3. The following pedigree represents the inheritance of a rare disorder.

Based on the above pedigree, what is the most likely mode of inheritance?
(1) Autosomal Dominant
(2) X-linked recessive
(3) X-linked dominant
(4) Autosomal recessive

The pedigree most likely shows an autosomal recessive mode of inheritance.​

Introduction

In competitive exams, questions on pedigree analysis of rare disorders test understanding of how to distinguish autosomal dominant, autosomal recessive, X‑linked dominant and X‑linked recessive inheritance patterns. Recognizing features such as affected offspring from unaffected parents, equal involvement of both sexes, and skipping of generations is crucial for solving these pedigree problems quickly and accurately.​

Reading the given pedigree

In the given pedigree, an affected male appears in the second generation, while both of his parents are phenotypically normal. The disorder is rare, and there is no strong sex bias because only a single affected male is seen and the sample size is very small.​

Key observations:

• Unaffected parents have an affected child → the trait must be recessive.​

• The parents are unrelated (no consanguinity shown), so they are most reasonably interpreted as heterozygous carriers of a rare mutant allele.​

From these core rules, autosomal recessive inheritance fits best.

Why option (4) Autosomal recessive is correct

For rare autosomal recessive traits, affected individuals are often born to clinically normal parents who are carriers. Generations may be skipped, and the trait tends to appear sporadically, especially when both carrier parents are phenotypically normal.​

In this pedigree, the only affected person is a male whose parents show no disease, matching the classic pattern of a rare autosomal recessive condition. Because the trait is rare, the most economical explanation is that both parents carry one mutant allele, giving a 25% risk of an affected child, which is exactly what is observed.​

Why option (1) Autosomal dominant is unlikely

Autosomal dominant traits usually show vertical transmission, where every affected individual has at least one affected parent and the trait typically appears in every generation. If the trait were autosomal dominant, the affected male in generation II would need to have received the mutant allele from either his father or mother, so at least one parent should be affected.​

Because both parents are unaffected in the pedigree, autosomal dominant inheritance contradicts the core rule that dominant traits do not generally skip generations. Therefore, option (1) is inconsistent with the pedigree and can be excluded.​

Why option (2) X‑linked recessive is unlikely

X‑linked recessive traits characteristically affect males much more frequently than females and show no father‑to‑son transmission; affected males often arise from carrier mothers. Here, a single affected male with unaffected parents could indeed be explained by a carrier mother, but X‑linked recessive inheritance usually leaves characteristic patterns such as clusters of affected males in multiple generations or among maternal male relatives, which are not evident.​

Most importantly, when dealing with a rare disorder and only one affected male from unaffected non‑consanguineous parents, it is usually not possible to distinguish between X‑linked recessive and autosomal recessive purely from that one individual; in exam pedagogy, the default assumption with no supporting sex‑bias is autosomal recessive. Since no additional information (e.g., other affected maternal uncles or male‑only clustering) is present, autosomal recessive is considered the most likely pattern, making option (2) less favored.​

Why option (3) X‑linked dominant is ruled out

X‑linked dominant traits usually show:

• No father‑to‑son transmission.

• All daughters of an affected male are affected, whereas sons are unaffected.​

In the given pedigree, the only affected person is a male whose parents are both unaffected, which cannot be explained by a dominant X‑linked allele, because a male with a dominant mutant allele on his single X chromosome must show the phenotype and thus would transmit it to all daughters. The absence of affected females and the lack of an affected parent rule out X‑linked dominant inheritance, so option (3) is incompatible with the diagram.​

Summary of options in a table

Most likely inheritance pattern in the given pedigree