1. 1
   A gardener crosses a pure-breeding red-flowered four o'clock plant (Mirabilis jalapa) with a pure-breeding white-flowered plant. All of the F1 plants have pink flowers. She then self-pollinates the F1 plants and grows 240 F2 plants from the seeds.

   (a)

   How many of the 240 F2 plants would be expected to have pink flowers?

   [1 mark]

   • A60
   • B120
   • C180
   • D240

   (b)

   Which other example shows a heterozygote with a dual phenotype rather than an intermediate one?

   [1 mark]

   • ATall and dwarf pea plants
   • BCarriers of phenylketonuria
   • CBlood group AB ($I^AI^B$)
   • DHuman skin colour

   (c)

   Explain why the proportions of the flower colours in the F2 equal the proportions of the genotypes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two parents with no symptoms have a baby whose newborn heel-prick blood test shows a very high phenylalanine concentration, and she is diagnosed with phenylketonuria (PKU). PKU is caused by a recessive allele of the PAH gene, whose locus is on the long arm of chromosome 12 at position 12q23.2. The baby is put on a diet very low in phenylalanine and develops normally.

   (a)

   What is the probability that the couple's next child will have PKU?

   [1 mark]

   • A0
   • B3/4
   • C1/2
   • D1/4

   (b)

   Which gene locus would be most likely to be linked to PAH, so that its alleles fail to assort independently of PAH alleles?

   [1 mark]

   • A12q23.1, on chromosome 12 next to PAH
   • B1p36, on chromosome 1
   • CXq28, on the X chromosome
   • D12p13, on the other arm of chromosome 12, very far from PAH

   (c)

   Explain how the treatment of the baby shows that phenotype depends on both genotype and environment.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Mendel crossed pure-breeding pea plants with round, yellow seeds with pure-breeding plants with wrinkled, green seeds. Round (R) is dominant to wrinkled (r) and yellow (Y) is dominant to green (y); the two genes are on different chromosomes. All the F1 seeds were round and yellow. The F1 plants were self-pollinated and produced 556 F2 seeds: 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green. The critical value of chi-squared at p = 0.05 with 3 degrees of freedom is 7.815.

   (a)

   Explain how the movement of chromosomes during meiosis in the F1 plants leads to a 9 : 3 : 3 : 1 ratio in the F2.

   [3 marks]

   (b)

   Use a chi-squared test to evaluate whether these results support the hypothesis that the two genes assort independently.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the fruit fly Drosophila melanogaster, grey body (G) is dominant to black body (g) and normal wings (N) are dominant to vestigial wings (n). Pure-breeding grey, normal-winged flies were crossed with black, vestigial-winged flies. The F1 females, all grey with normal wings, were then crossed with black, vestigial-winged males. The 2300 offspring were: 965 grey normal, 944 black vestigial, 206 grey vestigial and 185 black normal.

   (a)

   Analyse the results to show that the body-colour and wing genes are linked, identifying the recombinant offspring.

   [6 marks]

   (b)

   Discuss whether Mendel's law of independent assortment should be regarded as a biological law, using evidence from the pea and fruit fly crosses.

   [6 marks]

   Total for question 4: 12 marks

End of questions