Meiosis and chromosome mutationsEdexcel A-Level Biology B: Subtopic test

1. 1
   A fruit fly, Drosophila, has 8 chromosomes in each of its body cells. Sperm are made by meiosis in the testes of the male fly.

   (a)

   How many chromosomes are present in each sperm cell of the fruit fly?

   [1 mark]

   • A2
   • B4
   • C8
   • D16

   (b)

   Ignoring crossing over, how many different combinations of maternal and paternal chromosomes can be present in the sperm of one male fly?

   [1 mark]

   • A4
   • B8
   • C16
   • D64

   (c)

   Explain why meiosis I, rather than meiosis II, halves the chromosome number.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A baby girl is born with Down's syndrome. A karyotype of her cells shows 47 chromosomes, including three copies of chromosome 21. Both of her parents have a normal karyotype of 46 chromosomes.

   (a)

   Which term describes this baby's karyotype?

   [1 mark]

   • APolysomy, because one chromosome is present in three copies
   • BMonosomy, because one chromosome is missing
   • CTranslocation, because part of one chromosome is attached to another
   • DPolyploidy, because there are three complete sets of chromosomes

   (b)

   Assume the error occurred in the formation of the egg. How many chromosomes did the egg contain?

   [1 mark]

   • A22
   • B23
   • C47
   • D24

   (c)

   Explain how non-disjunction during meiosis could have caused this baby's condition.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Many patients with chronic myeloid leukaemia have a Philadelphia chromosome in their white blood cell precursors. It forms when a piece of chromosome 22 breaks off and joins chromosome 9, while a piece of chromosome 9 joins chromosome 22. The change is found in the leukaemia cells but not in the patient's other body cells, such as skin cells.

   (a)

   Explain why the Philadelphia chromosome is unlikely to be passed on to the patient's children.

   [3 marks]

   (b)

   Compare the effect on the karyotype of a translocation with the effect of non-disjunction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Meiosis in the gonads normally produces eggs and sperm that each contain 23 chromosomes, and no two gametes from one person are exactly alike. Occasionally errors occur: one child is born with 47 chromosomes and another girl is born with 45.

   (a)

   Explain how meiosis produces genetically different haploid gametes.

   [6 marks]

   (b)

   Compare the causes and consequences of Down's syndrome and Turner's syndrome.

   [6 marks]

   Total for question 4: 12 marks

End of questions