4. Given below is a pedigree indicating a pattern of inheritance:

Which one of the following options correctly describes the pattern of inheritance shown in the above pedigree?
(1) X-linked recessive
(2) Autosomal recessive
(3) X-linked dominant
(4) Autosomal dominant

The pedigree shows an X‑linked dominant pattern of inheritance. The correct option is (3) X‑linked dominant.

Understanding the pedigree

In the pedigree, an affected female (filled circle) and an unaffected male (empty square) have two children, one affected daughter and one affected son. Because an affected mother with an unaffected father can have both affected sons and affected daughters, this strongly suggests an X‑linked dominant trait, where a single mutant allele on the X chromosome is sufficient to cause the phenotype in both sexes.​​

Why option (3) X‑linked dominant is correct

• In X‑linked dominant inheritance, both males and females are affected, often with a slight excess of females, and the trait usually does not skip generations.​

• An affected heterozygous mother (X*X) crossed with a normal father (XY) can produce:

  • 50% affected daughters (X*X) and 50% normal daughters (XX)

  • 50% affected sons (X*Y) and 50% normal sons (XY).​
    This matches the structure of the given pedigree: an affected mother with an unaffected father giving rise to affected offspring of both sexes.

Why option (1) X‑linked recessive is wrong

• In classic X‑linked recessive inheritance, males are affected much more often than females, and affected females almost always have an affected father and a carrier or affected mother.​

• For an affected daughter to appear in an X‑linked recessive pedigree, she must inherit the mutant allele from both parents, so her father must be affected; however, in the given pedigree the father is unaffected while the daughter is affected, which excludes X‑linked recessive inheritance.​

Why option (2) Autosomal recessive is wrong

• Autosomal recessive traits often skip generations, and affected individuals typically arise from unaffected carrier parents; both sexes are affected equally.​

• In the pedigree shown, there is no evidence that unaffected parents are producing affected children; instead, the trait is clearly associated with an affected parent, which is inconsistent with autosomal recessive inheritance.​

Why option (4) Autosomal dominant is wrong

• Autosomal dominant traits usually show vertical transmission, where every affected individual has at least one affected parent, and males and females are affected equally.​

• However, autosomal dominant traits do not show a sex‑chromosome–linked pattern; in particular, they lack the characteristic daughter/son patterns seen in X‑linked pedigrees. In the present pedigree, the involvement of an affected female parent producing both affected son and daughter fits better with X‑linked dominant than with a purely autosomal dominant trait.​

Short SEO‑optimized introduction

Pedigree analysis is a high‑yield topic in CSIR NET and other life‑science exams, and a common task is to distinguish between autosomal and X‑linked, dominant and recessive inheritance patterns using minimal information. In this question, the pedigree reveals an X‑linked dominant mode of inheritance, and careful application of standard rules allows systematic elimination of X‑linked recessive, autosomal recessive, and autosomal dominant options.​