These revision notes provide an in depth summary of this specific chapter of AQA Biology in the A Level Specification -
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Biology revision – Populations and evolution (Unit 18)
Population genetics
- A population is a group of organisms of the same species that occupies a particular
space at a particular time and that can potentially interbreed
- Any species exists as one or more populations
- All the alleles of all the genes of all the individuals in a population at a given time are
known as the gene pool
- The number of times an allele occurs within the pool is referred to as allelic
frequency
Cystic fibrosis:
- Diseases has a dom allele (F) and recessive allele (f) that leads to cystic fibrosis
- Any individual human has two of these alleles in every one of their cells, one on each
of the pair of homologous chromosomes on which the gene is found
- As these alleles are the same in every cell of a single person, we only count one pair
of alleles per gene per individual when considering a gene pool
- If there are 10,000 people in a population there will be twice as many alleles in the
gene pool of this gene
- In our population of 10,000 people, if all 10,000 had FF:
o The probability of anyone being FF would be 1.0 and ff would be 0.0
o The frequency of the F allele would be 100% and the frequency of the f allele
would be 0%
- If everyone in our population was Ff:
o Probability of anyone being Ff would be 1.0 and frequency of F allele would be
50% and f allele would be 50%
- In reality, population is not made up of one genotype but a mix
- To work out the allele frequency of these mixed populations we can use the Hardy-
Weinberg principle
Hardy-Weinberg principle
- HWP provides a mathematical equation used to calculate frequencies of alleles of a
particular gene in a population
- The principle makes the assumption that the proportion of dom and recess alleles of
any gene in a population remains the same from one gen to the next
This is the case if these 5 conditions are met:
- No mutations arise
- The population is isolated – there is no flow of alleles into or out of the population
- There is no selection – all alleles are equally likely to be passed to the next gen
- Population is large
- Mating within the population is random
- Although these conditions are probably never totally met in a natural population
o HWP is still useful when studying gene frequencies
- To help understand principle consider a gene that has two alleles
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