Determine Skin Color? Financial analyst Catherine Howarth, who is of Nigerian heritage, believed a rare recessive white gene resulted in her two children being completely white. This case will explore the genetics of skin color and determine whether Catherine is correct about her “recessive white gene.”

Recall that in pea plants, yellow is dominant to green. YY = yellow

Yy = yellow yy = green

1. If skin color were inherited in a simple dominant or recessive pattern, like seed color in pea plants. What would be the genotypes of darkskinned parents that could produce fair-skinned children? Jonah (right) and Sophia both have fair skin and blue eyes.

Many human traits are controlled by multiple genes. This type of pattern is called POLYGENIC INHERITANCE. Examples include: skin color, eye color, and hair color. Many human traits are thought to be controlled by many genes, particularly those that show a range of phenotypes. 2. What other traits are likely to be controlled by multiple genes?

Skin color is controlled by three genes that interact additively to influence the amount of melanin produced by cells. The more melanin that is produced, the darker the pigmentation of the skin. It is also called quantitative inheritance, because each gene “adds” to the amount of melanin. AABBCC = darkest level of pigmentation aabbcc = lightest level of pigmentation. 3. What color skin would a person with AaBbCc genotype have?

If skin color is controlled by 3 genes, these are the possible phenotypes. AABBCC

=

Dark brown

AaBBCC AABbCC AABBCc

Medium browns

AAbbCC AABBcc

Light browns

AABbCc AaBBCc

Dark tans

4. Examine the genotypes shown, there is a pattern that explains why polygenes are also called “ADDITIVE.” What is the pattern?

More phenotypes... AABBCC

Dark brown

AaBBCC AABbCC AABBCc

Medium browns

AAbbCC AABBcc

Light browns

AABbCc AaBBCc AaBbCc aaBbCC AabbCC AABbcc aaBBCc

Dark tans

AAbbcc aaBBcc aabbCC aaBbCc AaBbcc AabbCc

Light tans

Aabbcc aaBbcc aabbCc

Fair

aabbcc Medium tans

Pale (albino)

5. What genotypes would you assign to Jonah and Sophia?

Activity: Modeling Skin Color Inheritance You will need three coins to flip to model the inheritance of skin color. Each coin represents one of the three genes. Heads is the dominant allele, tails is the recessive. For example: Heads - Heads - Tails = ABc Tails - Heads - Tails = aBc

But each individual has two parents, so you need to flip them twice to determine the full genotype of your offspring.

6. Create five offspring by flipping your coins and record them on a table.

Polygenic Inheritance Data Table (Skin Pigmentation)

Child 1 2 3 4 5

Genotype

Phenotype

Here is another way to look at your cross. The coin flipping activity assumes that each parent is a heterozygote. This chart shows the 64 different ways the alleles can combine. 7. What do each of the colored circles on the chart represent?

Now that you have practiced with how skin color is inherited, consider this problem….. If Richard Howarth has the “fair” phenotype: aaBbcc And Katherine has the “light brown” phenotype: AAbbCC

8. What genotypes and phenotypes are possible in their offspring?

Consider this quote from Catherine Howland regarding her light skinned babies. “The only explanation seems to be there must have been a white gene in my family that has remained dormant for years until now. And now it seems to be very dominant.” 9. Do you agree with this quote? Explain your reasoning using your understanding of the genetics of skin color.

Final Thoughts... Skin color is likely controlled than more than three genes and there may be other yet unknown factors that can influence skin color. This case presents a rather simplified version of a very complex inheritance pattern.

10. Summarize key vocabulary. a) Dominant and Recessive b) Polygenic (Quantitative) inheritance c) Melanin d) Genotype and Phenotype e) Heterozygote