Population genetics Flashcards
hardy weinberg model
2 alleles in diploid individual are randomly and independently sampled from an infinitely large pool of gametes
p(A)=p
p(a)=q
assumptions of hardy weinberg
large population
random mating
no migration
no mutation
no selection
all so that nothing is changing allele frequencies
no mutation
no new alleles
random mating
no assortative, inbreeding or outbreeding
no migration
allele frequencies not influences by variation in other populations
infinitely large population
no sampling error=constant allele frequencies=mathematically simple model
no selection
alleles dont affect fitness
no alleles selected for
prediction 2
after one generation of random mating, genotype frequencies will align to p^2+2pq+q^2
prediction 1
allele frequencies of a population dont change solely due to random mating
where are rare alleles mainly found
heterozygotes
when is the frequency of heterozygotes greatest
when p=q=0.5
when the frequency of one allele is high, most individuals are…
homozygotes
allele frequencies calculation
a diploid population of N individuals will have 2N alleles
f(A)=(2nAA+nAa) / 2N
genotypic frequencies calculation
f(AA)=number of AA individuals / N
non random mating
can affect genotype frequencies but not allele frequencies unless accompanied by other evolutionary forces
some combinations of alleles will occur at a higher frequency than expected
positive assortative mating
genetically/phenotypically similar individuals tend to mate with each other
negative assortative mating
genetically or phenotypically dissimilar
individuals tend to mate with each other
promotes heterozygosity
inbreeding
individuals tend to mate with relatives
increases homozygosity and reduces heterozygosity
assortative mating
Phenotype-biased mating means frequencies of certain genotypes, in loci underlying the phenotype, will be altered
Affects heterozygosity
This effect is not genome-wide, though it can be multiple loci
measuring inbreeding
considering whether an individual has a pair of alleles that are identical by descent
identical by state alleles
by random chance, individuals can have a pair of alleles that are the same but not because they are from a recent ancestor
inbreeding coefficient, F
probability that two alleles sampled at a locus are identical by descent
inbred has to be homozygote for trait
genotypic frequencies equations for inbreeding
f(AA)=p^2+pqF
f(Aa)=2pq-2pqF
f(aa)=q^2+pqF
where F is the proportional reduction in the frequency of heterozygotes compared to that expected in the hardy weinberg model
inbreeding depression
when inbreeding results in reduced viability and fecundity (fitness)
Must be caused by a general pattern of lower fitness of homozygotes compared to heterozygotes
cause of inbreeding depression
Inbreeding increases homozygosity across the genome. Leads to homozygosity at some loci harbouring deleterious recessive alleles
many generations of inbreeding can allow natural selection to purge such alleles
genetic load
represents the reduction in a population’s average fitness due to the presence of deleterious alleles. Even though deleterious alleles may reduce fitness when homozygous, they persist because they are often “hidden” in heterozygous individuals where selection cannot effectively remove them.
mutation-selection balance
This is the equilibrium between:
Mutation, which introduces new deleterious alleles into the population.
Selection, which removes these alleles when they appear in homozygous individuals.
The balance ensures that deleterious alleles are not completely eliminated but remain at low frequencies in the population.
why do allele frequencies change
genetic drift
natural selection
migration
genetic drift
random changes in allele frequencies
smaller populations more affected
for any allele frequency, the expected amount of ‘error’ is proportional to
1/2N
where 2N is the total number of alleles
bottlenecks
huge reduction in population size
can cause drift events
founder effect
allele frequencies very different from original population
reduces heterozygosity
fitness
the average contribution to the next generation made by an individual or genotype
overdominance
heterozygous genotype has a higher fitness than either of the homozygous genotypes
neither allele is favoured/selected for
selection maintains both alleles in the population
migration
Isolated populations will tend to become more different from each other in allele frequencies over time (differences in local pattern of selection, differences caused by drift)
Migration opposes this process and reduces genetic differences between populations, through exchange
factors increasing genetic variation within populations (summary)
mutation
migration
some types of natural selection
factors increasing genetic variation between populations (summary)
mutation
genetic drift
some types of natural selection
factors decreasing genetic variation within populations (summary)
genetic drift
some types of natural selection
factors decreasing genetic variation between populations (summary)
migration
some types of natural selection
