Meiosis and genetic variationEdexcel International A Level Biology: Subtopic test

1. 1
   A diploid human cell in the testis contains 46 chromosomes and begins meiosis to produce sperm cells.

   (a)

   How many bivalents are formed in this cell during prophase I?

   [1 mark]

   • A46
   • B23
   • C92
   • D12

   (b)

   Which event in meiosis halves the chromosome number from diploid to haploid?

   [1 mark]

   • AReplication of the DNA during interphase
   • BCrossing over in prophase I
   • CSeparation of homologous chromosomes in anaphase I
   • DSeparation of sister chromatids in anaphase II

   (c)

   Ignoring crossing over, calculate the number of genetically different combinations of maternal and paternal chromosomes that could be present in the sperm cells produced by this person.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In the fruit fly Drosophila the gene for body colour (grey G, dominant to black g) and the gene for wing length (long L, dominant to vestigial l) are close together on the same chromosome. A fly heterozygous for both genes, with G and L on one chromosome and g and l on its homologue, was crossed with a black, vestigial-winged fly. Out of 1000 offspring, 462 were grey with long wings, 458 were black with vestigial wings, 42 were grey with vestigial wings and 38 were black with long wings.

   (a)

   What is the term for the position of a gene on a chromosome?

   [1 mark]

   • AAllele
   • BCodon
   • CChiasma
   • DLocus

   (b)

   Which statement best explains why most of the offspring were parental types?

   [1 mark]

   • AThe genes are linked on the same chromosome, so the alleles are usually inherited together
   • BThe genes are on different chromosomes and assort independently
   • CCrossing over occurs between these genes in every meiosis
   • DThe alleles for the two genes are codominant

   (c)

   Calculate the percentage of the offspring that were recombinants.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A diploid cell has the genotype AaBb. Alleles A and B are on one chromosome of a homologous pair, and alleles a and b are on the other chromosome of the pair. In this cell, one chiasma forms between the loci of the two genes during prophase I of meiosis.

   (a)

   Describe what happens during crossing over in this cell.

   [3 marks]

   (b)

   Give the genotypes of the four gametes this cell produces, state which are recombinants, and explain why only two types of gamete would be produced if no crossing over had occurred.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A plant breeder works with a diploid crop species with 2n = 14. The gene for disease resistance (R, dominant to susceptible r) and the gene for high yield (Y, dominant to low yield y) are on the same pair of chromosomes, close together, with a recombination frequency of 5%. One parent plant has alleles R and y on one chromosome and alleles r and Y on its homologue. The breeder wants plants with R and Y on the same chromosome.

   (a)

   Explain how meiosis in this crop plant produces genetically different gametes.

   [6 marks]

   (b)

   Explain why it is difficult to obtain a chromosome carrying both R and Y from this parent plant, and calculate the percentage of its gametes that carry both R and Y.

   [6 marks]

   Total for question 4: 12 marks

End of questions