8. population genetics Flashcards
population genetics def
study of the properties of genes in a population, inculidng genetic variation and natural selection
what was the old view of evolution and genetic variation
scientists thought that selection shoul always favour the optimal form and eliminate variation, but this was not the case in an actual population
population def
a localised group of interbreeding individuals
gene pool def
collection of alleles in the population
allele frequency
how common a specific allele is within the population (ie ratio between A:a for a trait)
calculated by number of copies of that allele/total number of all alleles
genotypic frequency
the frequency of a particulat genotype appearing in a population (ie AA, Aa and aa ratio)
calculated by number of times that genotype exits in population / total number of genotypes
what is the sum of all allele frequencies / all genotype frequencies
1 (100%)
what happens to the frequency of an allele across generations
HARDY & WEINBERG PRINCIPLE: shows that frequency of genotypes will stay the same from one generation to the next
(ie the dominant alleles DO NOT replace recessive ones)
!! this occurs as long as certain conditions are met
what are the conditions that are required for the hardy weinberg principle to hold? ()
- pop is large (so sample is representative)
- random mating (no selection of genotype crossing)
- no mutations / mutational equilibrium
- no migration of the population (geographically, so there is no introduction or loss of genotypes)
- no natural selection (no genotype is more likely to survive than another based on random advantage)
equations linked to a population following the hardy weinberg principle
p + q = 1
(p + q) ^2 = 1
p^2 + 2pq + q^2 = 1
*where p and q are the two alleles of the trait being studied
how are the hardy weinberg equations derived
- p+q=1: the two traits need to add to one to make up 100% of the population
- (p+q)^2=1: demostrated using a punnet square (pq x pq), to show that there is 25% chance of pp and qq, and 50% chance of pq for this cross. Hence it follows a binomial representation
Link between allelic and genotypic frequency in the HW equilibrium
GENOTYPIC PROPORTIONS ARE DETERMINED BY ALLELIC FREQUENCIES:
- max % of heterozygotes is 50% (when frequency of p = q = 0,5) - Aa genotype hence is modlled with an inverse parabola with a peak at 0.5
- the frequency of aa decreases non linearly from 1 to 0 as the f(A) increases and f(a) decreases
- the frequency of AA increases non linearly from 0 to 1 as the f(A) increases and f(a) decreases
what does it indicate if the frequency of an allele is very low (ie q)
that the majority of individuals that express that trait are heterozygous
how are allelic and genotype frequencies used for genetic councelling
genetic councelling is done to calculate the prob of a child being affected by a hereditary disease
the prob of this occuring is a function of the genotype frequencies in the population, and is hence equal to the PRODUCT OF THEIR RESPECTIVE FREQUENCIES
the geneticist then uses an equilibrium check - a statistical analysis between the observed genotype FR in a pop and the expected value based on the HWE.
how does the HWE extend to sex linked genes
FOR FEMALES: nothing changes bcos they have 2X chromosomes, sp alleles behave the same as any other locus
!! for an affected female the genotype is q^2
FOR MALES: only have 1X chromosome, hence the frequency of genotypes is:
p+q = 1
!!! for an affected male, the genotype is q
(this is also another explanation for why sex linked affects males at a much higher % than autosomal diseases)
how is the HWE applied in the case of multiple alleles
if p/q/r represents the frequency of the alleles then (p + q + r)^2 = 1
this results in a genotype ratio of
p^2: 2pq: q^2: 2pr: r^2: 2qr
what is the reality in normal populations
that the frequency of alleles DO change slightly, due to the fact that there ARE mutations ad there IS natural selection
hence the HWE is a good baseline against which to measure these slight changes
what does a diff in HWE expected allele frequency vs observed allele frequency indicate
MICROEVOLUTION
-> inbreeding effects
-> genetic drift
-> mutations
-> migration
-> natural selection
assortative mating def
NON RANDOM MATING: when the choice of partner for reproduction isnt by chance in regards to genotype
2 types:
1. POSITIVE: when the genotypes selected are genetically similar
2. NEGATIVE: when the genotypes selected are genetically different
what is genetic drift
the change in frequency of an existing gene variant in the population due to CHANCE ALONE
due to 2 main effects:
1. founder effect: small n of ppl eastablish a new population (hence create a new, small gene pool)
- bottleneck effect: a population experiences a large reduction in number (due to random death for eg by environmental disaster) hence contracting the existing gene pool significantly
eg of founder effect
polydactyly in amish population: around 200 people who are mainly endogamous within the community
eg for bottleneck effect
frequent for ANIMAL populations after intensive hunting or an environmental disaster than wipes out a large number of the individuals
what is fitness and what is it used for
FITNESS: an individual’s genotypic reproductive success.
Relative fitness (W) is 1 for the most successful genotype and less than 1 for other less favoured genotypes, in wuch a way where
W = 1 - s (s is the selection coefficient)
!! SOS: this selection has the potential to act against either a dominant OR recessive trait (which is called unbalancing) OR lead to a heterozygous advantage (eg sickle cell disorder in regards to malaria).
