Genetics Ch 14

1
Genetics - Ch 14

• Population Genetics
• and Evolution

2
Population Genetics
studies the pattern of genetic variation in
natural populations

• Gene pool all alleles in population

• Genotypic frequency proportion of indiv. in a
  pop. with a specific genotype

P observed indiv. with AA / total indiv. in
pop. H Aa / total Q aa / total
P H Q 1
3
Genotypic Frequency Example
pop. of 400 320 have AA, 72 have Aa, 8 have aa
P H Q
320/400 0.8
72/400 0.18
8/400 0.02
P H Q 0.8 0.18 0.02 1
(p)
freq. A 712/800 0.89 (89)
A a
320 320 72 712
8 8 72 88
freq a 88/800 0.11 (11)
(q)
total alleles 712 88 800
4
Allele Frequency
proportion (frequency) of an allele in a
population
freq. AA P freq. Aa H freq. aa
Q freq. A allele P H/2 p
freq. a allele Q H/2 q
p q 1
p 0.8 0.18/2 0.89
q 0.02 0.18/2 0.11
p q 0.89 0.11 1 gene is polymorphic
5
Hardy-Weinberg Principle
estimates genotypic freq. for a gene with 2
different alleles in ideal conditions
p2 expected freq. of AA (P obs. freq.
AA)
2pq exp. freq. Aa (H obs. freq. Aa)
q2 expected freq. of aa (Q obs.
freq. aa)
p2 2pq q2 1
6
Hardy-Weinberg Example
P obs. freq. AA
p2 exp. freq. AA
H obs. freq. Aa
2pq exp. freq. Aa
Q obs. freq. aa
q2 exp. freq. aa
1 in 40,000 indivs. in a pop. are born with
albinism (aa) estimate the of carriers (Aa) of
a recessive disease in pop.
(solve for estimated H)
Q 1/40,000 0.000025
then q2 0.000025
0.005
p 1 - 0.005 0.995
2 (0.995) ( 0.005) 0.00995 0.001
estim. freq. Aa (H) 2pq
0.001 X 40,000 40 individuals estimated to be
carriers (Aa)