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.
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.
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.
i)
Identify the type of cell division by which B. pheonicis produces eggs.
[1 mark]
ii)
Suggest why scientists might think that diploidy provides an evolutionary advantage.
[1 mark]
iii)
B. phoenicisis 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.
While studying the cells of B. phoenicis scientists discovered that the cells of all individuals contained bacteria. Treatment with antibiotics caused female B. phoenicisto lay eggs that hatched into haploid males.
i)
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 mark]
ii)
Suggest why treatment with antibiotics might have enabled the production of male offspring.
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.
Characteristics
Number of offspring
Short legs and brown eyes
545
Long legs and red eyes
182
Short legs and red eyes
14
Long legs and brown eyes
12
Calculate the offspring ratios for the cross shown. Give your answers to the nearest whole number.
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).
