1. 1
   A forensic science trainer explains to a group of trainee crime-scene investigators how a tiny sample of skin cells left at a crime scene can be used to help identify who was present, by comparing sections of DNA extracted from the cells with a reference sample.

   (a)

   What shape does a molecule of DNA form?

   [1 mark]

   • ADouble helix
   • BSingle strand
   • CTriangular lattice
   • DBranched chain

   (b)

   Each different base along a DNA strand is one of four types. Which of the following is one of these four bases?

   [1 mark]

   • AUracil
   • BSucrose
   • CRibose
   • DAdenine

   (c)

   A gene is described as 'a small section of DNA'. Explain what a gene codes for, and how this relates to the way DNA profiling can be used to distinguish between individuals.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A vet is explaining to a dog breeder why a litter of Labrador puppies has a mixture of black and brown coat colours, even though both parent dogs have a black coat.

   (a)

   In dogs, the allele for black coat colour (B) is dominant over the allele for brown coat colour (b). Both parent dogs have the genotype Bb. What is the probability that a puppy from this cross has a brown coat?

   [1 mark]

   • A0%
   • B75%
   • C25%
   • D50%

   (b)

   What term describes a dog with the genotype Bb, which has two different alleles for coat colour?

   [1 mark]

   • AHomozygous dominant
   • BHeterozygous
   • CHomozygous recessive
   • DCodominant

   (c)

   Explain why two black-coated parent dogs can produce a brown-coated puppy.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A city environmental agency is investigating why fish caught from a lake near an old chemical factory show a higher rate of unusual fin shapes and skin markings than fish from a clean lake nearby.

   (a)

   Suggest reasons why the fish from the lake near the old chemical factory show more variation in fin shape and skin markings than fish from the clean reference lake.

   [3 marks]

   (b)

   Most mutations that occur in the fish's DNA have no effect on their phenotype. Explain why this is, and describe what happens in the rare cases when a mutation does affect the phenotype.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A pest control company has noticed that a chemical spray that used to kill 95% of cockroaches in a large warehouse now only kills around 40% of the cockroach population, even though the same spray and dose are used each time.

   (a)

   Explain, using the theory of natural selection, why the chemical spray has become less effective at killing cockroaches in the warehouse over time.

   [6 marks]

   (b)

   Suggest and evaluate two strategies the pest control company could use to slow down the development of further resistance in cockroach populations.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A biology teacher demonstrates fertilisation using a model, explaining how a sperm cell and an egg cell join together in humans to form a new cell called a zygote, which then develops into an embryo.

   (a)

   Human body cells contain 46 chromosomes. How many chromosomes does a human sperm cell contain?

   [1 mark]

   • A46
   • B92
   • C12
   • D23

   (b)

   Which type of cell division produces sperm and egg cells, each with half the normal number of chromosomes?

   [1 mark]

   • AMeiosis
   • BMitosis
   • CBinary fission
   • DBudding

   (c)

   Explain how the fusion of a sperm cell and an egg cell at fertilisation restores the normal number of chromosomes in the new cell.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A biology class is using a genetic diagram to work out the probability of a baby's biological sex, given that egg cells always carry an X chromosome and sperm cells carry either an X or a Y chromosome.

   (a)

   Which combination of sex chromosomes normally results in a male baby?

   [1 mark]

   • AXX
   • BXY
   • CYY
   • DXO

   (b)

   What is the probability, from a genetic cross, that a baby conceived will be female (XX)?

   [1 mark]

   • A0%
   • B25%
   • C50%
   • D100%

   (c)

   Using a genetic diagram, explain why it is the father's sperm, rather than the mother's egg, that determines the biological sex of a baby.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Taxonomists studying two groups of tree frogs living in neighbouring, unconnected valleys in a mountain range have found that although the frogs look very similar, breeding experiments show that frogs from the two valleys almost never produce offspring together, and any offspring that do result are infertile.

   (a)

   Explain what this breeding evidence suggests about whether the tree frogs living in the two valleys should be classified as the same species or as two different species.

   [3 marks]

   (b)

   Suggest and explain how the tree frog populations in the two valleys could have become separate species over time.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Biologists studying a population of wild rabbits introduced to a remote island 150 years ago have found that the island rabbits are now noticeably smaller, with shorter ears, than the mainland rabbit population they came from, and DNA analysis confirms the two populations are now genetically distinct.

   (a)

   Explain, using the theory of evolution by natural selection, how the island rabbit population could have evolved to have a smaller body size and shorter ears than the mainland population over 150 years.

   [6 marks]

   (b)

   The DNA analysis shows the island and mainland rabbit populations are now genetically distinct. Evaluate whether this evidence is enough to conclude that the island rabbits have become a separate species, and describe what further evidence would help to test this.

   [6 marks]

   Total for question 8: 12 marks

End of questions