Rotifers are multicellular, aquatic animals that range in size from 50 μm to 3 mm. Their reproduction can be either asexual, resulting in the production of genetically identical females, or sexual, resulting in the production of eggs that can remain dormant for many years. A representation of a rotifer life cycle is shown in the diagram below.
Annotate the diagram as follows:
i)
Use the letter A to indicate one location within the asexual phase where mitosis is occurring.
[1 mark]
ii)
Use the letter B to indicate one location within the sexual phase where mitosis is occurring.
[1 mark]
iii)
Use the letter C to indicate two locations where meiosis is occurring.
[1 mark]
iv)
Use the letter D to indicate one location where fertilisation is occurring.
Rotifers mainly reproduce asexually, switching to sexual reproduction for brief periods. The X in the diagram in part a) indicates the occurrence of a stimulus that shifts the rotifers from asexual to sexual reproduction. This stimulus could be a change in the environmental conditions, such as a drought that reduces the size of their habitat.
Use the information provided here and in part a) to explain why it is advantageous to rotifers to switch to sexual reproduction when environmental conditions change.
Polyploidy is a condition in which cells have a chromosome number that is greater than the normal diploid (2n) number. Polyploidy is considered to be a useful characteristic in crop plants as it gives rise to bigger plant organs and provides cells which contain a larger variety of alleles for breeding programmes. While polyploidy can occur naturally in plants, it can also be induced artificially using a chemical called colchicine. Colchicine works by preventing the formation of the microtubules that make up the spindle fibres inside cells.
Suggest how colchicine gives rise to tetraploid (4n) cells after mitosis in plants.
Plant scientists have been able to breed seedless watermelons by crossing tetraploid watermelons with regular, diploid plants to produce infertile, triploid (3n) plants. The diagram below illustrates this process. Note that diploid watermelons contain 22 chromosomes.
Suggest why the offspring of the tetraploid-diploid cross are infertile.
For the triploid offspring shown in part b) to start producing fruits, the triploid plants must be pollinated with pollen from diploid plants. Pollination involves the transfer of a plant gamete, pollen, from the flower of one plant to the flower of another. While the diploid pollen does not fertilise the triploid plants (they are infertile), it does trigger fruit production, and the resulting watermelon fruits are seedless. The graphic below shows a recommended planting plan for farmers that will ensure pollination of the triploid plants with pollen from diploid plants.
i)
State why diploid plants need to be used for pollination rather than other triploid plants.
[1 mark]
ii)
Suggest two disadvantages to farmers of the method described here and in part b) for growing seedless watermelons.
A watermelon variety that naturally produces fewer seeds has been identified by scientists, and observation of its cells indicates that an event known as reciprocal translocation of chromosomes occurs in the cells of the watermelon variety. Reciprocal translocation of chromosomes involves the exchange of entire sections of chromosomes between non-homologous chromosomes during meiosis.
i)
Contrast reciprocal translocation of chromosomes and crossing over with each other.
[1 mark]
ii)
Suggest how reciprocal translocation of chromosomes could result in a watermelon plant that produces fruits containing fewer seeds.
Calculate the number of different combinations of chromosomes that could be generated when this cell divides by meiosis.
[1 mark]
ii)
Explain why the number calculated in part i) is not a true representation of the amount of genetic variation that can be generated from this cell during sexual reproduction.
Edwards syndrome is a rare but serious condition that influences birth weight and development. Death rates during infancy are high. The image below shows the karyogram of an individual with Edwards syndrome.
i)
Use the karyogram to suggest the cause of Edwards syndrome.
[1 mark]
ii)
Describe the events that have led to the feature noted in part i).
One of the life-threatening complications that babies with Edwards syndrome can be born with is a structural abnormality in the heart. The image below shows a heart abnormality that can be seen in babies born with Edwards syndrome.
Explain why the abnormality shown in the image can be life-threatening.
