QUESTION 26 (4 marks)
Karyotypes for two human gametes are shown.
Karyotype A (sperm cell)
Karyotype B (egg cell)
a) Identify which cell exhibits aneuploidy. Refer to evidence from the karyotype.
[1 mark]
b) Explain how this chromosome abnormality may have occurred.
[2 marks]
This table lists some genetic conditions resulting from chromosomal abnormalities.
Genetic condition | Common name |
Monosomy 5 | Cri du chat syndrome |
Monosomy X | Turner syndrome |
Trisomy 13 | Patau syndrome |
Trisomy 18 | Edwards syndrome |
Trisomy X | Triple X syndrome |
c) Predict which genetic condition would occur if the two gametes produced a zygote.
[1 mark]
QCAA guide · typeset solution
QCAA sample response and mark allocation
26a) | The sperm cell is missing a sex chromosome. | • identifies the sperm cell is missing a chromosome [1 mark] |
26b) | The abnormality may have occurred due to non- disjunction during meiosis, where the sex chromosomes failed to separate at anaphase and ended up in the same daughter cell. This results in the daughter cells (gametes) containing either two copies of the sex chromosome/s or no copies, as is the case for karyotype A. | • identifies the abnormality may be due to non- disjunction [1 mark] • explains that non-disjunction occurs when chromosomes fail to separate during meiosis [1 mark] |
26c) | Turner syndrome. | • predicts Turner syndrome [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.