1. 1
   Horses (Equus caballus) have 64 chromosomes in their body cells and donkeys (Equus asinus) have 62. A female horse mated with a male donkey produces a mule, which has 63 chromosomes. Mules are healthy and strong, but they almost never produce offspring of their own.

   (a)

   What is the most likely reason that mules are sterile?

   [1 mark]

   • AChromosomes from the two parents cannot all form homologous pairs in meiosis, so viable gametes are rarely formed
   • BMules cannot carry out mitosis
   • CMules are haploid
   • DMules have no genes from the horse

   (b)

   How should horses and donkeys be classified according to the biological species concept?

   [1 mark]

   • AAs one species, because they can mate and produce offspring
   • BAs one species, because they share most of their genome
   • CAs separate species, because their offspring are not fertile
   • DAs one species, because they are in the same genus

   (c)

   Deduce the number of chromosomes in a horse egg and in a donkey sperm, and explain how the chromosome number of the mule arises.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A hospital uses whole genome sequencing for patients about to receive clopidogrel, a drug that prevents blood clots after heart attacks. The drug only works after it is activated in the liver by the enzyme CYP2C19. About 30% of the patients sequenced carried at least one copy of a single-nucleotide polymorphism (SNP) in the CYP2C19 gene that produces an enzyme with little or no activity. Apart from such differences, the patients' genomes were almost identical.

   (a)

   What is a single-nucleotide polymorphism?

   [1 mark]

   • AA difference in the number of chromosomes between individuals
   • BA difference in a single base at a particular position in the genome between individuals
   • CA gene found in one species but not another
   • DThe insertion of a whole new gene into a genome

   (b)

   Which change has made whole genome sequencing of individual patients possible?

   [1 mark]

   • AGenomes have been found to be smaller than expected
   • BDNA can now be read with a light microscope
   • CEach patient's genome contains unique genes
   • DSequencing has become much faster and cheaper

   (c)

   Suggest how the hospital could use these sequencing results in personalised medicine.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ecologists surveyed fish in a lake using environmental DNA (eDNA). They filtered 1 litre of water at each of five sites, extracted the DNA, copied a short barcode section of a mitochondrial gene by PCR, sequenced it and compared the sequences with a reference database. eDNA identified 14 fish species, including the critically endangered European eel (Anguilla anguilla). Three days of netting at the same sites caught 9 species, all of which were also detected by eDNA; no eels were caught. Two further eDNA sequences matched no species in the database.

   (a)

   Outline how barcodes allow species to be identified from environmental DNA.

   [3 marks]

   (b)

   Evaluate the use of eDNA compared with netting for investigating the biodiversity of the lake.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two oak species grow in British woodland. Pedunculate oak (Quercus robur) has acorns on long stalks and leaves with very short stalks. Sessile oak (Quercus petraea) has acorns with no stalks and leaves with long stalks. Both have 24 chromosomes in their body cells. Where they grow together they frequently hybridise and the hybrids are fertile, yet across Britain the two remain recognisably different and tend to grow on different soils. A student wants to make a dichotomous key to identify these two oaks and four other tree species growing in a local park.

   (a)

   Discuss whether Quercus robur and Quercus petraea should be regarded as separate species.

   [6 marks]

   (b)

   Explain how the student should construct and test a dichotomous key for the six tree species.

   [6 marks]

   Total for question 4: 12 marks

End of questions