What is the inbreeding coefficient for third degree relatives, like first cousins?

The inbreeding coefficient quantifies the probability that two alleles at a locus are identical by descent, based on the degree of relatedness between individuals. For third degree relatives, such as first cousins, the correct inbreeding coefficient is calculated as follows:

First cousins share grandparents, which means they are two generations away from a common ancestor. The calculation involves the formula for calculating the inbreeding coefficient, which is 1/2 to the power of the number of generations separating the two individuals from their most recent common ancestor, multiplied by the number of common ancestors.

In the case of first cousins, the calculation is:

1. The common ancestor (the grandparent) is two generations back.

2. Since first cousins have a common grandparent, the calculation involves squaring the half of the familial relation:

1/2^2 = 1/4.

However, when referencing the inbreeding coefficient specifically for third degree relatives, the correct value observed in genetic studies shows that first cousins indeed have a coefficient of inbreeding of 1/8. This reflects the cumulative relationship from both sides of the family.

The answer provided does not align with the standard definition and calculation commonly used in genetics, where 1/