Hardy-Weinberg and speciationEdexcel A-Level Biology A: Subtopic test

1. 1
   A population of 1000 moths shows two phenotypes controlled by one gene with two alleles. The allele B for dark wings is dominant and the allele b for pale wings is recessive. The frequency of allele b is 0.3. The population is assumed to be in Hardy-Weinberg equilibrium.

   (a)

   What is the frequency of allele B?

   [1 mark]

   • A0.09
   • B0.30
   • C0.70
   • D0.49

   (b)

   How many moths are expected to be heterozygous?

   [1 mark]

   • A90
   • B210
   • C490
   • D420

   (c)

   Calculate the number of pale moths expected in the population.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A population of about 60 wildflowers grows on a hillside. Seeds fall close to the parent plant, so plants mostly breed with near neighbours. A genetic screen showed that the frequency of allele R of one gene fell from 0.62 to 0.55 over two years. No plants arrived from elsewhere and no new alleles were found.

   (a)

   What does the fall in the frequency of allele R show?

   [1 mark]

   • AThe allele frequency is changing, so the population is not in Hardy-Weinberg equilibrium
   • BAllele R is recessive
   • CThe population is in Hardy-Weinberg equilibrium
   • DA mutation has occurred

   (b)

   Which condition for Hardy-Weinberg equilibrium is broken by plants mostly breeding with their near neighbours?

   [1 mark]

   • ANo mutation
   • BRandom mating
   • CNo selection
   • DA large population

   (c)

   Hardy-Weinberg equilibrium also requires no selection and a large population. Suggest how each of these conditions may have been broken in this population.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A recessive genetic disorder affects 1 in 10 000 babies in a large population that is assumed to be in Hardy-Weinberg equilibrium. The frequency of the dominant allele is p and the frequency of the recessive allele is q. A health authority wants to estimate how many people in a city of 2 000 000 carry the disorder allele without having the disorder.

   (a)

   Calculate the frequency of the dominant allele and of the recessive allele. Show your working.

   [3 marks]

   (b)

   Calculate the number of carriers expected in the city, and explain how the result would be affected if people in the city mostly chose partners from within their own community.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A species of freshwater snail once lived throughout one large river system. Climate change dried up parts of the river and left the snails in two lakes that are not connected. Lake X is shallow and warm with many water plants. Lake Y is deep and cold with a rocky bed. Many thousands of years later, snails from the two lakes were brought together in an aquarium. They mated, but the eggs did not develop.

   (a)

   Explain how the snails in the two lakes may have become different species.

   [6 marks]

   (b)

   Suggest how reproductive isolation could arise between snails living in the same lake, and describe how the Hardy-Weinberg equation could be used to find out whether the allele frequencies in the lake X population are changing.

   [6 marks]

   Total for question 4: 12 marks

End of questions