Question

Let's suppose you have pedigree data from thousands of different families involving a particular genetic disease. How would you decide whether the disease is inherited as a recessive trait as opposed to one that is dominant but shows incomplete penetrance?

Short answer

The distinction between a recessive trait and a dominant one with incomplete penetrance depends on the pattern of the disease's spread across generations. If the disease often skips generations and appears in individuals without a parental case, it's likely recessive. If the disease usually passes directly from parent to child, with only occasional non-affected instances despite the presence of the gene, it's likely a dominant trait with incomplete penetrance.

Step-by-step solution

Understanding Recessive and Dominant Traits

Recessive traits are observed only when two copies of the gene, one from each parent, are present. Dominant traits express themselves even if only one copy of the gene is inherited. Incomplete penetrance occurs when an individual inherits a dominant gene but the trait is not expressed.

Analyzing Pedigree Data

Examine the pedigree data. For a recessive trait, the disease is likely to skip generations as it only exhibits when an individual has two copies of the disease gene. The disease would exhibit in individuals who may not have any ancestors with the disease, because each parent can carry a single recessive copy without manifestation. In a dominant trait with incomplete penetrance, the affected individuals will frequently have an affected parent, since the disease can be expressed even if only one copy of the gene is inherited. The disease does not typically skip generations unless it exhibits incomplete penetrance. In this case, an affected individual may have an affected parent who does not show the disease because the gene is not expressing itself (penetrance is incomplete).

Making Determination

Based on the analysis of the pedigree data, a determination can be made regarding the nature of the disease trait. If the disease strikes even when the parent generation does not show symptoms, it is likely a recessive trait. However, if the disease is present in consecutive generations but isn't ubiquitous (some carrying gene do not show it), it is indicative of a dominant trait with incomplete penetrance.