Chapter 21, Problem 1CS

CASE STUDY |A genetic flip of the coin

On July 11, 2008, twin sons were born to Stephan Gerth from Germany and AddoGerth from Ghana. Stephan is very fair-skinned with blue eyes and straight hair; Addo is dark-skinned, with brown eyes and curly hair. The first born of the twins, Ryan, is fair-skinned, with blue eyes and straight hair; his brother, Leo, has light brown skin, brown eyes, and curly hair. Although the twins' hair texture and eye color were the same as those of one or the other parent, the twins had different skin colors, intermediate to that of their parents. Experts explained that the blending effect of skin color in the twins resulted from quantitative inheritance involving at least three different gene pairs, whereas hair texture and eye color are not quantitatively inherited. Using thisas an example of quantitative genetics, we can ask the following questions:

What approach is used in estimating how many gene pairs are involved in a quantitative trait? Why would this be extremely difficult in the case of skin color in humans?

Summary Introduction

Case summary:

Twin sons were born to Stephan Gerth from Germany, who has fair-skinned with blue eyes and straight hair; and Addo Gerth from Ghana, who has dark-skinned with brown eyes and curly hair. One of their twins have fair skin, blue eyes, and straight hair whereas another boy has light brown skin, brown eyes, and curly hair. Experts explained that blending effect of skin color in the babies is resulted from quantitative inheritance involving at least three different gene pairs. Hair texture and eye color are not quantitatively inherited.

Characters in the case:

Stephen Gerth and Addo Gerth, who have twins and one of the twins have blended skin color of the parents.

Adequate information:

Among the two twin sons of Stephen and Addo, one son is born with light brown colored skin. That is considered as a blended trait.

To determine:

The approach used to estimate number of genes involve in the quantitative trait and whether this is difficult in case of skin color in human beings.

Given information:

The blending effect of skin color in the babies is resulted from quantitative inheritance involving at least three different gene pairs. Hair texture and eye color are not quantitatively inherited.

Explanation of Solution

When many different factors combine to produce a distinctive trait then it is known as quantitative inheritance. Skin color can be considered as an example of polygenic inheritance. This is because many genes collectively influence the phenotypic expression of the trait.

The number of gene pairs can be estimated by determining the number of different phenotypic categories found. The number can be determined by using following equation that is 2n+1, where n is the number of polygenes.

In case of human skin color, it would be extremely difficult because of presence of so many distinct phenotypic characters. It is hard to determine the number of shades found in human beings. There are nearly eight loci are found in human beings for skin colors.

Conclusion

Therefore, it can be concluded that number of gene pairs can be estimated by using equation 2n+1. It is hard to determine the number of shades for skin color in human due to presence distinct phenotypic categories.