Fig. 1 shows what can happen during meiosis in a person producing egg cells. Normally, the two copies of chromosome 21 separate, so each gamete has one copy...
Question 1 Report
Fig. 1 shows what can happen during meiosis in a person producing egg cells. Normally, the two copies of chromosome 21 separate, so each gamete has one copy. In the diagram, the two chromosome 21 copies fail to separate. A doctor uses this diagram when explaining why a child may have Down syndrome even when there is no known family history of the condition.
(a) State the usual number of chromosomes in a human body cell. [1]
(b) Describe the difference between the two gametes shown in Fig. 1. [2]
(c) Explain how fertilisation of the gamete with two copies of chromosome 21 can lead to Down syndrome in a child. [4]
(d) Complete the chromosome numbers of the two possible zygotes when each gamete shown is fertilised by a normal sperm cell containing 23 chromosomes. [3]
(e) Explain why this chromosome condition can occur in a child with no previous family history. [4]
Answer Details
(a) A human body cell normally contains 46 chromosomes. [1 mark]
(b) One gamete has two copies of chromosome 21, while the other has no copy of chromosome 21. [2 marks]
(c) A normal sperm contributes one copy of chromosome 21. If it fertilises the egg with two copies, the zygote has three copies of chromosome 21, called trisomy 21. This chromosome imbalance causes Down syndrome. [4 marks]
(d) The egg with two copies has 24 chromosomes, so \(24+23=47\) chromosomes in the zygote. The egg with no chromosome 21 has 22 chromosomes, so \(22+23=45\) chromosomes in the zygote. [3 marks]
(e) The condition can arise through an error during meiosis, when chromosome copies fail to separate. This can happen randomly in an egg or sperm cell, so it is not necessarily caused by an inherited disease allele or a previous family history. [4 marks]
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