How is the inbreeding coefficient defined for second degree relatives like aunts or uncles?

The inbreeding coefficient quantifies the likelihood that two alleles at a locus are identical by descent. For second-degree relatives, such as aunts or uncles, this coefficient is calculated based on the degree of genetic relatedness.

Aunt or uncle relationships indicate that there is a one-generation separation from the common ancestor, which is typically a grandparent. Each parent contributes half of their genetic material to their offspring, meaning that a niece or nephew shares approximately 25% (or 1/4) of their genes with an aunt or uncle. However, the inbreeding coefficient specifically represents the probability that two alleles for a given trait are inherited from a common ancestor rather than through unrelated branches of the family tree.

In this case, the inbreeding coefficient for second-degree relatives is 1/8, reflecting the fact that the genetic similarity is calculated not from one side of the relationship (the direct link between parent and child) but rather on the broader pedigree connections, specifically considering both parents' contributions. This coefficient tells us there’s an averaged expectation of inherited alleles being identical due to shared ancestry which is seen in aunts or uncles at the 1/8 ratio.

The correct calculation ultimately leads to