QUESTION 77 (5 marks)
The pedigree traces a known X-linked recessive trait. Assume complete penetrance and no new mutations. Use for the normal allele and for the trait allele.
a) Determine the genotype of I-2. [1 marks]
b) Explain why II-2’s trait allele was inherited from I-2 rather than I-1. [2 marks]
c) Calculate the probability that the parents’ next child has the trait, assuming equal probabilities of male and female children. [2 marks]
Practice marking scheme
Answer
a) XᴺXⁿ. b) A son receives his X from his mother and Y from his father. c) 25%.
Working
An unaffected mother of an affected son must carry the allele, giving . The unaffected father is . Sons inherit the maternal X, while their father supplies Y. The four equally likely outcomes are , , and ; only the last is affected, so the probability is .
Marking criteria
- States I-2 is XᴺXⁿ. [1 mark]
- States that sons inherit their X from their mother. [1 mark]
- States that the father supplies Y to sons. [1 mark]
- Shows a valid cross or the product of 1/2 male and 1/2 affected among sons. [1 mark]
- Calculates 25% of all children. [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.