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Colour
blindness and Haemophilia (Bleeder’s disease) are two common example of sex
linked diseases in human beings.
Ø COLOUR BLINDNESS: This is a human
disease which causes the loss of ability to differentiate between red and green
colour. The gene for this red-green colour is present in X-chromosome. Colour
blindness is recessive to normal vision. If a colour blind man (XCY)
marries a girl with normal vision (XX), the daughters would have normal vision
but would be carrier, while sons would also be normal. The CROSS is shown
below:
If the carrier girl (heterozygous for blindness, XC X) now
marries a colour blind man (XCY), the offspring would show 50%
females and 50% males. Of the 50% females, 50% would be carrier for colour
blindness and rest 50% would be colour blind. Of the males, 50% would have
normal vision and rest 50% would be colour blind. The CROSS is shown below:
Ø HAEMOPHILIA-A (Bleeder’s disease): The
person suffering from this disease cannot synthesize a normal blood protein
called antihemophilic globulin (AHG) required for normal blood clotting.
Therefore, even a very small cut may lead to continuous bleeding for a long
time. This gene is located on X-chromosome and is recessive. IT REMAINS LATENT
IN CARRIER FEMALES. The CROSS is shown below:
If a normal man marries a girl who is carrier for haemophilia, the
progeny would consists of 50% females and 50% males. Of the females, 50% would
be normal and rest 50% would be haemophilia carrier. Of the males, 50% would be
normal and the rest would be haemophilics.
Ø HAEMOPHILIA-B (Christmas disease): A
person suffering from Haemophilia-B lacks plasma thromboplastin. It is
inherited just like Haemophilia-A.
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