A prenatal laboratory examined cells collected during pregnancy because a scan suggested that the fetus might have a chromosome condition. Fig. 1 is a simpl...
Question 1 Report
A prenatal laboratory examined cells collected during pregnancy because a scan suggested that the fetus might have a chromosome condition. Fig. 1 is a simplified chromosome diagram from one cell. The chromosomes labelled 21 are shown separately from the other chromosome pairs.
(a) Name the structure labelled X. [1]
(b) Describe the chromosome difference shown in Fig. 1. [2]
(c) Explain how an egg cell with two copies of chromosome 21 can be formed during meiosis. [3]
(d) Use the diagram to explain why fertilisation of this egg by a normal sperm produces a cell with three copies of chromosome 21. [2]
Answer Details
(a) X labels a chromosome. [1]
(b) The cell has three copies of chromosome 21. [1] Normally there are two copies, so this is one extra copy. [1]
(c) During meiosis, the homologous chromosome 21 pair can fail to separate. [1] Both copies then move into the same egg cell. [1] The other egg cell receives no copy of chromosome 21. [1]
(d) This abnormal egg contributes two copies of chromosome 21. [1] A normal sperm contributes one copy, giving the fertilised cell three copies in total. [1]
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