colour blind boy can’t receive a colour blind ‘gene’ from his father, even if his father is colour blind, because his father can only pass an X chromosome to his daughters.

A colour blind daughter therefore must have a father who is colour blind and a mother who is a carrier (who has also passed the faulty ‘gene’ to her daughter). If her father is not colour blind, a ‘carrier’ daughter won’t be colour blind. A daughter can become a carrier in one of two ways – she can acquire the ‘gene’ from a carrier mother or from a colour blind father.

This is why red/green colour blindness is far more common in men than women.

Blue colour blindness affects both men and women equally, because it is carried on a non-sex chromosome.

For the sake of the following explanation a normal X chromosome is shown as (X) whilst a colour blind carrying X chromosome is shown in bold (X).

The colour blind ‘gene’ is carried on one of the X chromosomes. Since men have only one X chromosome, if his X chromosome carries the colour blind ‘gene’ (X) he will be colour blind (XY). A woman can have either:-(i) two normal X chromosomes, so that she will not be colour blind or be a carrier (XX),(ii) or, one normal X and one colour blind carryingXchromosome, in which case she will be a carrier (XX), or rarely(iii)she will inherit a colour blindXfrom her father and a colour blindXfrom her mother and be colour blind herself (XX). She will pass on colour blindness to all of her sons if this is the case.

See the tables below to understand how people can become colour blind and how colour blindness is passed on to future generations.

Option A

25% is correct answer