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Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation4 Marks
Question
Explain any two X-linked recessive disorder?
✓

Answer

X-Linked recessive disorder:
→ (i) colourblindness, (ii) Haemophilia.
→ (i) Colour Blindness:
→ It is a sex-linked recessive disorder due to defect in either red or green cone of eye resulting in failure do discriminate between red and green colour.
→ This defect is due to mutation in certain genes present in the X-chromosomes.
→ It occurs in about 8 per cent of males and only about 0.4 per cent of females.
→ This is because the genes that lead to red-green colour blindness are on the X-chromosomes. Males have only one X-chromosomes and females have two.
→ The son of a woman who carries the gene has a 50 per cent chance of being colour blind.
→ The mother is not herself colour blind because the gene is recessive. That means that its effect is suppressed by her matching dominant normal
gene.
→ A daughter will not normally be colour blind, unless her mother is a carrier and her father is colour blind.
→ (ii) Haemophilia:
→ This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied.
→ In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected.
→ Due to this, in an affected individual a simple cut will result in non-stop bleeding.
→ The heterozygous female (carrier) haemophilia may transmit the disease to sons.
→ The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least a carrier and the father should be haemophilic (unviable in the later stage of life.)
→ The family pedigree of Queen Victoria shows a number of haemophilic descendents as she was a carrier of the disease.

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