QUESTION 92 (9 marks)
A researcher marks 30 frogs and releases them. In a later capture of 45 frogs, 9 are marked. These 45 frogs comprise 9 AA, 24 Aa and 12 aa individuals at one diploid locus. Assume a closed, well-mixed population and a genotype-representative sample. After both captures, 20 AA frogs immigrate; no other population changes occur.
a) Estimate the original population using the Lincoln index. [2 marks]
b) Calculate the sample frequency of A and estimate the original number of A alleles in the entire population. [3 marks]
c) Estimate A frequency after immigration. [2 marks]
d) Name the process that changed the local allele frequency and explain why it need not create a new allele. [2 marks]
Practice marking scheme
Answer
a) 150 frogs. b) f(A) = 0.467; estimated A copies = 140. c) f(A) = 0.529. d) Gene flow transfers existing alleles.
Working
. The sample contains A copies out of 90, giving . Applying this frequency to 300 population alleles gives an estimated 140 A copies. The immigrants add 40 A copies and 40 total alleles: . These are model-based estimates; gene flow moves an existing A allele into the local gene pool rather than changing its base sequence.
Marking criteria
- Substitutes the capture data correctly. [1 mark]
- Calculates N = 150. [1 mark]
- Calculates sample A frequency 42/90. [1 mark]
- Uses 300 total original alleles. [1 mark]
- Estimates 140 original A copies. [1 mark]
- Adds 40 A copies and 40 total alleles. [1 mark]
- Calculates post-immigration frequency approximately 0.529. [1 mark]
- Names gene flow. [1 mark]
- Distinguishes movement of existing alleles from 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.