15.

Find the pattern of inheritance of the trait showing incomplete penetrance from the figure shown above.
(1) Autosomal dominant.
(2) Autosomal recessive.
(3) Mitochondrial inheritance.
(4) X-linked recessive.

The trait in this pedigree is best explained by autosomal dominant inheritance with incomplete penetrance.​

Reading the pedigree

• Both males and females are affected in roughly equal numbers, so the trait is autosomal, not sex‑linked.​

• Affected individuals appear in every generation, indicating a dominant pattern rather than a recessive one.​

• Some obligate carriers (individuals who must carry the mutant allele because they have affected children) are phenotypically normal, which indicates incomplete penetrance of a dominant allele.​

These points together match autosomal dominant inheritance with incomplete penetrance, corresponding to option (1).​

Why not autosomal recessive?

• In true autosomal recessive pedigrees, affected children can be born to unaffected carrier parents, and affected individuals are usually fewer and may appear to “skip” generations.​

• Here, affected individuals consistently have at least one affected parent, and there is extensive vertical transmission across generations, which is typical for dominant, not recessive traits.​

Therefore option (2), autosomal recessive, does not fit this pedigree.​

Why not mitochondrial inheritance?

• Mitochondrial traits show strict maternal inheritance: all children of an affected mother are usually affected, while children of an affected father are typically unaffected.​

• In the given pedigree, affected fathers clearly have affected sons and unaffected children, contradicting the uniform maternal transmission expected for mitochondrial inheritance.​

Hence option (3), mitochondrial inheritance, is ruled out.​

Why not X‑linked recessive?

• X‑linked recessive traits primarily affect males, and affected males usually inherit the allele from carrier mothers; father‑to‑son transmission does not occur because sons receive the Y chromosome from their fathers.​

• The pedigree shows affected males and females and also male‑to‑male transmission, which is incompatible with X‑linked recessive inheritance.​

Thus option (4), X‑linked recessive, is inconsistent with the pattern seen.​

Key genetic concept: incomplete penetrance

• Penetrance is the proportion of individuals with a particular genotype who actually express the associated phenotype.​

• Incomplete penetrance means that some individuals inherit the disease‑causing variant but remain clinically unaffected, a feature commonly associated with autosomal dominant disorders.​

This explains why some individuals in the pedigree who must carry the dominant allele (because of affected offspring) do not themselves show the trait.​

SEO‑optimized introduction:
Autosomal dominant inheritance with incomplete penetrance is a frequently tested concept in human pedigree analysis, especially in competitive life‑science examinations. Understanding how incomplete penetrance alters the expected vertical pattern of a dominant trait helps students correctly differentiate autosomal dominant pedigrees from autosomal recessive, mitochondrial, and X‑linked recessive inheritance in exam questions.​