The diagram shows chromosomes from one cell of a developing fetus. Scientists arranged the chromosomes in numbered pairs after analysing cells collected dur...
Question 1 Report
The diagram shows chromosomes from one cell of a developing fetus. Scientists arranged the chromosomes in numbered pairs after analysing cells collected during an amniocentesis test. Most chromosome pairs contain two chromosomes. In the final box, chromosome 21 is present three times. The parents are given genetic counselling before deciding what further support they may need after the child is born.
(a) Name the structure in a cell that carries genes. [1]
(b) Use Fig. 1 to calculate the total number of chromosome 21 copies in each cell shown. [1]
(c) Describe how this chromosome number differs from the usual number in a human body cell. [2]
(d) Explain how an error during meiosis in either the male or female parent could produce this chromosome pattern in the child. [4]
Answer Details
(a) A chromosome is the cell structure that carries genes. [1 mark]
(b) There are 3 copies of chromosome 21 shown. [1 mark]
(c) The cell has one extra chromosome 21. [1 mark] A usual human body cell has two copies of chromosome 21. [1 mark]
(d) Meiosis produces gametes. [1 mark] During meiosis, chromosome 21 may fail to separate into different gametes, an error called nondisjunction. [1 mark] One gamete then receives two copies of chromosome 21. [1 mark] If this gamete fuses with a normal gamete containing one copy during fertilisation, the child has three copies of chromosome 21. [1 mark]
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