Sources of genetic variationEdexcel A-Level Biology B: Subtopic test

1. 1
   An orchard owner notices one branch of an apple tree bearing red apples while the rest of the tree bears green apples. Cuttings taken from the red branch and grown into new trees all produce red apples. Genetic testing shows a new allele for fruit colour is present in every cell of the red branch.

   (a)

   Which process is the only source of completely new alleles?

   [1 mark]

   • ACrossing over
   • BMutation
   • CRandom fertilisation
   • DIndependent assortment

   (b)

   Which statement about mutations is correct?

   [1 mark]

   • AMutations only occur when cells are exposed to a mutagen such as ultraviolet radiation
   • BMutations always produce a harmful phenotype
   • CMutations arise spontaneously and at random, so they can produce new alleles
   • DA mutation in a leaf cell can be passed to offspring made by sexual reproduction

   (c)

   Explain why every tree grown from a cutting of the red branch produces red apples.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A species of beetle has a diploid number of 8 (four pairs of homologous chromosomes). A breeder wants to know how many genetically different offspring two unrelated beetles could produce. Ignore crossing over unless told otherwise.

   (a)

   Ignoring crossing over, how many different combinations of chromosomes can be present in the gametes of one beetle?

   [1 mark]

   • A8
   • B4
   • C32
   • D16

   (b)

   Which event in meiosis I gives rise to independent assortment?

   [1 mark]

   • ARandom orientation of the homologous pairs at the equator of the spindle
   • BExchange of sections between non-sister chromatids
   • CSeparation of sister chromatids
   • DReplication of the DNA

   (c)

   Two unrelated beetles of this species mate. Calculate the number of genetically different zygotes that random fertilisation could produce from their gametes, ignoring crossing over.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two parents who are both heterozygous at many gene loci have four children. No two of the children, apart from any identical twins, would be expected to have the same genotype, even though all of the children have the same two parents.

   (a)

   Explain how meiosis produces gametes that differ genetically from each other.

   [3 marks]

   (b)

   Explain how random fertilisation, and the origin of new alleles, add to the variation among the children.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A conservation team studies a small captive population of an endangered parrot. One chick has a feather colour allele that was not found in either parent or in any earlier generation of the breeding programme. The chick's siblings, from the same parents, differ from each other in plumage.

   (a)

   Explain how a new allele could have appeared in the chick and how it could be passed on to later generations.

   [6 marks]

   (b)

   Compare the contributions of crossing over, independent assortment and random fertilisation to the plumage differences among the chick's siblings.

   [6 marks]

   Total for question 4: 12 marks

End of questions