QUESTION 137 (5 marks)
A hypothetical species has three homologous chromosome pairs. Consider independent assortment without crossing over. Each pair provides a choice of one maternally or one paternally inherited homologue in a gamete. Assume those choices are independent and homologues can be distinguished.
a) Determine the number of chromosome combinations in gametes from one parent. Show the choices used. [2 marks]
b) Determine the number of combinations from random fertilisation between two such parents. [1 mark]
c) Explain why crossing over would make this whole-chromosome count an incomplete description of possible genetic variation. [2 marks]
Practice marking scheme
Answer
Eight chromosome combinations per parent and 64 parental chromosome-combination pairings at fertilisation. Crossing over can add combinations of alleles within a chromosome.
Working
For each of three pairs there are two choices, giving 2×2×2 = 8 distinct chromosome combinations. Random union of one of eight egg combinations and one of eight sperm combinations gives 8×8 = 64 pairings. Crossing over exchanges segments between non-sister chromatids, adding allele combinations not counted in this whole-homologue model.
Marking criteria
- Uses two independent choices for each of three pairs. [1 mark]
- Obtains eight combinations. [1 mark]
- Obtains 64 fertilisation pairings. [1 mark]
- Describes exchange between non-sister chromatids of homologues. [1 mark]
- Links the exchange to additional combinations of alleles within a chromosome. [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.