Question

If a woman with normal vision, whose father was colorblind, married a man whose mother was colorblind, the probability of their first child being a colorblind girl is

• A. 0 percent
• B. 12.5 percent
• C. 25 percent
• D. 50 percent
• E. 100 percent

Answer 1

The correct option is C 25 percent

Colorblindness is X-linked recessive disorder and follows criss-cross inheritance. The woman has a colorblind father who passed affected X chromosome to her, rendering her carrier for the disease. The man also has colorblind mother making him colorblind. The genotype of carrier woman is $X^{c}X$ and that of colorblind man is $X^{c}Y$.
There are 1/4 or 25% chances of their first child to be colorblind. Thus, the correct answer is option C.

$X^{c}X$ ----> $X^{c}Y$	$X^c$	X
$X^c$	$X^c{X}^c$ colorblind daughter	$X^{c}X$ Carrier daughter
Y	$X^{c}Y$ Colorblind son	XY Normal Son