If a colour blind man marries a daughter of a colour blind man and normal woman. Given the child is a daughter, what is the probability that she is colour blind?

The correct option is B 0.5
Cross - 1:
Colourblind man - $X^{c}Y$
Normal woman - XX
$X^{c}Y$ x X X

	X	X
$X^c$	$X^{c}X$	$X^{c}X$
Y	XY	XY

All the daughters would be carriers for the disease but none of them would be affected since colour blindness is an X-linked recessive disorder.

Cross - 2:
Daughter from cross - 1: $X^{c}X$
Colour blind man : $X^{c}Y$
$X^{c}Y$ x $X^{c}X$

	$X^c$	X
$X^c$	$X^c{X}^c$	$X^{c}X$
Y	$X^{c}Y$	XY

1 affected daughter : 1 carrier daughter : 1 affected son : 1 normal son
Hence, probability of an affected daughter is one out of two or 1/2 or 0.50.