17. 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 of inheritance can explain the pedigree shown below?
(1) A and C (2) B and C
(3) C and D (4) D only
The pedigree can be explained only by autosomal dominant inheritance (D only).
Understanding the pedigree
In the given pedigree, unaffected parents have unaffected children, but whenever an individual is affected, at least one of the parents is also affected, which is the hallmark of a dominant trait. The trait appears in both males and females, and there is clear male‑to‑male transmission (affected father to affected son), which strongly supports autosomal rather than X‑linked inheritance.
Why X‑linked recessive (A) is not possible
For an X‑linked recessive trait, affected males often have carrier mothers who are usually unaffected, and affected females must have an affected father and at least a carrier mother. In this pedigree, there is male‑to‑male transmission of the trait, which cannot occur for an X‑linked recessive allele because sons obtain their X chromosome from their mothers, not their fathers.
Why X‑linked dominant (B) is not possible
X‑linked dominant pedigrees typically show no male‑to‑male transmission, and all daughters of an affected father are affected, while none of his sons are. In the present pedigree, an affected male transmits the trait to a son, violating the basic rule of X‑linked dominant inheritance and therefore excluding this mode.
Why autosomal recessive (C) is not possible
Autosomal recessive traits often appear in the progeny of unaffected parents, with the trait frequently skipping generations. Here, every affected individual has at least one affected parent, and there is no clear example of unaffected parents producing an affected child, so a recessive model is inconsistent with the observed pattern.
Why autosomal dominant (D) fits the pedigree
Autosomal dominant traits usually show vertical transmission, with no generational skipping; each affected person has at least one affected parent. The equal occurrence in males and females and the presence of father‑to‑son transmission match classic autosomal dominant inheritance, so option (4) D only is correct.
Introduction
Pedigree analysis is a high‑yield concept in CSIR NET life science, and many questions test the ability to distinguish X‑linked and autosomal modes of inheritance using small family trees. This CSIR NET June 2012 pedigree problem compares four modes—X‑linked recessive, X‑linked dominant, autosomal recessive and autosomal dominant—and asks which can explain the observed pattern, making it an excellent practice question for exam‑oriented preparation.
