Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The possibility of a female becoming a haemophilic is extremely rare.
Reason (R): For a female to be haemophilic, her mother has to be at least a carrier and her father should be haemophilic.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A): The possibility of a female becoming a haemophilic is extremely rare. — True. A is about how rare haemophilia is in females. Haemophilia is a sex-linked recessive disease; its gene lies on the X-chromosome. A female has two X-chromosomes, so she shows the disease only if both carry the gene.
Reason (R): For a female to be haemophilic, her mother has to be at least a carrier and her father should be haemophilic. — True. R is about what parents a haemophilic daughter needs. She gets one X from her mother and one from her father. The father’s only X must carry the gene, so he must be haemophilic. The mother must pass on the gene, so she must be at least a carrier.
Why R explains A? A haemophilic female needs both conditions at once: a carrier (or affected) mother and a haemophilic father. A haemophilic father is uncommon; NCERT adds that he is unviable in the later stage of life. So such a pair of parents is seldom found. This is why a haemophilic female is extremely rare, and R is the correct explanation of A.
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. The heterozygous female (carrier) for 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 carrier and the father should be haemophilic (unviable in the later stage of life).”