16. Following are four modes of inheritance:
(A) X-Linked recessive
(B) X-Linked dominant
(C) Autosomal recessive
(D) Autosomal dominant
Which of the above modes can explain the inheritance of a common trait shown in the pedigree below?
(1) B and D only
(2) A, C and D only
(3) c only
(4) D only
The pedigree is consistent with autosomal dominant and X‑linked dominant inheritance; autosomal recessive and X‑linked recessive are not compatible.
Correct answer and MCQ option
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The trait can be explained by modes:
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(B) X‑linked dominant
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(D) Autosomal dominant
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Therefore, the correct MCQ option is: (1) B and D only.
Both patterns fit because affected individuals appear in every generation and both sexes are affected, but the small size of the pedigree does not allow a clear distinction between an autosomal and an X‑linked dominant locus.
Why A and C are rejected
Option A – X‑linked recessive
In X‑linked recessive traits, affected males are usually much more common than affected females because a single mutant allele on the X makes a male affected, whereas females must be homozygous.
In the pedigree, an affected female appears, and she is born to an unaffected father, which would require the father to carry the recessive allele on his single X chromosome and therefore be affected, so X‑linked recessive cannot explain this pedigree.
Option C – Autosomal recessive
Autosomal recessive traits characteristically skip generations and are often seen when unaffected carrier parents produce affected offspring.
Here, the trait shows a vertical pattern with at least one affected individual in successive generations and no clear unaffected‑parent → affected‑child combination, so autosomal recessive inheritance is unlikely.
Why B and D fit the pedigree
Option B – X‑linked dominant
In X‑linked dominant inheritance, an affected individual usually has an affected parent, and the trait does not skip generations.
The pedigree shows affected individuals in successive generations and both sexes are affected, which is compatible with an X‑linked dominant trait; with this small sample, the expected father‑to‑all‑daughters pattern or absence of father‑to‑son transmission cannot be conclusively checked, so X‑linked dominant remains possible.
Option D – Autosomal dominant
Autosomal dominant traits also show a vertical pattern: every affected person generally has at least one affected parent, and males and females are affected with similar frequency.
The pedigree shows affected individuals in each generation and no obvious sex bias, so autosomal dominant inheritance explains the pattern equally well, which is why both B and D are included in the correct option.
SEO‑optimised introduction
Pedigree analysis questions in CSIR NET Life Sciences frequently ask students to identify the correct mode of inheritance by comparing X‑linked dominant, autosomal dominant, autosomal recessive and X‑linked recessive patterns.
This article breaks down a representative pedigree problem where the correct answer involves recognising that the pattern fits both X‑linked dominant and autosomal dominant inheritance, while autosomal recessive and X‑linked recessive options are ruled out using classic pedigree rules.
