Q.48 A pedigree of an inheritable disease is shown below.
What type of inheritance does the disease follow?
(A) Autosomal dominant (B) X-linked dominant
(C) X-linked recessive (D) Autosomal recessive
Understanding the Question
The pedigree shows:
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Affected males (filled squares)
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Carrier females (half shaded circles)
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Unaffected males and females
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Only males show the disease
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Females appear as carriers but rarely affected
From this pattern, we must determine the mode of inheritance.
Correct Answer: (C) X-linked recessive
Why the Inheritance is X-linked Recessive
✔ 1. Mostly males are affected
X-linked recessive diseases primarily affect males because:
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Males have one X chromosome
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A single defective allele causes disease
✔ 2. Females are carriers
Half-shaded females indicate they carry one mutant allele, but the other normal allele protects them.
✔ 3. Carrier mothers pass disease to sons
Carrier females transmit the gene to:
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50% of sons → affected
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50% of daughters → carriers
This pattern matches the pedigree perfectly.
✔ 4. No male-to-male transmission
Males pass their Y chromosome to sons (not X),
So affected fathers cannot produce affected sons → Fits X-linked inheritance.
Why Other Options Are Incorrect
(A) Autosomal dominant
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Would show affected individuals in every generation
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Both males and females would be affected
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Carriers would not exist
This pedigree shows carriers → Not autosomal dominant
(B) X-linked dominant
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Affected females would appear frequently
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Affected fathers must pass the disease to all daughters
Not seen here → Females are mostly unaffected
(D) Autosomal recessive
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Affects both sexes equally
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Carrier parents often produce both affected sons and daughters
Here, almost exclusively males are affected → So not autosomal recessive
Conclusion
Pedigree patterns showing:
✔ Affected males
✔ Carrier females
✔ No male-to-male inheritance
are hallmark features of X-linked recessive disorders like hemophilia and red–green color blindness.
Final Correct Answer: (C) X-linked recessive
