Example 1
A population has 20 AA, 30 Aa and 50 aa individuals. Find A frequency.
A copies =2(20)+30=70. Total copies =2(100)=200. Frequency A=20070=0.35.
BUILD FLUENCY. EARN YOUR ASCENT.Sign in
Year 12 · SACE Stage 2
Variation, alleles and natural selection
Calculate allele frequencies and identify selection sequences.
Method and worked examples
A population’s gene pool contains its alleles. Mutations create new sequence variants, while sexual reproduction reshuffles existing variants through meiosis and fertilisation. In a diploid population of N individuals, an autosomal locus normally has 2N allele copies. Count two A copies for each AA individual and one for each Aa individual; divide by 2N to find the A frequency. Frequencies refer to populations, not to an individual deciding to adapt.
Natural selection occurs when heritable differences affect survival or reproductive success in particular conditions. Individuals with a favourable variant can contribute more offspring, changing allele frequencies across generations. Selection does not generate a needed mutation on demand. The same allele may have different effects under different conditions. Use the stated environmental and inheritance information when choosing a conclusion.
Example 1
Example 2
A resistant bacterial variant exists before treatment and leaves more descendants during treatment. What follows?
The resistant variant can increase in frequency. Treatment selects among variants rather than intentionally causing the needed mutation.
Watch out: Evolution describes population change across generations; mutation does not anticipate a future need.