Question

red-green color - blindness is x - linked in humans. some people are unable to see red or green colors. this condition, colorblindness, is a recessive trait carried by the x chromosome, x^b. color - blindness is primarily in males, but can show up in females as well. it occurs when abnormal red and green photopigments in the eye are inherited from parents. it causes the person to not be able to distinguish between certain colors with red and green in them. the following pedigree shows a family in which some individuals are affected by colorblindness.

family members 6 & 7 in generation ii are expecting a child. the fathers genotype is x^by. the mothers genotype is x^bx^b. use the pedigree and provided punnet square to answer the following question:
which of the following describes the possibility of this son being colorblind?
(a) zero percent, because the father is not colorblind
(b) twenty - five percent, because in the punnett square, only one box out of four shows an affected male
(c) fifty percent, because only one of the two males in the punnett square has the genotype for being affected

Explanation:

Step1: Analyze the Punnett square for male offspring

In a Punnett square for \(X^{B}X^{b}\) (mother) and \(X^{B}Y\) (father), the male genotypes are \(X^{B}Y\) and \(X^{b}Y\).

Step2: Calculate the probability of color - blind male

There are 2 male genotypes in total (\(X^{B}Y\) and \(X^{b}Y\)). The number of color - blind male genotypes (\(X^{b}Y\)) is 1. The probability \(P=\frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}}\). For male children, the total number of male genotypes from the cross is 2, and the number of color - blind male genotypes is 1. So the probability \(P = \frac{1}{2}=50\%\)

Answer:

C. Fifty percent, because only one of the two males in the Punnett square has the genotype for being affected