Sex linked Inheritance

Colour blindness. A particular traits in human beings renders them unable to differentiate between red colour and green colour. The gene for this red green colour blindness is located on X-chromosome. Colour blindness is recessive to normal vision so that if colour blind man marries a girl who is normal (homozygous) for this character, sons will be normal, but daughters will be heterozygous (normal phenotype), which means that these daughters would be carriers of this trait. If such a carrier girl marries a colour blind man, 50% of female progeny and 50% of male progeny would be colour blind (Fig. 16.9).
Hemophilia. Hemophilia is another popular example of sex linked inheritance in human beings. It is recessive character and is, therefore, masked in heterozygous condition. Individuals suffering with this disease lack a factor responsible for clotting of blood. Consequently, even a minor cut may cause prolonged bleeding leading to death. Since it is a recessive character, a lady may carry the disease and would transmit the disease to 50% of her sons, even if the father is normal (Fig. 16.10).