Q.In the given pedigree chart, a cross between a normal couple resulted in a son who was haemophilic and a normal daughter. In course of time, when the daughter was married to a normal man, to their surprise the grandson was also haemophilic. [Pedigree chart] Choose the option that indicates the correct inheritance of trait in the above pedigree chart : (A) Autosome linked dominant trait (B) Sex-linked dominant trait (C) Autosomal recessive trait (D) Sex-linked recessive trait
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Start your 14-day free trial to unlock the full solution →The pedigree shows haemophilia skipping a generation and passing from a carrier mother to her son — the classic pattern of an X-linked recessive trait. The correct option is (D).
The concept: why sex-linked recessive fits
Haemophilia is a well-known X-linked recessive disorder. The key signature of such a trait is that it appears almost exclusively in males, and it is transmitted from a carrier mother (who is unaffected) to her sons. A father never passes his X chromosome to his sons — he gives them a Y — so an affected male always inherits the defective X from his mother. In a pedigree, this creates a pattern where the disease seems to skip a generation: an unaffected woman can carry the allele and pass it to her son, who then expresses it.
Let’s map the given family onto this logic.
Step-by-step reasoning
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The first couple: normal parents, one affected son
A normal man (XY with normal X) and a normal woman have a haemophilic son. Since the son is affected, he must have received an X chromosome carrying the haemophilia allele. His father gave him a Y, so the defective X came from his mother. The mother is phenotypically normal, so she must be a carrier — heterozygous for the recessive allele. This immediately rules out any dominant inheritance (if it were dominant, she would be affected) and any autosomal recessive pattern (where both parents would need to be carriers, but the father would then be normal — possible, but we’ll see why X-linked fits better).
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The daughter: normal phenotype, but a carrier
The couple also has a normal daughter. She received one X from her father (normal) and one X from her mother. Since her mother is a carrier, the daughter has a 50% chance of being a carrier herself. The pedigree tells us she later marries a normal man and has a haemophilic son — so she must indeed be a carrier. This is exactly what we expect for an X-linked recessive: the daughter is unaffected because she has one normal X, but she can pass the defective X to her children.
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The grandson: affected son of a carrier mother and normal father
The daughter (carrier) marries a normal man. Their son inherits his mother’s defective X and his father’s Y — and therefore has haemophilia. The father is normal, so he could not have contributed the disease allele. This pattern — affected son born to unaffected parents, where the mother is the carrier — is the hallmark of X-linked recessive inheritance.
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Why the other options fail
- (A) Autosome linked dominant: If it were autosomal dominant, at least one parent of every affected child would be affected. Here, both parents of the affected son are normal — impossible. …
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