Question

Why can transmission of an autosomal dominant trait with incomplete penetrance look like autosomal recessive inheritance?

Short answer

Transmission of an autosomal dominant trait with incomplete penetrance can appear like autosomal recessive inheritance due to the chance of skipping generations. Incomplete penetrance may result in individuals who carry the dominant gene but do not express the trait, thus creating the impression of autosomal recessive inheritance where traits can skip generations and only appear when two recessive alleles are present.

Step-by-step solution

Understanding Autosomal Dominant Traits

Autosomal dominant traits are those that are expressed or seen even when only one copy of the gene is present in the genotype. So, if even a single parent carries the autosomal dominant allele, their offspring can express the trait. Thus, this pattern of inheritance can be observed from parent to offspring in every generation.

Understanding Incomplete Penetrance

Incomplete penetrance refers to the phenomenon when an individual inherits a dominant allele but does not express the trait. This means that, despite carrying the gene, the person may not show the traits associated with it. As a result, this could break the continuous transmission usually seen in autosomal dominant traits.

Understanding Autosomal Recessive Inheritance

In the case of autosomal recessive inheritance, the trait is typically not expressed unless both alleles are the recessive variant. So, a person has to inherit the recessive gene from both parents to express the trait.

Connecting the Concepts

An autosomal dominant trait with incomplete penetrance can appear to be autosomal recessive because of the potential discontinuity in the trait's expression from generation to generation. Offspring can inherit the dominant gene but due to incomplete penetrance, they not may show the trait, thus giving the appearance of skipping generations which is a characteristic of autosomal recessive inheritance.