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alleles will usually be retained in such founding populations as indicated
in Fig. 7.1. So, with just 10-20 founders, it is highly likely that 100% of the
source population allelic diversity will be present. But what about diversity
in terms of heterozygosity?
It is also expected that the level of heterozygosity from trial to trial
should be more variable when low numbers of founders are involved. For
example, when only one founder is drawn from the source population with
only two alleles of equal frequency, the sampled individual will either be
homozygous for one allele, heterozygous, or homozygous for the other
allele. Under Hardy-Weinberg expectations, the expected frequencies of
individuals with these different phenotypes will be:
homozygote for the fi rst allele phenotype frequency = p
2
= 0.25
heterozygote phenotype frequency = 2pq = 2*0.5*0.5 = 0.5
homozygote for the second allele phenotype frequency = q
2
= 0.25
and thus the chances of drawing a heterozygous individual is 50%. One
would expect, then, that as the size of the founding population drawn
from the source population increases, it becomes more likely that the
average observed heterozygosity for that population is 0.5. NEWGARDEN
analyses demonstrate this to be the case. In Fig. 7.3, as more founders are
drawn, the average heterozgosity for founding populations converges
on 0.5. Note, however, that even in founding populations of size 100, the
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10 100
FOUNDERS
Fig. 7.3
Variation in average observed heterozygosity for founding populations of different
sizes (results for the same trials described for Fig. 7.1). In the source population from which
founders are randomly drawn, there is only one locus with two alleles of equal frequency (both
0.5). Each letter represents the average value for one trial (25 replicate runs per trial) with a
particular number of founders. Overlapping values are shown as one letter. As the number of
founders increases, the average heterozygosity more consistently converges on 0.5.
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