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The graph below shows changes in allele frequencies at a single locus with two alleles B and b in two very large populations - VCE - SSCE Biology - Question 7 - 2005 - Paper 1
Question 7
The graph below shows changes in allele frequencies at a single locus with two alleles B and b in two very large populations. The phenotype resulting from allele B i... show full transcript
Worked Solution & Example Answer:The graph below shows changes in allele frequencies at a single locus with two alleles B and b in two very large populations - VCE - SSCE Biology - Question 7 - 2005 - Paper 1
Step 1
In which population, X or Y, is selection occurring against the homozygous recessive phenotype? Explain your response.
Answer
Selection is occurring against the homozygous recessive phenotype in population X. This is evident from the graph, where the frequency of the b allele is decreasing over time. As the frequency of the b allele declines, it indicates that the homozygous recessive individuals, represented by the b allele, are being selected against.
Step 2
Describe what has occurred in population Y by the 60th generation.
Answer
By the 60th generation, the frequency of allele B in population Y has become fixed, meaning that the frequency of the b allele has decreased to zero. This suggests that only the dominant phenotype, resulting from allele B, is present in this population.
Step 3
Given the results in population Y, why has a similar pattern of events not occurred in population X?
Answer
In population X, a similar pattern of fixation has not occurred because there remains a substantial frequency of the b allele, which implies that either the homozygous recessive individuals are still surviving or that there are other selective pressures at play that prevent the b allele from being eliminated entirely.