nbreeding depression is defined as the phenomenon of
reduced survival & fertility of offsprings of related individuals.
As seen above, proportion of heterozygotes within a population is reduced with each round of inbreeding.
In other words, progeny of inbreeding are more likely to homozygous at a given locus as compared to progeny of non-inbred matings.
Recessive, deleterious alleles are more likely to be expressed in inbred progeny.
This effect is even more evident if population size is small (think about it, smaller the population, higher the chances of non-random mating).
System of mating F
Half-sibling (figure above) 1/8
First Cousins 1/16
2nd Cousins 1/64
FI = (1/2)^N * (1+Fa) where N is the number of individuas in the breeding loop not counting I.
Fa is the inbreeding coefficient in individual A.
Usually assume Fa is zero if no inbreeding there
Note- Both inbreeding and genetic drift are more evident in smaller populations. Therefore, one of the conditions for HW equilibrium is large population size.