Genetic variation from meiosis and fertilisationAQA A-Level Biology: Subtopic test

1. 1
   A species of beetle has eight chromosomes in each body cell, arranged as four pairs of homologous chromosomes. Researchers are studying how many genetically different gametes one beetle can produce.

   (a)

   Ignoring crossing over, how many different combinations of maternal and paternal chromosomes are possible in the gametes of one beetle?

   [1 mark]

   • A16
   • B8
   • C4
   • D32

   (b)

   Between which structures does crossing over occur during meiosis?

   [1 mark]

   • ASister chromatids of the same chromosome
   • BNon-sister chromatids of homologous chromosomes
   • CChromosomes of different pairs
   • DChromatids in different gametes

   (c)

   Explain how independent segregation of homologous chromosomes produces genetic variation in gametes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Humans have 23 pairs of homologous chromosomes in their body cells. A student compares the possible variation in human gametes with that in the fruit fly, which has four pairs of homologous chromosomes.

   (a)

   Which expression gives the number of different chromosome combinations in a human gamete, ignoring crossing over?

   [1 mark]

   • A$2 \times 23$
   • B$23^2$
   • C$23$
   • D$2^{23}$

   (b)

   Which expression gives the number of different chromosome combinations in a zygote formed by random fertilisation of two gametes, ignoring crossing over, where $n$ is the number of homologous pairs?

   [1 mark]

   • A$2^n$
   • B$2n$
   • C$2^{2n}$
   • D$n^2$

   (c)

   Calculate the number of different chromosome combinations possible in the gametes of a fruit fly, and in the zygotes formed by random fertilisation, ignoring crossing over.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A plant is heterozygous for two genes that are on the same pair of homologous chromosomes. One chromosome of the pair carries alleles $A$ and $B$ and the other carries alleles $a$ and $b$. The plant is allowed to undergo meiosis to make gametes.

   (a)

   Describe how crossing over leads to new combinations of alleles on the chromatids of the plant.

   [3 marks]

   (b)

   Explain how crossing over allows this plant to produce gametes with four different combinations of the alleles, and why this increases genetic variation among its offspring.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A species of antelope has 30 chromosomes in each body cell, arranged as 15 pairs of homologous chromosomes. A conservation biologist is assessing how much genetic variation could be produced in a small breeding herd.

   (a)

   Explain how meiosis produces gametes that are genetically different from each other in the antelope. Include a calculation of the number of chromosome combinations from independent segregation alone.

   [6 marks]

   (b)

   Explain how random fertilisation increases the genetic variation among offspring of this herd, including a calculation of the number of chromosome combinations possible in a zygote, ignoring crossing over.

   [6 marks]

   Total for question 4: 12 marks

End of questions