Inbreeding loosely understood as
mating of relatives (individuals
with at least one common ancestor)
but in finite populations, all individuals
related, more or less closely
"Inbreeding" in
population genetics more precisely defined as
mating of related individuals at a higher frequency
than random expectation
F (Inbreeding
Coefficient) = prob. of "Identity by Descent":
Probability
that
two alleles are exact genetic copies of allele in
common ancestor
Determined
by consanguinity
(relatedness) of
parents [HOMEWORK]
Inbreeding reduces H_{exp} by proportion
F
(&
increases
proportion
of homozygotes) [see derivation]
f(AB) = 2pq (1F)
f(BB) = q^{2} + Fpq
f(AA)
=
p^{2} + Fpq
Inbreeding
affects genotype proportions,
not allele frequencies
HOMEWORK
Inbreeding increases frequency of individuals
with
deleterious
recessive genetic diseases by factor F/q [see derivation]
Ex.: if f(B) = q = 10^{3}
and F = 0.10 , then F/q = 100
=> 100fold increase in BB births
Inbreeding coefficient of population estimated from experimental data:
F = ( 2pq  H_{obs} ) / 2pq = (H_{exp}  H_{obs}) / H_{exp} [see derivation]
Obs 
Exp 

AA 
0.226 
0.181 
AB 
0.400 
0.489 
BB 
0.374 
0.329 
& q = 0.374 + (1/2)(0.400) = 0.574 ( = 1  0.426 )
Then F = (0.489  0.400) /
(0.489) = 0.182
intermediate
between F_{fullsib}
= 0.250
& F_{1stcousin}
= 0.125
=> Mice
live in small family groups with close inbreeding
[typical
for small mammals]
Inbreeding "advantageous" to population in
longterm, In
combination with Natural Selection,
increases rate of evolution (q 0 more quickly)
deleterious homozygotes
exposed to selection in higher proportion
=>
deleterious alleles eliminated more quickly
increases phenotypic variance (homozygotes more
common)
advantageous alleles reinforced as
homozygotes
karyotypic
rearrangements common in small populations:
source of Peripatric
Speciation