Question

In a pedigree chart, if two unaffected individuals have a child with the trait, what is the most likely mode of inheritance for this trait ?

• A. \( \text{Autosomal dominant} \)
• B. \( \text{Autosomal recessive} \)
• C. \( \text{X-linked dominant} \)
• D. \( \text{X-linked recessive} \)

Hint:

When analyzing pedigrees, remember that if unaffected parents have affected offspring, the trait is usually recessive. If the trait appears in both sexes equally, it is likely autosomal.

Answer 1

The Correct Option is B

If two unaffected individuals have an affected child, it means that the parents must be heterozygous carriers of the recessive allele for the trait. Let's denote the dominant allele as 'A' and the recessive allele as 'a'. The unaffected parents would have the genotype 'Aa'. When two heterozygous parents (Aa x Aa) reproduce, there is a 25\% chance of having an offspring with the homozygous recessive genotype 'aa', which would express the trait. This pattern is characteristic of autosomal recessive inheritance, where the affected trait appears only when the individual has two copies of the recessive allele. In autosomal dominant inheritance, if a parent is affected (carrying at least one dominant allele), there is a high probability that their offspring will also be affected. The scenario described contradicts this. In X-linked recessive inheritance, the pattern of inheritance is different for males and females due to the difference in their sex chromosomes. If the trait were X-linked recessive, an unaffected mother (carrier) and an unaffected father would have a higher chance of having affected sons. While it's possible to have an affected child, autosomal recessive is the most likely mode in a general case of two unaffected parents having an affected child. Similarly, X-linked dominant inheritance would typically show an affected father passing the trait to all his daughters. The given scenario does not align with this pattern.