QUESTION 76 (6 marks)
The pedigree shows a fully penetrant trait with no new mutations. Choose among autosomal dominant, autosomal recessive and X-linked recessive inheritance. Use A for the normal dominant allele if a recessive model applies.
a) Identify the inheritance pattern and justify using the affected daughter and her parents. [2 marks]
b) State the genotypes of I-1 and I-2. [2 marks]
c) II-2 has a partner with genotype Aa. Calculate the probability that their child has the trait. Show your reasoning. [2 marks]
Practice marking scheme
Answer
a) Autosomal recessive. b) Both parents are Aa. c) 1/6.
Working
Unaffected parents having an affected child exclude a fully penetrant dominant model without a new mutation. An affected daughter under an X-linked recessive model would require an affected father, so that model is also excluded. Thus both parents are and the affected daughter is . An unaffected child of has genotype with conditional probability and with probability . Only a carrier II-2 can produce an affected child with an partner: .
Marking criteria
- Identifies autosomal recessive inheritance. [1 mark]
- Uses unaffected parents and the affected daughter to exclude the alternative models. [1 mark]
- States I-1 is Aa. [1 mark]
- States I-2 is Aa. [1 mark]
- Determines the conditional carrier probability for II-2 as 2/3. [1 mark]
- Calculates affected-child probability as 1/6. [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.