IBD-value and IBD-fraction

IBD-value and IBD-fraction

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I have some questions about the IBD-value and IBD-fraction for two of the condensed identity states, $S_1$ and $S_7$.

My questions:

1) Is the IBD-value for the individuals i and j two (2) in $S_1$ and 2 in $S_7$ as well? (We can form the pairs aa aa in $S_1$ and the pairs aa and AA in $S_7$. The number of alleles in $S_1$ (and similarly for $S_7$) for individual i that are IBD with alleles of individual j is therefore 2)

2) Is the IBD-fraction ( or coefficient of relationship) 1 in $S_1$ and 1 in $S_7$ ? (We have the fraction 2/2 in each case since both alleles of each individual are IBD with the alleles of the other individual as described in 1) )

Picture source:">IBD-value in pedigree with inbreeding

Coefficient of relationship: "… is the fraction of genes in two individuals that are identical by descent,… "


If I've understood the graphs correctly, then I think you are right on both questions. The difference between 1 and 7 is just that the alleles are in LD in 1 but not in 7