Question

A normal man marries a woman who is a carrier for colour-blindness. What will be the phenotypes of the child born to them? Depict the phenotypes through a possible cross.

Answer 1

A normal man (XY, with normal vision X^N Y) marries a woman who is a carrier for colour blindness (XX^N X^c, where X^c is the recessive colour-blind allele). Their children will have the following phenotypes:

• Daughters will be either normal (X^N XN) or carriers like their mother (X^N X^c), both having normal vision.
• Sons will be either normal (X^N Y) or colour blind (X^c Y).

Punnett square:

	XN (male)	Y (male)
XN (female)	XN XN (normal daughter)	XN Y (normal son)
Xc (female)	XN Xc (carrier daughter)	Xc Y (colour blind son)

Thus, daughters are normal or carriers with normal vision, and sons can be normal or colour blind. This occurs because colour blindness is an X-linked recessive trait. The male inherits his single X chromosome from the mother, determining whether he is colour blind or not.