GENE222 Mod 7 - Population Genetics

What are the three principles informing Natural Selection?

1. Principle of Variation

Individual differences in phenotype

2. Principle of Heredity

Offspring phenotypes resemble their parents phenotypes

3. Principle of Selection

In a certain environment, some phenotypes are more successful at surviving and reproducing than other

What are the two equations to actually remember from Hardy-Weinberg equations?

p+q = 1 and P² + 2pq + q² = 1, the second just comes from squaring the first to get a test cross, so really just one.

What are the 8 assumptions of the Hardy-Weinberg principle

1. Organism is diploid

2. Reproduction is sexual

3. Generations are separate

4. Mating is at random

5. Population is large

6. Migration is negligible

7. Mutation is negligible

8. Natural selection can be ignored

What are the derivatives of the principles assumptions of the Hardy-weinberg princple?

1. Genetic variation is maintained from generation to generation. (i.e., p and q don’t change.)

2. Genotype frequencies don’t change after the first generation. They reach a Hardy-Weinberg Equilibrium. (p2 + 2pq + q2 = 1)

What are the two most basic models of fitness?

Fitness can be either

(i) constant or frequency-independent, or

(ii) frequency-dependent.

From population genetics data, how would you determine if a phenotype-affecting allele was dominant/recessive?

Whichever homozygote the heterozygote has the same fitness level/reproductive success will indicate which allele is dominant, since the heterozygous and homozygous dominant have the same phenotype in simple genetics.

How does different zygote advantages (via selection pressures) affect the genetic variation of a population?

Homozygous advantage, whether it is recessive or dominant, will decrease genetic diversity as it pushes the population towards losing one allele and fixing the other.
Heterozygote disadvantage is similar, it will push the population towards either extreme, depending on which is closer and the relative fitness of homozygous recessive and dominant.
Heterozygos advantage is the only situation where genetic variation is maintained in a population, it will reach an equilibrium in the middle.

What is the most general rule that inform Pop-gen with single loci?

Mean fitness increases each generation until equilibrium is reached.

What are some other factors that alter the effect of genetic drift? (5)

fluctuating population size

unequal sex ratio

age structure

unequal family sizes

geographical structure

What variables in a population has a simple calculation that allows you to adjust the effective population?

Unequal sex ratio, can be done to find the number of sexually viable parents.

What were the three main schools of thought to explain genetic variation?

The balance school of thought held that most genes were heterozygous, and this balance was held by the population which preserved genetic variation in the community. This may have been a selection pressure for heterozygosity.

The classical school of thought is that DNA is mostly homologous and homozygous; rare mutants were carried mostly by heterozygotes.

Neutral school. Holds that most variation in genes - alleles - is selectively neutral. This is why so much variation is possible in the genome, most of it doesn’t affect fitness. Supplemented/amplified by genetic drift.

Describe the molecular clock idea used in population genetics

Over generations, there is a somewhat steady rate of alleles being replaced with others. This effect is enhanced by selection pressures. This causes a rate of turnover of alleles in a population over generations, called the molecular clock.