Meiosis and genetic variationEdexcel A-Level Biology A: Subtopic test

1. 1
   The gene for the beta-globin polypeptide of haemoglobin is found at one position on chromosome 11 in humans. Two alleles of this gene are common in some populations: HbA, which codes for normal beta-globin, and HbS, which codes for the beta-globin found in sickle cell haemoglobin.

   (a)

   Which term describes the position of the beta-globin gene on chromosome 11?

   [1 mark]

   • AAn allele
   • BA genotype
   • CA locus
   • DA gene pool

   (b)

   A person has the genotype HbA HbS. How many copies of the beta-globin gene do their body cells contain, and where are they found?

   [1 mark]

   • ATwo, one on each chromosome 11 of the homologous pair
   • BTwo, both on the same chromosome 11
   • COne, on one of the two chromosomes 11
   • DTwo, one on chromosome 11 and one on chromosome 12

   (c)

   State what is meant by an allele and explain why HbA and HbS are found at the same locus.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Red-green colour blindness is caused by a recessive allele on the X chromosome. A woman who is a carrier of the allele and a man with normal colour vision have a family together.

   (a)

   Why is red-green colour blindness more common in men than in women?

   [1 mark]

   • AMen have two X chromosomes, so the allele has a double effect
   • BThe allele is carried on the Y chromosome
   • CMen inherit their X chromosome from their father
   • DMen have only one X chromosome, so a single recessive allele is expressed

   (b)

   What is the probability that a son of this couple is colour blind?

   [1 mark]

   • A1 in 4
   • B1 in 2
   • C3 in 4
   • D0

   (c)

   Explain why none of the daughters of this couple will be colour blind, although some may be carriers.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In fruit flies, the gene for body colour (grey G, dominant, or black g) and the gene for wing length (long L, dominant, or vestigial l) are on the same autosome. A heterozygous grey, long-winged fly, with G and L on one chromosome and g and l on its homologue, was crossed with a black, vestigial-winged fly. Among 1000 offspring, 462 were grey and long-winged, 458 were black and vestigial-winged, 41 were grey and vestigial-winged and 39 were black and long-winged.

   (a)

   Explain the results of this cross.

   [3 marks]

   (b)

   Calculate the percentage of the offspring that are recombinants. Explain why the two recombinant phenotypes occur in similar numbers, and predict how the percentage would change if the two loci were further apart on the chromosome.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Humans have 23 pairs of homologous chromosomes in each diploid body cell. Meiosis produces gametes, and brothers and sisters with the same two parents are not genetically identical to one another. Sexually reproducing species are often better able to survive the arrival of a new disease than species that reproduce only by asexual reproduction.

   (a)

   Explain how independent assortment and crossing over during meiosis produce gametes that are genetically different from one another. In your answer, calculate the number of different combinations of chromosomes that independent assortment alone could produce in the gametes of one person.

   [6 marks]

   (b)

   Explain why meiosis is necessary in a sexual life cycle and evaluate how the genetic variation it produces helps a population to survive a new disease. Include a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions