Answer:
= [tex]\frac{1}{4}[/tex]
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 = [tex]\frac{1}{2}[/tex] Â
So if a carrier woman cross with a normal man
Let say the gene of the carrier woman is: [tex]XX^c[/tex]
and the normal man = [tex]XY[/tex]
so if we have a cross between  [tex]XY[/tex] and [tex]XX^c[/tex]; we have:
           [tex]X[/tex]           [tex]Y[/tex]
[tex]X[/tex] Â Â Â Â Â Â Â Â Â [tex]XX[/tex] Â Â Â Â Â Â Â Â [tex]XY[/tex]
[tex]X^c[/tex] Â Â Â Â Â Â Â Â [tex]XX^c[/tex] Â Â Â Â Â Â Â Â [tex]X^cY[/tex]
we can see the offspring to be :
[tex]XX[/tex] =normal unaffected female
[tex]XY[/tex] = normal unaffected male
[tex]XX^c[/tex] = carrier female for color-blind
[tex]X^cY[/tex] = carrier male for color-bind
Then probability that of  inheriting defective allele = [tex]\frac{2}{4}[/tex] = [tex]\frac{1}{2}[/tex]
Let's not forget that the probability that the woman is a carrier of color-blind allele is = [tex]\frac{1}{2}[/tex] Â
∴  the probability that the son is colorblind = [tex]\frac{1}{2}*\frac{1}{2}[/tex]
= [tex]\frac{1}{4}[/tex]
