Read the given passage and answer the following questions:
Colour blindness is a sex-linked disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome. 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 chromosome.
Which among the following is a sex-linked recessive disease?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Haemophilia
A sex-linked recessive disease is caused by a recessive gene on the X chromosome. It passes from an unaffected carrier mother to some of her sons. Most Mendelian disorders, however, are caused by genes on autosomes (non-sex chromosomes).
Option (1) Sickle cell anaemia — Incorrect. Sickle-cell anaemia is an autosome-linked recessive trait controlled by the alleles HbA and HbS. Only HbSHbS individuals show the disease.
Option (2) Phenylketonuria — Incorrect. Phenylketonuria is an inborn error of metabolism inherited as an autosomal recessive trait. The affected person lacks the enzyme that converts phenylalanine into tyrosine.
Option (3) Haemophilia — Correct. Haemophilia is a sex-linked recessive disease. A protein of the blood-clotting cascade is affected, so a small cut can bleed without stopping. A carrier mother may pass it to her sons; an affected female is extremely rare.
Option (4) Thalassemia — Incorrect. Thalassemia is an autosome-linked recessive blood disease that reduces the synthesis of one of the globin chains of haemoglobin.
NCERT Extract:
Mendelian Disorders – “… 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.”
Mendelian Disorders – “… Sickle-cell anaemia : This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous).”