Topic 8.2 Gene Pools Flashcards
Gene pool
The sum total of all the alleles in a population at a given time.
Allele frequency
- As the environment changes, the frequency at which different alleles occur changes.
- The number of individuals carrying a certain allele in a population determines the allele frequency.
Hardy-Weinburg equations: Allele frequency
p + q = 1
p= frequency of dominant allele
q= frequency of recessive allele
Hardy-Weinburg equations: Genotype frequency
p^2 + 2pq + q^2 = 1
p^2= proportion on dominant (AA)
2pq= proportion of heterozygous (Aa)
q^2= proportion of homozygous (aa)
Hardy-Weiburg Assumptions
1) Large populations
2) Mating is random
3) No immigration or immigration (isolated population)
4) No mutations occur
5) No natural selection taking place
Mutations
- For the allele frequency to remain stable in a population, no mutations must occur.
- In a single generation, each gene has between 1 in 10^4 and 1 in 10^9 chance of mutation.
- The vast majority of these will be recessive and will never be expressed; but occasionally mutations will arise.
Non-random mating
- Random mating must occur to keep gene pool in equilibrium.
- Non-random mating occurs when some features of the phenotype affects the probability of two organisms mating.
- More attractive male will attract a female and pass on their genes, ensuring their offspring are likely to carry the alleles for the attractive genes.
- In humans, non-random mating is the normal situation.
Populations of varying sizes
- Large populations containing many individuals usually have large gene pools; the chance of losing an allele by random events is reduced in a large population.
- Eg. if a population is smaller and the occurrence of the allele is low, if the individuals carrying the alleles die, the allele is also lost.
- So a bigger population will have more individuals carrying the allele, so it is less likely to lose the allele.
Isolation
- If the Hardy-Weinburg genetic equilibrium is to be maintained, the population must exist in isolation.
- There should be no migration.
- If animals leave their familial groups (etc), migration of genetic material out (or into) of the population takes place.
- As a result of this, gene flow occurs; tending to make the different populations more alike.
Selection pressure
For Hardy-Weinburg equilibrium to apply, all alleles would have the same level of reproductive advantage, or disadvantage.
(This however does not happen).
Population bottlenecks
- When a population’s size becomes very small very quickly.
- Causes a severe decrease in the gene pool of the population and the allele frequency changes dramatically.
- Genetic diversity is greatly reduced.
(Usually due to a catastrophic environmental event, hunting a species to near extinction, or habitat destruction).
The founder effect
The founder effect is the loss of genetic variation that occurs when a small number of individuals leave the population and set up a new population.
Stabilising selection
- Most common type of selection.
- Average survives.
- Eg. Birthweight.
- Eg. Flower height (short flowers die from little sunlight, tall flowers die from wind damage).
Directional selection
One extreme trait is favoured.
- Eg. Giraffe neck length in natural selection (long neck can reach tall trees- survived, short necks died- no food)
Disruptive selection
Opposite extreme traits are favoured.
- Average tart is eliminated.
- Eg. Colour of rabbits (white and black rabbits live- camouflaged, grey rabbit dies- seen by predators).
- Eg. Humming bird beaks.
