QUESTION 141 (7 marks)
One autosomal coat-colour locus has three alleles with the dominance order . A parent is crossed with a parent. The phenotype is determined by the highest-ranking allele present.
a) Use a genetic cross to determine offspring genotypes and their probabilities. [2 marks]
b) Determine the probabilities of the three phenotypes. [2 marks]
c) Explain how C³ can be present in offspring although its phenotype is absent. [1 mark]
d) Distinguish multiple-allele inheritance from polygenic inheritance. [2 marks]
Practice marking scheme
Answer
Genotypes C¹C², C¹C³, C²C² and C²C³ each have probability 1/4. Half express the C¹ phenotype and half the C² phenotype; none express the C³ phenotype. Each individual still has only two alleles at this diploid locus.
Working
The first parent supplies C¹ or C²; the second supplies C² or C³. Their four equally likely combinations have the stated genotypes. C³ is transmitted in some offspring, but is masked by a higher-ranking allele. Multiple alleles describes variation across the population; it does not give a diploid individual three copies at one locus.
Marking criteria
- Identifies both parents’ possible gametes. [1 mark]
- Gives the four genotypes with probability 1/4 each. [1 mark]
- Gives C¹ phenotype 1/2 and C² phenotype 1/2. [1 mark]
- Gives C³ phenotype zero. [1 mark]
- Explains masking of C³ by C¹ or C². [1 mark]
- States multiple alleles occur at one locus across a population. [1 mark]
- States polygenic traits involve contributions from multiple loci. [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.