Question

A woman with normal vision has a son. The woman’s maternal grandfather is color-blind. What is the probability that the son is colorblind, if his father is normal? Assume there are no other color-blind individuals in the pedigree.A. 1/2B. 1/3C. 1/4D. 1/6E. 1/8

Answer 1

Answer:

= $\frac{1}{4}$

Explanation:

Given that the woman’s maternal grandfather is color-blind, then it is possible that the woman is a carrier of color-blind allele.

Then probability that the woman is a carrier of color-blind allele is = $\frac{1}{2}$  

So if a carrier woman cross with a normal man

Let say the gene of the carrier woman is: $XX^c$

and the normal man = $XY$

so if we have a cross between  $XY$ and $XX^c$; we have:

                     $X$                     $Y$

$X$                   $XX$                 $XY$

$X^c$                 $XX^c$                 $X^cY$

we can see the offspring to be :

$XX$ =normal unaffected female

$XY$ = normal unaffected male

$XX^c$ = carrier female for color-blind

$X^cY$ = carrier male for color-bind

Then probability that of  inheriting defective allele = $\frac{2}{4}$ = $\frac{1}{2}$

Let's not forget that the probability that the woman is a carrier of color-blind allele is = $\frac{1}{2}$  

∴  the probability that the son is colorblind = $\frac{1}{2}*\frac{1}{2}$

= $\frac{1}{4}$