QUESTION 155 (9 marks)
A flowering-plant population initially contains 20 RR, 20 Rr and 10 rr plants. A flood kills plants independently of genotype. Only two Rr and two rr plants survive. Genomic profiles also show loss of several rare alleles at other loci. A nursery makes 3000 rooted cuttings from each survivor, producing 12 000 plants; no mutation occurs and all clones are equally represented. Each cutting can later flower.
a) Calculate R frequency before the flood and among the survivors. [3 marks]
b) Calculate R frequency after clonal multiplication. [1 mark]
c) Identify the process accounting for the flood-associated change under the stated survival model. [1 mark]
d) Explain why recovery to 12 000 plants does not demonstrate restoration of genetic diversity. [2 marks]
e) Explain one resulting extinction risk and one way breeding immigration could alter that risk. [2 marks]
Practice marking scheme
Answer
R frequency is 0.60 before the flood and 0.25 among survivors. It remains 0.25 after equal clonal multiplication. Drift through a bottleneck can reduce genetic diversity; rapid recovery in plant number does not restore lost alleles. Breeding immigrants with additional alleles could introduce gene flow.
Working
Initially there are 2(20)+20 = 60 R copies among 100 alleles. Survivors have two R copies among eight, giving 0.25. Equal multiplication preserves genotype proportions: 6000 Rr and 6000 rr give 6000 R among 24 000 alleles. The chance loss of individuals is a bottleneck, and copying survivors cannot recreate alleles they lack. Reduced genome-wide variation can reduce the chance that some plants tolerate a future pathogen or environmental change.
Marking criteria
- Uses 60 R copies among 100 alleles before the flood. [1 mark]
- Calculates initial frequency 0.60. [1 mark]
- Calculates survivor frequency 2/8 = 0.25. [1 mark]
- Calculates clone frequency 0.25. [1 mark]
- Identifies genetic drift through a bottleneck. [1 mark]
- States that the cuttings copy the surviving genotypes. [1 mark]
- Explains that copying does not restore alleles lost in the flood. [1 mark]
- Links reduced variation to lower potential tolerance of a future change/pathogen. [1 mark]
- Explains that breeding immigrants can introduce additional alleles by gene flow. [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.