Question

Work out the following crosses and mention the phenotypic ratio of their
progeny.
(a) A colour blind man marries a carrier woman
(b) A man with normal colour vision marries a carrier woman..

Answer 1

Answer:

Explanation:

Colour blindness is a usually a genetic (hereditary) condition (you are born with it). Red/green and blue colour blindness is usually passed down from your parents. The gene which is responsible for the condition is carried on the X chromosome and this is the reason why many more men are affected than women.

Answer 2

Colour blindness is X- linked recessive.  

So,

A colour blind female has two X chromosomes- both of which are recessive- XrXr

A colour blind male has one recessive gene and one normal Y chromosome- XrYn.

So any progeny of the above couple will be colour blind since colour blindness is decided by the X chromosome.  

Combinations there of:

XrXr- female colour-blind

XrYn- male colour-blind

XrYn- male colour-blind

XrXr- female colour-blind

This mode of inheritance makes sure that females rarely are ever colour blind.