Variation and selection in populations

What is Population genetics?

Extension of Mendel's basic principals. Tool to learn about biological function, mechanisms, and human history

Which entity is studied in population genetics

Humans

Population

Group of interbreeding individuals of specific species at the same time and place

Gene pool

total number of alleles carried by members of population

Sample

number of individuals to make inferences about the entire population

Phenotype frequency

proportion of individuals in population that have particular phenotype

Genotype frequency

proportion of individuals in population that have particular genotype

Allele frequency

Proportion of gene copies in population of given allele type

How to calculate allele frequencies

A/total = % A

Hardy-Weinberg equilibrium (HWE): It correlates allele and genotype

frequencies in a given population

Allele frequencies should be same in adults as in gametes, allele frequencies in gametes can calculate expected genotype freq in zygotes of next gen

Assumptions of HWE

5 assumptions: population has infinite individuals, individuals are random, no new mutations appear, no migration in or out of population, random mating, genotypes have no effect of ability to survive and mate

 HWE binomial equation and simple calculations using this equation

P²+2pq+q²=1

 What do p, q, 2pq, p2 and q2 represent in HWE binomial

P A dominant allele freq, q recessive allele, p2 dominant trait, 2pq hetero, q2 recessive

Genotype frequencies change but allele frequencies stay constant across

Generations.

In population not at HWE one generation of random mating can reshuffle alleles into eq.

CODIS

combined DNA indexing system. Loci are simple sequence repeat loci SSRs, 13 loci are unlocked and highly variable, HWE can be used to calculate likelihood of match

Product rule

 P(match at locus 1 and locus 2)

Do natural populations always follow HWE

No, mutations can occur, migration, and more. More useful short term than longterm

 No population is infinitely large

o Migrations of small groups of individuals does occur

o Mating is not random (in real life)

o There are genotype-specific differences in fitness

Bottleneck

disease or natural calamity occurs and survivors make up new population

Genetic drift

Variation in frequency of diff genotypes in small population with chance of gene disappearing if individual dies or does not reproduce or migrate or other external cause

Founders effect

Few individuals separate from larger population and establish new population

Fixation

When only one allele in pop has survived and all individuals are homozygous for that allele. Short term allele frequencies changes are small, many generations may be large changes.

Fitness

individuals relative ability to survive and transmit genes to next gen

Natural selection

Process that progressively eliminates individuals with lower fitness

Does natural selection work in isolation?

No. Environment plays a role

Natural selection may lead to changes in allele frequencies

Alleles with better fitness survive

What is heterozygote advantage?

Individuals that are carriers have resistance

Heterozygotes are silenced carriers of recessive allele.

Balancing selection

maintains genetic polymorphisms and contribute to heterozygote advantage

Genetic ancestry is the inheritance of segments of DNA from biological ancestors

Y chromosome inherited paternally mitochondria inherited maternally

How many segments are possible at

autosomes, Y chr and mitochondrial genome

there are only two genetic

ancestors at each locus- one allele from each parent