1. The following pedigree tree represents

(1) Autosomal Dominant
(2) Autosomal Recessive
(3) X-linked Dominant
(4) X-linked Recessive

Introduction

Pedigree analysis questions are common in CSIR NET and other life‑science exams, and the key to scoring lies in quickly recognizing the inheritance pattern from a small family tree. This article breaks down a classic CSIR NET‑style pedigree problem and shows how to distinguish autosomal recessive inheritance from autosomal dominant and X‑linked modes using simple rules.​

Reading the pedigree

• Unshaded symbols are unaffected; shaded symbols are affected.

• In the given tree, both parents in the first generation are unaffected, yet two sons in the second generation are affected, and in the third generation affected children again arise from unaffected parents.

• Both males and females appear in the pedigree, but affected individuals are relatively few and appear when two unaffected parents mate, indicating a recessive trait.​

These features strongly point towards a recessive autosomal condition where carriers are phenotypically normal but can produce affected offspring when mated with another carrier.​

Why Option 2 (Autosomal Recessive) is correct

Key diagnostic points for autosomal recessive traits are:

• Affected individuals often have unaffected parents, who are obligate carriers.

• The trait can skip generations (i.e., an unaffected generation between affected individuals).

• Both sexes are affected in roughly similar proportions when enough individuals are present.​

In the pedigree:

• Unaffected parents (first generation) have affected sons in the second generation, which is classic for autosomal recessive inheritance.

• Another pair of unaffected parents in the second generation produce an affected child in the third generation, again matching autosomal recessive behavior.

• There is no bias towards only males or only females, supporting autosomal, not sex‑linked, inheritance.​

Therefore, the most consistent explanation is autosomal recessive inheritance (Option 2).

Why Option 1 (Autosomal Dominant) is wrong

Autosomal dominant traits show these hallmarks:

• Every affected individual has at least one affected parent; the trait usually appears in every generation (no skipping).

• Unaffected individuals do not pass the trait to offspring.​

In the given pedigree:

• Affected individuals clearly arise from unaffected parents, which cannot happen under a simple autosomal dominant model with complete penetrance.

• There is an apparent generation gap where some parents are unaffected but produce affected children, contradicting autosomal dominant expectations.​

Because these conditions are violated, autosomal dominant inheritance (Option 1) is excluded.

Why Option 3 (X‑linked Dominant) is wrong

Typical X‑linked dominant patterns include:

• An affected father transmits the trait to all daughters and no sons, because daughters receive his X chromosome while sons receive his Y.

• The trait usually appears in every generation, with affected females often more common.​

In this pedigree:

• There is no consistent pattern where an affected male passes the trait to all of his daughters and none of his sons.

• Affected individuals arise from unaffected fathers and mothers, which does not match X‑linked dominant rules.​

Hence, the pedigree does not support an X‑linked dominant mode (Option 3).

Why Option 4 (X‑linked Recessive) is wrong

X‑linked recessive traits have characteristic features:

• Males are affected more frequently than females, because a single mutant allele on the X chromosome is sufficient in males.

• Affected males are usually born to carrier mothers; affected males do not pass the trait to their sons (no male‑to‑male transmission).

• There is often a criss‑cross pattern: affected grandfather → carrier daughter → affected grandson.​

In the problem pedigree:

• Both sexes can be affected from unaffected parents, and the pattern does not clearly show excess affected males over females (given the small sample).

• The arrangement of affected individuals does not strongly fit the classic criss‑cross X‑linked recessive pattern; instead, it fits the simpler expectation of a recessive allele on an autosome present in both parents.​

Therefore, X‑linked recessive inheritance (Option 4) is inconsistent with the pedigree.

Quick comparison table

The only mode that fully explains all observations is autosomal recessive inheritance (Option 2).