QUESTION 144 (7 marks)
A diploid population has 10 AA, 20 Aa and 30 aa adults. All survive to reproduce. In a supplied model, each AA adult contributes 6 gametes, each Aa adult 4, and each aa adult 2 to a pooled sample. Aa gametes contain A or a with equal probability; use expected allele counts. No mutation or migration occurs.
a) Calculate A frequency among the adults. [2 marks]
b) Calculate the expected A frequency in the pooled gametes. Show contributions from each genotype. [3 marks]
c) Explain how the reproductive difference can change what is passed to offspring without a difference in adult survival. [2 marks]
Practice marking scheme
Answer
Adult A frequency is 1/3; expected A frequency in the pooled gametes is 1/2. Unequal reproductive contribution can change the allele composition passed to offspring even without unequal adult survival.
Working
Adults contain 2(10)+20 = 40 A copies among 120 alleles. The gamete pool totals 10×6 + 20×4 + 30×2 = 200. Expected A copies are 60 + 40 + 0 = 100, giving 0.50. AA adults contribute more gametes per adult, so the A allele is disproportionately represented in the reproductive pool. No new allele has to be created.
Marking criteria
- Calculates 40 A copies among 120 adult alleles. [1 mark]
- Obtains adult frequency 1/3. [1 mark]
- Calculates 200 pooled gametes. [1 mark]
- Calculates 100 expected A gametes, including 40 from heterozygotes. [1 mark]
- Obtains pooled frequency 0.50. [1 mark]
- Identifies unequal reproductive contribution/differential reproductive success. [1 mark]
- Links disproportionate A transmission to allele change rather than a necessary new mutation. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.