Lecture 11 - Inbreeding, Mutation and Migration and Genetic Drift

What does inbreeding do to Hardy-Weinberg?

it causes significant deviation because there is non-random mating

What happened when two cousins have a child?

the child could end up with two alleles that are exactly the same = identical by descent

grandparent allele just happens to be passed down in both halves of the pedigree to both parents of the child and it ends up in the child

What is the inbreeding coefficient F?

the overall probability that the two alleles are inherited by a given individual will be identical by descent

How is the probability of identical by descent calculated (F)?

What are the equations for the genotype frequencies in inbreeding?

f(A1A1) = p² + Fpq

f(A1A2) = 2pq(1-F)

f(A2A2) = q² + Fpq

What happens with the genotype frequencies in inbreeding?

• there are more homozygotes

• there are less heterozygotes

*compared to random mating

What else can be seen in inbreeding populations? (think recessive)

inbreeding depression (reduction in the viability of inbred individuals) → occurs because most deleterious conditions are recessive and require two copes of the mutation to be expressed

• with these populations we know that the number of homozygotes increases so you see an increase in recessive diseases

• morphological and physiological defects are also prevalent

What are the evolutionary forces that change allele frequencies?

• mutation

• migration

• genetic drift

• natural selection

What is the effect of mutations?

• mutations can usually be ignored

• they have more of an effect on genetic variation

• the rate at which they effect populations as a whole is very low because the number of new mutations that come into the population generation after generation is very small compared to other forces

What is the rate of mutation per base pair of DNA per generation in humans?

10^-9 → very low

and so because of this we don’t need to worry too much about it changing allele frequencies from one generation to the next

What is the effect of migration?

• migration can be unidirectional or bi-directional

• this has a higher effect compared to mutation rates because migration rates can be high in some cases → they can enter the population more frequently than an allele entering a population due to mutation

• unidirectional migration can have the same effect as a mutation and can greatly be ignored unless the migration rate is relatively high

• bidirectional migration has more of an effect because it may be trying to tie two potentially isolated populations → lots of migration back and forth between the two (high migration rates) can make it harder to think of them as isolated populations and they may end up being treated as one population

Can migration have a negative effect?

yes, for example with a mouse that has a colour polymorphism

colour type may make a difference depending on the soil type → light coloured mouse on black soil may be in more danger of predation compared to the black coloured mouse

but also the mouse may not know the colour of the soil so theres no way for them to take that into account

What is the effect of genetic drift?

• genetic drift happens in small populations

• there can be cycles of up and down (dying and then resurfacing) due to predation

  • moves allele frequencies up or down with equal probability

• in the first generation the frequency may be around the same but as we move to further generations just by chance alone the allele frequencies can change

  • there is the possibility of completely losing the allele

  • can also cause fluctuations

• when populations become small, chance dominates, alleles are lost and overall genetic variability is lost