Hardy Weinberg Flashcards
the hardy Weinberg law
- Important principle of population genetics
- Is a mathematical representation of the relationship between genotype and allele frequencies in an ideal population
Covers an autosomal locus with two alleles
- Makes several key assumptions about the population
- To deliver 2 key predictions about genotype and allele frequencies
assumptions
- The population is infinitely large
- Random mating occurs within the population
- The population is free from the effects of migration
- There is no natural selection
- No mutations occur
predictions
- When the assumptions are met:
- The genotype and allele frequencies are constant
- In the proportions p2, 2pq, q2 where p is the frequency of allele A and q is the frequency of allele a
planet square
view ppt
how to do hardy Weinberg law
- Given any set of initial genotype frequencies after one generation of random mating the genotype frequencies in the progeny are in the proportions:
- p2, 2pq, q2
- Where
- f(AA) = pxp = p2
f(Aa) = pxq + pxq = 2pq
f(aa) = qxq = q2 - These are the expected frequencies of genotypes under HWE giving the relationship
- p2 + 2pq + q2 = 1
- They will stay in these proportions generation after generation given random mating and the absence of factors that change allelic or genotype frequencies
hardy Weinberg equilibrium
- When a population obeys HW law, it is said to be in Hardy Weinberg Equilibrium (HWE)
- When a population is in HWE, genotypes frequencies can be predicted from allele frequencies
- Important for forensic science
HW law is exactly true when
- all the assumptions are met
- approximately true in some populations where assumptions are not met
linkage equlibrium
- HWE describes a state of independence between alleles at one locus
- Linkage equilibrium describes a state of independence between alleles at different loci
linkage
- Linkage describes when alleles are not passed independently to the next generation
- Linkage violates the law of independent assortment
- Usually happens with alleles that are physically close on the same chromosome
Linkage is not the same as linkage equilibrium/disequilibrium
- Linkage disequilibrium is the non-random association of alleles at different loci
- If there is no linkage disequilibrium between alleles at different loci they are said to be in linkage equilibrium
- Cannot say that if there is no linkage, it follows that there is no linkage disequilibrium
- Linkage disequilibrium can be caused by linkage or by other population genetic effects such as population subdivision
HW recap
- p + q = 1
- p2 + 2pq + q2 = 1
- p = frequency of the dominant allele in the population
q = frequency of the recessive allele in the population - p2 = percentage of homozygous dominant individuals
q2 = percentage of homozygous recessive individuals
2pq = percentage of heterozygous individuals
assumptions of HW
- Large population size
- Random mating
- No external forces acting to change allele frequencies
- i.e. no mutation, migration, or selection!
- H-W is an equilibrium condition- frequencies will not change
- If a population is not in equilibrium, one generation of random mating will return the population to equilibrium.
worked example
- Let us consider a gene with two alleles A1 A2 and how they behave in a population: what are the frequencies of the genotypes?
- p= frequency A1 q= frequency A2
- In a population of individuals 50 % of the alleles are A1 and 50% are A2
- ie the gametes in the population containing these alleles have the following frequencies:
- A1 0.5 A2 0.5
for the rest of the answer look on ppt
practice question - 1) In a population segregating for two alleles A1 and A2 at a gene A with frequencies p and q the Hardy Weinberg principle applies (p2+2pq+q2= [p+q]2 = 1). If q = 0.4 what is the most common genotype and what is its frequency? What is the least frequent genotype and its frequency? Show your workings-out as credit will be given for the approach used.
answer in ppt
