13.

The above pedigree shows the inheritance of a rare allele.  The allele is:
(1) X-linked recessive
(2) Autosomal recessive
(3) Dominant with incomplete penetrance
(4) Autosomal recessive with incomplete penetrance

The pedigree is most consistent with an autosomal recessive inheritance of a rare allele.​​

Understanding the pedigree

In the pedigree, unaffected parents produce affected children in more than one sibship, which is a hallmark of a recessive trait. Both males and females are affected in roughly equal proportion, so the trait is most likely autosomal, not sex‑linked. Therefore, among the given options, autosomal recessive best explains the observed pattern.​​

Option (1) X‑linked recessive

X‑linked recessive traits typically show many more affected males than females, and an affected female must have an affected father (because she must inherit the mutant X from him). In this pedigree, there is an affected female whose father is clearly unaffected, which rules out X‑linked recessive inheritance.​

Option (2) Autosomal recessive (correct)

Autosomal recessive traits often appear in the offspring of two unaffected carrier parents, so affected individuals can appear in a sibship even though both parents are phenotypically normal. This matches the pedigree, where unaffected parents have affected sons and daughters, and the trait may appear to “skip” a generation.​

Option (3) Dominant with incomplete penetrance

For any dominant trait, almost every affected child must have at least one affected parent, even if penetrance is incomplete; incomplete penetrance explains missing phenotypes, not affected offspring from two definitely unaffected parents. In this pedigree, there are affected individuals whose parents are clearly unaffected, which is incompatible with a dominant model, so “dominant with incomplete penetrance” does not fit.​

Option (4) Autosomal recessive with incomplete penetrance

Incomplete penetrance is invoked when some individuals who carry a dominant or recessive genotype expected to be affected show no phenotype. The simple autosomal recessive model already explains all the observed affected and unaffected individuals without needing penetrance assumptions, so adding “incomplete penetrance” is unnecessary and not supported by the data.​

SEO‑optimized introduction

Pedigree analysis is a high‑yield topic in CSIR NET Life Sciences, and questions often test your ability to distinguish autosomal recessive inheritance from X‑linked and dominant traits in families carrying a rare allele. In this article, a CSIR NET pedigree problem on a rare allele is dissected step by step to show why the correct answer is autosomal recessive and how to systematically eliminate X‑linked recessive, dominant with incomplete penetrance, and autosomal recessive with incomplete penetrance options.​