10.1 Meiosis

The diagram below illustrates the life cycle of the pea aphid, Acyrthosiphon pisum. Note that the term autosome refers to any chromosome that is not a sex chromosome.

Identify the biological sex of the individuals labelled P and Q.

The diagram in part a) shows that aphids use a different type of reproduction in the spring and summer to the type used in the autumn.

Identify the type of reproduction used by aphids in the spring and summer, and in the autumn.

[1]

Explain your answer to part i).

[2]

Suggest the advantage to the aphids of switching their method of reproduction in the autumn.

[1]

Most eukaryotic chromosomes are described as being monocentric. Aphids have unusual chromosomes known as holocentric chromosomes. A holocentric chromosome after DNA replication is shown in the diagram below.

Contrast holocentric chromosomes with normal monocentric chromosomes.

While scientific understanding of aphid meiosis is still limited, the holocentric nature of their chromosomes means that aphids are thought to carry out a form of meiosis known as inverted meiosis. The possible behaviour of a homologous pair of aphid chromosomes during metaphase I is shown in the diagram below.

Suggest, with reasons, two ways in which meiosis in aphids could be different to conventional meiosis.

The spider mite Eutetranychus africanus has very few chromosomes (2n = 4).

The diagram below shows a series of cells undergoing cell division.

Identify, with reasons, which of the cells in the diagram above belong to E. africanus.

A sample of cells was taken from the reproductive organs of E. africanus and the mass of DNA in each cell was determined. Some of the cells’ DNA had a mass of 1.7 arbitrary units (a.u.) whilst other cells' DNA had a mass of 3.4 or 6.8 a.u..

Use your knowledge of the cell cycle to explain this observation.

A species of false spider mite, Brevipalpus phoenicis, is the only animal to have so far been identified as having exclusively haploid cells throughout its life cycle. B. phoenicis populations are entirely female, producing eggs which hatch into more females.

The discovery of the haploid nature of B. phoenicis was a surprise to scientists, who believed that being diploid was essential due to the evolutionary advantage that it provides.

Identify the type of cell division by which B. pheonicis produces eggs.

[1]

Suggest why scientists might think that diploidy provides an evolutionary advantage.

[1]

B. phoenicis is a highly successful pest of citrus, tea, and palm plantations. Suggest how B. phoenicis might have evolved to become such a successful pest despite the points covered in parts i) and ii) above.

[1]

While studying the cells of B. phoenicis scientists discovered that the cells of all individuals contained bacteria. Treatment with antibiotics caused female B. phoenicis to lay eggs that hatched into haploid males.

Suggest the experimental measure that the scientists would need to take in order to demonstrate that the link between antibiotic treatment and male egg development is causal.

[1]

[2]

The diagram below shows three cells in different stages of cell division. Note that all of the cells shown have the same 2n chromosome number.

Identify the cell(s) in the diagram above that show the following:

Homologous chromosomes

[1]

Meiosis

[1]

Reduction division

[1]

Fruit flies, Drosophila melanogaster, are frequently used in scientific studies. The diagram below shows the gene loci and alleles of two genes on a pair of chromosomes in a male and female D. melanogaster individual. Note that the dominant alleles are long legs and red eyes.

A cross was carried out between the two individuals shown above. The table below shows the number of offspring with short/long legs and brown/red eyes produced from the cross.

Calculate the offspring ratios for the cross shown. Give your answers to the nearest whole number.

Explain the offspring ratios shown in part b).

The diagram below shows the gene loci and alleles for a third gene on the chromosomes of the individuals in part b). Note that the grey body allele is dominant to the black body allele.

Suggest, with a reason, how the numbers of offspring with short/long legs and black/grey bodies would differ from the numbers with short/long legs and brown/red eyes shown in part b).

One mark is available for clarity of communication throughout this question. 

Compare and contrast meiosis I and meiosis II.

Use named examples to describe the roles of mitosis and meiosis in living organisms.

The image below shows a pair of chromosomes during meiosis, taking place in a testis of Drosophila melanogaster (a fruit fly). The position of the alleles of some genes is indicated.

State the name of the arrangement of chromosomes shown in the image above.

The diagram below shows two of the gamete nuclei that formed at the end of the meiosis process shown in part a). Assume that the parent cell only contains 2 chromosomes.

Draw the chromosomes in each of the nuclei shown in the diagram to give their appearance at the end of meiosis.

Outline the events that occur during anaphase I of meiosis.

Explain how independent assortment contributes to genetic variation in gametes.

European rabbits (Oryctolagus cuniculus) have a diploid (2n) chromosome number of 44.

The number of possible chromosome combinations that are possible as the result of independent assortment can be calculated using the formula 2ⁿ, where n is the haploid chromosome number.

Calculate the number of different possible chromosome combinations in the gametes of rabbits.

The graph below shows how the mass of DNA changes over time during two different types of cell division of a diploid organism.

Identify, with a reason, which of the division types represents meiosis.

Describe the two processes represented by the letters X and Y in the graph in part b).

The fruit fly (Drosophila melanogaster) has a diploid number (2n) of 8.

The image below shows some cells from different organisms undergoing cell division.

Identify, with a reason, the cell which would represent a fruit fly cell that has just completed meiosis I.

Explain what is meant by the phrase ‘non-Mendelian ratio’.

Explain how the work of Thomas Hunt Morgan provided a possible explanation for non-Mendelian ratios.

Hydra are small, freshwater invertebrates that can reproduce by forming buds on their cylinder-shaped bodies. These buds grow over time, developing tentacles like the adult hydra, and eventually detaching from the parent hydra’s body.

Suggest why meiosis would not occur in Hydra during the process described above.

One mark is available for clarity of communication throughout this question. 

Describe the process of crossing over in prophase I.

Compare and contrast meiosis II and mitosis.

Draw a diagram to show how chiasmata are formed.