Download Population genetics

Population genetics
It analyzes the genetic composition
of groups formed by members of
the same species, and how this
population changes in time and
space.
Population genetics is part of
evolutionary studies.
The population is the basis of evolutionary
changes.
The genotype of an individual member is fixed at
birth.
The population is the smallest unit in which the
evolutionary change takes place, allowing the
origin of new alleles and their change in
frequencies.
Evolution takes place not on individuals but in the
population and in the species.
Parameters.
Allelic frequency: the proportion of a specific allele
in a given locus, considering that the population can
have one or multiple alleles in that locus.
Genotypic frequency: the proportion of a specific
genotype in a given locus, considering that multiple
genotypes are possible.
Phenotypic frequency: the proportion of individuals
with a specific phenotype in the population.
Mendelian population
Group of individuals with sexual reproduction, mating
among them, characterized by a common gene pool
Genotypic frequency: n. individuals with a specific
genotype/total number of individuals
∑ freq. genotype locus = 1
F (AA) = n. individuals AA/N
F (Aa) = n. individuals Aa/N
F (aa) =n. individuals aa/N
Phenotypic frequency: n. individuals with a specific
phenotype / total number of individuals
Allelic frequency: n alleles in population/ total of alleles in
that population
In case 2 alleles are present:
p = F(A) = (2x homozygotes AA) + (n.heterozygotes Aa) / (2x
tot.individuals)
Or
p=F(A)= (F AA) + (1/2 F Aa)
q=F(a)= (F aa)+ (1/2 F Aa)
For multiple alleles A1, A2, A3
p = F(A1) = (2x A1A1) + (A1A2) + (A1A3) / (2x tot. indiv.)
q = F(A2) = (2x A2A2) + (A1A2) + (A2A3) / (2x tot. indiv.)
r = F(A3) = (2x A3A3) + (A1A3) + (A2A3) / (2x tot.indiv.)
Principle of Hardy-Weinberg
Apply mendelian principles of segregation to calculate allelic and genotypic
frequencies in a given population
Genotypic frequencies are calculated with the following proportions:
f(AA) = p2
f(Aa) = 2pq
f(aa) = q2
p= allelic frequency of A
q= allelic frequency of a
The sum of allelic frequencies = 1(p+q) = 1
CONDITIONS:
large population
random crosses,
no mutations, migrations and selection
no change in time
EQUILIBRIUM OF HARDY-WEINBERG
A
A
a
a
A
A
a
A
A
Gametes
A=14/20=0.7= f(A)
a=6/20=0.30 f(a)
A
A A
A
A
a
A a
A
f (AA)= P gamete A+gamete A=
0.7x0.7=0.49
A
a
f(Aa)= P gamete A+gamete a or gamete
a+gamete A = 2x f(A)x f(a)
f (aa)= P gamete a+gamete a=
0.3x0.3=0.09
AA=49%
Aa=42%
aa=9%
At equilibrium genotypic frequencies depend
upon allelic frequencies.
Are population always at equilibrium??
Mating NOT random
Inbreeding
Mutations
Cahnges allelic frequencies
Ex: Drosophyla red eye. Una sola alternativa p=1
If mutation induces A
a then p=1
a
A
A
A
A
A
A
Due elementi influenzano il n.di alleli in popolazione:
Freq. Alleliche p e q
u
Freq. di mutazione
A
a
v
Pierce, GENETICA, Zanichelli editore S.p.A. Copyright © 2005
Selection
External forces favor 1 trait over another
FITNESS= reproductive success of a certain genotype
FITNESS= 1- selection coefficient
AA
Aa
aa
1
1
1
no selection
1
1
1-s
select against recessive
1-s
1-s
1
select against dominant
1
1-s
1
select against heterozygote
1-s
1
1-t
select in favor heterozygote
Pierce, GENETICA, Zanichelli editore S.p.A. Copyright © 2005
Migration
Genic pool opened by arrival of new individuals
Parameters:
1. N. di migrants / n.individuals in a population
2. Difference in frequencies between residente and migrant popolation.
Genetic drift
i. Founder effect
ii. Bottleneck effect
iii. Small population
Pierce, GENETICA, Zanichelli editore S.p.A. Copyright © 2005