2. The Hardy-Weinberg Equilibrium Flashcards
1
Q
What is population genetics and why is it important?
A
- The study of how Mendel’s Law and other concepts apply to the population level
- Integrates genetics and population biology
- Vital for understanding evolutionary history and forces
- Practical applications in human genetics eg. personalised genomic medicine
- Applications in plant breeding eg. greater yield and resistance to solve food crisis
- Applications in conservation eg. maintain diversity during 6th mass extinction
2
Q
How do dominance and recessivity work in genetics?
A
- Present at phentotypic level, not genotypic
- Continuous traits are refulated by many different genes, not just one set of dominant/recessive alleles, as found in E. Castle’s rat experiments (Barton et al., 2007)
3
Q
How do we approach Mendelian inheritance in a population as opposed to in an individual?
A
- There are many pairs of matings at a population scale
- Mendelian inheritance doesn’t dilute variation, which allows natural selection and other forces to act (Barton et al., 2007)
- Random mating of individuals is treated as a random union of gametes ie. as a gene pool
4
Q
What assumptions do we make about Hardy-Weinberg Equilibrium at population level?
A
- Diploid organism
- Loci are autosomal - things must be tweaked for sex-linked loci (Futuyma, 2013)
- Sexual reproduction
- Generations non-overlapping ie. grow, breed, die, repeat
- Panmictic population
- Population is infinitely large- a finite population would skew the outcomes due to chance (Futuyma, 2013)
- No mutation so no creation/destruction of variance
- No migration
- Natural selection not operating ie. equal fitness across organisms to prevent genotype frrquency (Futuyma, 2013)
- Equal segregation of alleles into gametes to prevent segregation distortion/meiotic drive (Futuyma, 2013)
- Just because assumptions are true, doesn’t mean they are at HWE
5
Q
How do expected frequencies vary for Mendelian crosses between individuals and populations?
A
- In a single cross, the expected frequency of an allele, if there are 2, can be 0, 50 or 100%
- In a population, the freq. can range from 0-100%
- In a population of Mendelian crosses, allele frequencies do not change
- After one generation of tandom mating, HWE will be reached and frequency does not change
- System becomes stable until disturbed
6
Q
What is the Hardy-Weinberg equilibrium?
A
- Created independently by Godfrey Hardy and Wilhelm Weinberg in 1908
- Equilibrium genotype ratios: p2 : 2pq : q2 in F1
- Even in extreme examples eg. random mating in a population of homozygotes, HWE is reached by F1
- p2 + 2pq + q2 = 1
- p - q = 1
7
Q
How do we test for deviation from HWE values?
A
- In nature, expected HWE values often match what is observed
- Chi squared typically shows HWE freqs, eg. for the codominant M and N alleles of the glycophorin A gene in rbcs
- Chi squared value, p = 0.9, so HWE
- This doesn’t mean the model is correct, just consistent, but cannot reject either
8
Q
When is HWE useful in practice?
A
- Most human diseases are recessive
- If genes have a dominant and recessive allele, it is impossible to differentiate between heterozygotes and dominant homozygotes
- Assumption of HWE means recessive homozygote is at frequency q2
- eg. cystic fibrosis - there are 80x more carriers than sufferers
- This was used in NHS tactics for a long time but direct measurement is now possible
9
Q
Why does HWE carry so much importance?
A
- In absence of evolutionary forces, allele and genotype frequencies remain constant ie. evolution doesn’t exist
- Under HWE, a new mutation would remain at low freq. indefinately (Futuyma, 2013)
- Explains why, at most loci, genotype frequencies are usually in line with HWE frequencies
- Can predict frequency of disease carriers
- Example of a basic poopulation genetics model in which symbolic notation demonstrates how something works
- It is a null hypothesis that can be rejected and proves evolution occurs
