I am reading the classic Weir and Cockerham 1984 paper about $F_{ST}$ estimation. At the beginning (first page, right column), they define 3 statistics.
$F$ is the correlation of genes within individuals ("inbreeding")
$\theta$ is the correlation of genes of different individuals in the same population ("coancestry")
$f$ is the correlation of genes within individuals within populations.
They also state that the 3 statistics are related by
$$f = (F-\theta)(1-\theta)$$
I don't quite understand those 3 statistics and especially I don't understand why this relationship holds true. Can you help me with that?