2. Colour blindness

In human beings a dominant X – linked gene is necessary for the formation of colour sensitive cells, the cones. The recessive form of this gene is incapable of producing colour sensitive cone cells. Homozygous recessive females (X c X c ) and hemizygous recessive males (X c Y) are unable to distinguish red and green colour. The inheritance of colour blindness can be studied in the following two types of marriages.

(i) Marriage between colour blind man and normal visioned woman A marriage between a colour blind man and a normal visioned woman will produce normal visioned male and female individuals in F generation but the females are carriers . The marriage between a F normal visioned carrier woman and a normal visioned male will produce one normal visioned female, one carrier female, one normal visioned male and one colour blind male in F2 generation. Colour blind trait is inherited from the male parent to his grandson through carrier daughter, which is an example of criss-cross pattern of inheritance (Fig. .