QUESTION 2 (6 marks)
A human karyotype and list of genetic conditions are shown.
Genetic condition | Common name |
Monosomy 5 | Cri du chat syndrome |
Monosomy X | Turner syndrome |
Trisomy 13 | Patau syndrome |
Trisomy 21 | Down syndrome |
Trisomy X | Triple X syndrome |
a) Which genetic condition is indicated by the karyotype?
[1 mark]
b) Explain how errors in meiosis may have led to this condition.
[3 marks]
c) Describe how the karyotype would be different for a male with Down syndrome.
[2 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
2a) | Monosomy X | • identifies condition [1 mark] |
2b) | The condition may have occurred due to non-disjunction, where the sex chromosomes failed to separate at anaphase. This would have resulted in some daughter cells (gametes) containing no sex chromosomes and some containing two copies. If one of the gametes containing no sex chromosomes fused with another gamete containing one X chromosome, the resulting offspring would have Turner syndrome. | • recognises the condition is due to non-disjunction [1 mark] • describes non-disjunction [1 mark] • explains how non-disjunction leads to Turner syndrome/Monosomy X [1 mark] |
2c) | A male with Down syndrome would have an extra chromosome 21 and a Y chromosome next to the X. | • recognises there would be an extra chromosome 21 [1 mark] • recognises there would be a Y chromosome [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.