8. Epistasis: Scientists have determined hair color and hair production are each

Question

8. Epistasis: Scientists have determined hair color and hair production are each due to a single gene. The genes are on different chromosomes. A dominant allele B causes production of brown color hair. bb individuals have blond hair. A dominant allele H is also required for hair generation. hh individuals are bald. Answer the following questions about his situation:

A. What is an epistatic gene?

B. What is the phenotype of a BbHh individual?

C. What will be the expected phenotypic ratios of the offspring from a BbHh X BbHh (double heterozygous) cross? Be sure to list the phenotypes and their ratios.

Explanation / Answer

A) The phenomenon where expression of one gene is dependant on the presence of one or more genes at a different locus is known as epistasis. The gene that affects the expression of the other gene is termed as epistatic gene. For example, if genes A and B are mutated, and each mutation by itself produces a unique phenotype but the two mutations together show the same phenotype as the gene A mutation, then gene A is epistatic.

B) The phenotype of BbHh individual will be Brown hair. Since, 'B' allele is dominant to 'b', it will indicate hair and gene foe baldness, 'H' is dominant to 'h', which means that the individual will have brown hair.

C) The double heterozygous cross between BbHh x BbHh can be depicted by the following punnet square:

According to the punnet square, the genotypes listed in bold have a dominant 'B' allele and a dominant 'H' allele, therefore they have a phenotype of Brown hair.

The genotypes underlined have recessive 'b' allele only and at least one dominant 'H' allele, therefore they will have a phenotype of blond hair.

The genotypes listed in italics only have recessive 'h' alleles which is epistatic to 'B' or 'b' allele, therefore irrespective of the presence of 'B' or 'b' alllele, these individuals will be bald.

The ratio is given as follows:

Brown : Blond : Bald = 9 : 3 : 4

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