Transcript Population Genetics

Population Genetics
Mendelain populations and the gene pool
Inheritance and maintenance of alleles and genes
within a population of randomly breeding individuals
Study of how often or frequent genes and/or alleles
appear in the population
Genotypic frequencies – how often do certain allelic
combinations appear
Allelic frequencies - how often does an individual allele
appear
Genotypic frequencies
frequency a particular
genotype appears
(combination of alleles)
for moths at right
out of 497 moths collected
BB appears 452 times
Bb appears 43 times
bb appears 2 times
Frequencies
BB 452 ÷ 492 = 0.909
Bb 43 ÷ 492 = 0.087
bb 2 ÷ 492 = 0.004
Total
1.000
BB
Bb
Bb
bb
What about alleles that do show simple dominant recessive relationship?
How does genotypic frequency really demonstrate
flux or change in frequencies of the dominant
allele?
What if there are multiple alleles?
Allelic frequencies
Allelic frequency
BB
Allelic frequency = Number
of copies of a given allele
divided by sum of counts of
all alleles
BB appears 452 times
Bb appears 43 times
bb appears 2 times
492 moths
994 alleles
Frequencies
B (904 + 43) ÷ 994 = 0.953
b (43 + 4) ÷ 994 = 0.047
Total
1.000
Bb
Bb
bb
Can also calculate it from the genotypic frequencies
BB was 0.909
Bb was 0.087
bb was 0.004
Therefore frequency of B = Frequency of BB + ½
frequency of Bb
f(B) = .909 + ½ 0.087 = .909 + .0435 = .9525
F(b) = 0.004 + ½ 0.087 = 0.004 + 0.0435 = 0.047
What about multiple alleles?
Genotype
A1A1
A1A2
A2A2
A1A3
A2A3
A3A3
Total
Number
4
41
84
25
88
32
274
f(A1) = Total number of A1 in population divided by total
number of alleles
Genotype
A1A1
A1A2
A2A2
A1A3
A2A3
A3A3
Total
Number
4
41
84
25
88
32
274
f(A1) = Total number of A1 in population divided by total
number of alleles
Genotype
A1A1
A1A2
A2A2
A1A3
A2A3
A3A3
Total
Number
4
41
84
25
88
32
274
f(A1) = ((2 X 4) + 41 + 25) ÷ (2 X 274)
= (8 +41 + 25) ÷ 548
= 74 ÷ 548
= 0.135
Number of A1
2X4
41
25
Allelic frequencies at X linked locus
same principle
However remember for humans that males only have one X
So that
F(one allele = 2 X the homzygous genotype) + the number of
heterozygotes + the males with the phenotype all divided by the
number of alleles in the population (2 X females) plus males.
Hardy – Weinberg “law”
Frequencies of alleles and genotypes within a
population will remain in a particular balance or
equilibrium that is described by the equation
Consider a monohybrid cross, Aa X Aa
Frequency of A in population will be defined as p
Frequency of a in population will be defined as q
Gametes
A (p)
A (p)
AA(pp)
a (q)
Aa(pq)
a (q)
Aa(pq)
aa(qq)
Frequency of AA offspring is then p2
Frequency of aa offspring is then q2
Frequency of Aa offspring then 2pq
Frequency of an allele being present is = 1
p2 + 2pq + q2 = 1
Where p = frequency of “dominant” allele
and q = frequency of “recessive” allele
For the moth example
(0.9525)2 + (2 X (0.953 X 0.047)) + (0.047)2
0.907 + (2 x 0.045) + .002
.907 + .09 + .002 = .999
Is this good enough?
Can be extended to more than two alleles
Two alleles
(p + q)2 = 1
Three alleles
(p + q + r)2 = 1
And X – linked alleles
Can be used to det4ermine frequencies of one
allele if the presence of one allele is known
Conditions or assumptions for the Hardy –
Weinberg law to be true
Infinitely large population (?)
Randomly mating population (with respect to trait)
No mutation (with respect to locus or trait)
No migration (with respect to locus or trait)
No natural selection (with respect to locus or trait)
Frequencies of alleles do not change over time
Population variation
How is it quantitated?
Proportion of polymorphic loci
Heterozygosity
Population variation in space and time for alleles
Blue mussel
Cline –systematic variation in allele frequency
across geography
Temporal variation
Population variation
Variation at many loci
How is it detected?
PCR
Sequencing
Protein electrophoresis
VNTRs
SNTRs
Synonymous vs. non-synonymous variations or
chnages
How is population variation of loci obtained
Random events
Mutation
Gain and loss of genes from the gene pool
Founder effect
Bottleneck effect
Random genetic drift
Selection
Migration
Mutations may be lost or fixed within a
population
Selection and speciation
Selection coefficient
Heterozygote superiority
Selection against recessive lethal
Fitness
Problems
Text
22.1 - 22.5
Study Guide
pg 502 – 505
1-15
Terms
Mendelian population
Gene pool
Genotypic frequencies
Hardy-Weinberg law
Genetic drift
Random mating
Cline
Random genetic drift