Population Genetics TB Flashcards
equation for frequency of a genotype in a population, for example of AA
f(AA)=number of AA individuals/N where N is the total number of individuals within the population
equation for frequency of an allele
number of copes of the allele/number of copies of all alleles at the locus
equation for frequency of an allele in a population in which there are only 2 alleles for the gene/locus
p=f(A)=2nAA+nAa/2N
q=f(a)=2naa+nAa/2N
where N is the total number of individuals in the sample and n is the number of individuals with those alleles
a diploid population of N individuals will have 2N alleles
equation for frequency of an allele when given genotype frequencies only
p=f(A)=f(AA)+1/2f(Aa)
calculating allele frequencies for x linked loci concepts
females can either be homozygous or heterozygous, males are hemizygous
equation for allele frequencies for x linked loci from number of individuals with the genotypes
p=f(XA)=2nXAXA+nXAXa+nXAY/2nfem+nmale
calculating frequency of x linked alleles from genotypic frequencies
p=f(XA)=f(XAXA)+1/2f(XAXa)+f(XAY)
Hardy weinberg principle
Two alleles in a diploid individual are randomly and independently
sampled from an infinitely large pool of gametes
Probability of sampling the A allele is p
Probability of sampling the a allele is under certain conditions allelic frequencies of a population dont change and the genotypic frequencies stabilise after one generation in the proportions p^2, 2pq and q^2. in these proportions, the population is said to be in hardy weinberg equilibrium
hardy weinberg assumptions
large population
random mating
not affected by mutation
no migration
no natural selection
applies to a single locus
mendel’s principle of segregation
each individual organism possesses two alleles at a locus and each has an equal probability of passing into a gamete. frequencies of alleles in gametes=frequency of alleles in parents
if the frequencies of alleles in a randomly mating population are p and q then the frequencies of the genotypes in the next generation will be….
p^2 2pq and q^2
what does random mating mean
members of the population mate randomly with respect to genotype-each genotype mates relative to its frequency.
implications of hardy weinberg principle
reproduction alone doesnt cause evolution
genotypic frequencies are determined by the allelic frequencies
positive assortative mating
tendency for like individuals to mate
negative assortative mating
tendency for unlike individuals to mate
what type of assortative is inbreeding
positive assortative mating for relatedness
how does inbreeding differ from other types of assortative mating
it affects all genes, not just those that determine the trait for which the mating preference exists
effect of inbreeding on the population
increase in the proportion of homozygotes and a decrease in the proportion of heterozygotes
deviation from the hardy weinberg equilibrium frequencies of p^2, 2pq and q^2
outcrossing
preferential mating between unrelated individuals
what is meant by homozygous alleles being in the same state
the two alleles are like in structure and function but do not have a common origin
what is meant by homozygous alleles being identical by descent
the copies are descended from a single alleles that was present in an ancestor
inbreeding coefficient
F, 0-1
measure of the probability that two alleles are identical by descent
changes in alleles with inbreeding
f(AA)=p^2+Fpq
f(Aa)=2pq-2Fpq
f(aa)=q^2+Fpq
because the proportion of heterozygotes decreases by 2Fpq and half of this value (Fpq) is added to the proportion of each homozygote each generation
self fertilisation effects on allelic proportions
reduces proportion of heterozygotes by 1/2 each generation until all are homozygotes
inbreeding depression
decreased fitness arising from inbreeding due to the increased appearance of lethal/deleterious traits
how can inbreeding be favoured
helps preserve groups of genes (co-adapted gene complexes) that exhibit gene interaction and work well together in a specific environment (whereas outcrossing causes recombination)
can cause homozygotes that are beneficial as lethal alleles removed by natural selection
processes that bring about changes in allele frequencies
mutation
migration
genetic drift
natural selection
genetic drift
random effects due to small population size
equation for change in an allele’s frequency due to forward mutation
change in q=rate of forward mutation x frequency of p
how is an equilibrium of mutation reached
more alleles of p means rate of forward mutation faster. increases q so more reverse mutations until equilibrium reached
equation for change in q frequency due to reverse mutation
mutation rate of reverse mutation x q
equation for overall change in allelic frequencies
change in q = (rate of forward mutation x p) - (rate of reverse mutation x q)
equation for allelic frequency at equilibrium
rate of forward mutation/(rate of forward mutation + rate of backwards mutation)
what allelic equilibrium means
no net change in allele frequency
genotypic frequencies also remain the same
takes a long time to reach equilibrium as mutation rates low
migration: definition and 2 effects
movement of genes from one population to another (gene flow)
1. prevents populations becoming genetically different
2. increases genetic variation
equation for frequency of allele a in merged population once unidirectional migration has occurred
frequency = frequency of a in migrant population + frequency of a in resident population
equation for change in allelic frequency due to migration
amount of migration ( frequency of a in migrant pop - freq a in resident pop)
effect of migration over time
equilibrium reached where even though migration still occurs, the allelic frequencies are equal in the migrant and resident populations
gene pools of the two populations are now the same
keeps populations homogenous in allelic frequencies (counteracts effects of genetic drift and natural selection)
increase in genetic variation as mutations spread to new population
how genetic drift opposes hardy weinberg principle
composition of gametes deviates from gene pool of parents, more significant in smaller gametic samples
non-random mating
sampling error
variance (s^2)
measures genetic drift
pq/2N
genetic drift is maximal when p and q are equal and is greater when N is smaller
effective population size
number of breeding adults
influenced by sex ratio, fluctuations in population size, age structure of population, random mating and variation in individuals in number of offspring produced
causes of genetic drift
sampling error
reduction in population size
founder effect
genetic bottleneck
effects of genetic drift
change in allele frequencies
reduction in genetic variation-increase in homozygous
fixation (genetic drift)
when one allele has a frequency of 1 in a population so all individuals are homozygous for one allele
alleles with an initial higher frequency are more likely to become fixed
different populations diverge genetically from each other over time-genetic drift is random so different populations acquire different changes in frequencies. all populations reach fixation, but with different alleles
natural selection
differential reproduction of genotypes
individuals with an adaptive trait produce more offspring
fitness (W)
relative reproductive success of a genotype
0-1
fitness calculation
mean number of offspring produced by a genotype/mean number of offspring produced by most prolific genotype
selection coefficient (s)
relative intensity of selection against a genotype
1-W
general selection model
overdominance
selection in which the heterozygote has higher fitness than either homozygote
stable equilibrium is reached
underdominance
heterozygote has lower fitness than the homozygotes
unstable equilibrium reached
any disturbance will cause one allele to become fixed
