1. 1
   Forensic scientists are comparing a DNA profile extracted from a hair found at a crime scene with DNA samples taken from several suspects, to see whose profile matches the crime scene sample.

   (a)

   When comparing the DNA base sequences from the crime scene sample, which pair of bases would the forensic scientist expect to always find joined together across the double helix?

   [1 mark]

   • AAdenine with thymine, and cytosine with guanine
   • BAdenine with guanine, and cytosine with thymine
   • CAdenine with cytosine, and thymine with guanine
   • DAdenine with uracil, and cytosine with guanine

   (b)

   The scientists also make an RNA copy of part of the gene during their analysis. Which feature distinguishes this RNA molecule from the original DNA?

   [1 mark]

   • AIt is double-stranded and contains thymine
   • BIt is single-stranded and contains uracil instead of thymine
   • CIt is double-stranded and contains uracil instead of thymine
   • DIt is single-stranded and contains the same four bases as DNA

   (c)

   Describe the structure of the DNA molecule that the forensic scientists extracted.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A dog breeder crosses two Labradors that are both heterozygous for coat colour: black fur (allele B) is dominant over brown fur (allele b). The breeder wants to predict the coat colours of the puppies in the litter.

   (a)

   Both parent Labradors carry one allele for black coat colour (B, dominant) and one allele for brown coat colour (b, recessive). What term describes their genotype for this gene?

   [1 mark]

   • AHomozygous dominant
   • BHomozygous recessive
   • CHeterozygous
   • DCodominant

   (b)

   What is the expected ratio of black-furred to brown-furred puppies in a large litter from this cross?

   [1 mark]

   • A1 black : 1 brown
   • BAll puppies black
   • C1 black : 3 brown
   • D3 black : 1 brown

   (c)

   Explain, in terms of the alleles each parent can pass on, why some puppies in the litter are expected to have brown fur even though both parents have black fur.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A hospital ward experiences an outbreak of a bacterial infection. Most patients recover quickly after treatment with an antibiotic, but doctors notice that a small number of bacteria from some patients survive the treatment. Over the following months, as the antibiotic is used repeatedly on the ward, an increasing proportion of bacteria collected from patients are found to be resistant to it.

   (a)

   Explain why some bacteria in the ward were already resistant to the antibiotic even before it was given to patients.

   [3 marks]

   (b)

   Explain how the proportion of antibiotic-resistant bacteria on the ward increased over the following months as the antibiotic continued to be used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In a moth population living near a heavily industrialised town, most moths were originally pale-coloured, well camouflaged against the pale bark of local trees. Over several decades, as soot from factories blackened the tree bark, scientists recorded that the proportion of dark-coloured moths in the population rose sharply, while pale-coloured moths became increasingly rare.

   (a)

   Use Darwin's theory of natural selection to explain how, over many generations, the moth population in the industrialised, sooty area became predominantly dark-coloured, having originally been mostly pale-coloured.

   [6 marks]

   (b)

   Explain how meiosis and random fertilisation contributed to the genetic variation that was already present in the moth population before the environment changed.

   [6 marks]

   Total for question 4: 12 marks

End of questions