9. The inheritance of a given disorder is recorded in three small families shown below:

Based on the above limited information, which one of the following inheritance pattern best explain the observations?
(1) X-linked recessive
(2) X-linked dominant
(3) Autosomal recessive
(4) Autosomal dominant

The pedigree is best explained by an X‑linked dominant pattern of inheritance (option 2).​

Question recap

The pedigree shows three small families where in each family only one child is affected and that affected child is always a daughter, while both parents are unaffected. The options are:​

1. X‑linked recessive

2. X‑linked dominant

3. Autosomal recessive

4. Autosomal dominant​

Why X‑linked dominant fits best

In X‑linked dominant traits, a single mutant allele on one X chromosome is sufficient to cause the disorder, and females are often more frequently affected because they have two X chromosomes. A key feature, especially for rare disorders, is that an affected female can appear in a small family without other affected relatives if there is a new mutation or incomplete penetrance in parents. The consistent involvement of only daughters in all three families supports a dominant X‑linked effect rather than an autosomal one.​

Why X‑linked recessive is unlikely (Option 1)

X‑linked recessive traits classically affect many more males than females because males express the phenotype with just one mutant X, while females need two copies. In the given pedigrees, only females (daughters) are affected and no affected males are seen, which is opposite to the typical expectation for X‑linked recessive inheritance, so this option is ruled out.​

Why autosomal recessive is unlikely (Option 3)

Autosomal recessive disorders typically show affected offspring born to phenotypically normal carrier parents, but they affect males and females with approximately equal frequency. In the question, every affected child is a daughter and no sons are affected across three different families, which is statistically unlikely for an autosomal recessive trait with equal risk for both sexes.​

Why autosomal dominant is unlikely (Option 4)

Autosomal dominant traits usually show vertical transmission, where an affected child has at least one affected parent in most generations, and both sexes are affected equally. Here, all parents are unaffected and only single daughters are affected in each family, so the pattern does not match the typical autosomal dominant expectation and this option is excluded.​