Question

Which of the following scenarios will be observed if a colour blind man is married to a normal woman?
[1 mark]

• A. All sons will be colour blind
• B. All daughters will be colour blind
• C. All sons and daughters will be normal
• D. All sons and daughters will be colour blind

Answer 1

The correct option is C All sons and daughters will be normal

Colour blindness is an X-linked recessive disorder. One copy of the affected gene in males in each cell is sufficient to cause the disorder (X${}^c$Y). Females with two copies of the affected gene show the disorder
(X${}^c$X${}^c$). Females heterozygous (X${}^c$X) for this trait be normal but serve as a carrier of the disease. According to the question, the female is normal (XX) and the father is colour blind (X${}^c$Y). The affected father will inherit the affected allele (X${}^c$) to all daughters who will have normal vision but serve as carriers. The cross is given below:

	X	X
$X^c$	$X^c$X (Carrier girl)	$X^c$X (Carrier girl)
Y	XY (normal boy)	XY (normal boy)

Hence, all sons and daughters will be normal but the daughters will be carriers.