Introduction

Pedigree charts are used to determine how genetic disorders are transmitted through families.
The diagram here shows affected and unaffected males and females across multiple generations.
By recognizing sex-linked and autosomal patterns, we can identify the most likely mode of inheritance.

Pedigree Features in the Question

• Only males are affected (black squares)

• No affected females

• Disease skips females despite transmission

• Affected males do not pass the trait to their sons

• Unaffected females produce affected sons

These observations strongly indicate X-linked recessive inheritance.

Why It Must Be X-Linked Recessive

1. Only males are affected → males have only one X chromosome; a single mutated allele causes disease

2. Carrier mothers produce affected sons → classic hallmark

3. No father-to-son transmission → males pass Y to sons, not X

4. Trait appears across generations through carrier females → consistent with skipping pattern

Option-by-Option Explanation

(a) X-linked dominant

❌ Incorrect

• In X-linked dominant disorders, affected males pass the disease to all daughters, but none are affected here.

• Also, affected females should be common, but the pedigree shows none.

(b) X-linked recessive or Y-linked

❌ Incorrect

• Y-linked traits affect all males in every generation (no skipping), because Y is passed father → son

• The pedigree clearly skips generations, ruling out Y-linked

• Only X-linked recessive fits

(c) Only Y-linked

❌ Incorrect

• If Y-linked, every affected father must have all affected sons

• Here, affected males have unaffected sons

• Not possible for Y-linked inheritance

(d) Only X-linked recessive

✔ Correct

• Affected males only

• No affected females

• Carrier females transmitting to sons

• No male-to-male inheritance
  All criteria perfectly match X-linked recessive.

Final Answer

🎯 (d) Only X-linked recessive

Conclusion

This pedigree illustrates the classic pattern of X-linked recessive inheritance, where:

• Females are silent carriers

• Males express disease due to a single defective X

• Fathers cannot pass the disorder to sons

• Carrier mothers pass it to 50% of sons