What is the inbreeding coefficient for first degree relatives such as siblings or parents?

The inbreeding coefficient is a measure of the probability that an individual has two alleles at a given locus that are identical by descent, meaning they have inherited the same allele from both parents through their family lineage. For first-degree relatives like siblings or parents, the inbreeding coefficient is calculated based on their genetic relationship.

Siblings share, on average, 50% of their genetic material because they receive half of their genes from each parent. Since each parent contributes one allele at a locus, the probability that two alleles inherited from siblings are identical by descent is 1/4, or 25%. This is also reflected in the inbreeding coefficient, which quantifies the likelihood that an individual has two identical alleles inherited from a common ancestor.

Parents, being the direct contributors of genetic material to their offspring, also have an inbreeding coefficient of 1/4 when considering their relationship to their children. The genetic sharing is symmetric—whether considering two siblings or a parent and child, the mathematical representation of their common genetic overlap resolves to this quarter sharing at the allelic level.

Thus, an inbreeding coefficient of 1/4 corresponds correctly to the genetic relationship seen in first-degree relatives.