Genetics, populations, evolution and ecosystems (AQA A2 Biology) PART 2 of 4 TOPICS
|
Populations:
Gene pool is all the different genes and alleles found in a population at a given time.
Allele frequency is a proportion of organisms in a population with the same allele.
The Hardy-Weinberg principal provides a mathematical model, which predicts that allele frequencies will not change from generation to generation. The model assumes that a population is large, random mating occurs, no mutations therefore no selection and no migration. The Hardy-Weinberg equation/equilibrium is derived from the monohybrid genetic cross of two heterozygous parents with the genotype pq. Four offspring are made where one will be homozygous pp, one will be homozygous for q and two will be heterozygous pq. This is then turned into the following equation:
p = dominant allele
q = recessive allele
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive
Also
NB: In exam questions the allele frequency may be given as a percentage of a population. It is your responsibility to change these percentages into decimals so that it can be used in the Hardy-Weinberg equilibrium as p and q are decimals. Allele frequencies are usually decimals.
EXAMPLE QUESTION ON HARDY-WEINBERG EQUILIBRIUM: Sea otters were close to extinction at the start of the 20th century. Following a ban on hunting sea otters, their population sizes started to increase. Scientists studied the frequency of alleles in one population of sea otters. The dominant allele, T, codes for an enzyme. The other allele, t, is recessive and does not produce a functional enzyme. In a population of sea otters, the allele frequency for the recessive allele, t, was found to be 0.2.
- Use the Hardy-Weinberg equation to calculate the percentage of homozygous recessive sea otters in this population. Show your working.
NB: In this question the allele frequency for the recessive allele is already a decimal so no converting is needed.
As t is the recessive allele it is represented by the letter q in the Hardy-Weinberg equilibrium so q = 0.2
Therefore q2 = 0.22 so q2 = 0.04
0.04 x 100 = 4% (this is your answer)
- Calculate the percentage of homozygous dominant sea otters in this population. Show your working.
Homozygous allele, T, is represented by the letter p.
To work out allele frequency of p you need to use the equation p + q = 1. q = 0.2
Therefore p = 1 – q so p = 1 – 0.2 = 0.8
Therefore p = 1 – q so p = 1 – 0.2 = 0.8
0.64 x 100 = 64% (this is your answer)
- Calculate the percentage of heterozygous sea otters in this population. Show your working
NB: Yes, the answer will be 32% but let us presume that we do not know the percentages of the homozygous recessive and dominant sea otters. We only the frequency of the homozygous recessive allele.
t is recessive therefore q = 0.2
To work out p you need to use the equation p + q = 1. q = 0.2
Therefore p = 1 – q so p = 1 – 0.2 = 0.8
Therefore p = 1 – q so p = 1 – 0.2 = 0.8
2pq is heterozygous therefore 2 x 0.2 x 0.8 = 0.32
0.32 x 100 = 32% (this is your answer)