Question

A couple with normal vision bear a colour blind child. Work out a cross to show how it is possible and mention the sex of the affected child.

Answer 1

Colour blindness is an X-linked recessive disease.

Condition: 

Male:  XY - Normal
         X^cY - Colour-blind

Female: XX - Normal
            X^cX - Carrier
            X^cX^c - Colour blind

According to the given situation, both the parents are normal but the child is colour blind. This means that the male parent is normal and the female parent is the carrier. The cross can be represented as follows:
 
Parents        XY      ☓      X^cX
Gametes      X,Y             X^c,X

Offsprings    X^cX    XX     X^cY    XY

Result: 50% sons are colour blind.
Thus, the affected child will be male.