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例题 1 · A-Level · Meiosis, Genetic Variation, Evolution
The diagram shows stages of meiosis in a human testis. Each circle represents a cell. [See Page 1 image]
(a) In each empty circle, write the number of chromosomes that would be found in the cell. (1 mark)
(b) Describe two ways in which meiosis contributes to genetic variation. (2 marks)
(c) Explain the importance of genetic variation in the process of evolution. (2 marks)
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(a) The first empty circle (after Meiosis I) should contain ’23’. The four empty circles (after Meiosis II) should each contain ’23’. (1 mark for all correct)
(b) Two from:
1. Crossing over (of chromatids) / exchange of alleles between homologous chromosomes during prophase I.
2. Independent assortment / random segregation of homologous chromosomes in metaphase I / anaphase I.
3. Random fusion of gametes (fertilisation) – though this is not …
例题 2 · A-Level · Meiosis, Polyploidy, Speciation
An old form of wheat, emmer wheat (Triticum turgidum), has a diploid chromosome number of 28 (2n = 28). A wild wheat, einkorn wheat (Triticum tauschii), has a diploid chromosome number of 14 (2n = 14). These two species occasionally crossed and produced sterile hybrid plants. Due to an error during cell division, one of these hybrid plants formed male and female gametes with 21 chromosomes. Fusion of these gametes resulted in viable offspring. These plants were a new spe…
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(i) 21 chromosomes (haploid from each parent: 14 + 7 = 21). (1 mark)
(ii) The original hybrid had 21 chromosomes (one set from each parent). These chromosomes are not homologous, so they cannot pair up properly during meiosis I, leading to non-disjunction and inviable gametes – hence sterility. Triticum aestivum (2n = 42) is a result of chromosome doubling (allopolyploidy). It has two sets of chromosomes from each parent (14 + 7 = 21, doubled t…
例题 3 · A-Level · Variation, Meiosis, Statistics
The graph shows the variation in length of 86 Atlantic salmon. [See Page 4 image]
(a) (i) What type of variation is shown by the lengths of the salmon in group R? Give the evidence to support your answer. (1 mark)
(ii) Give two possible causes of this variation that result from meiosis during gamete formation. (2 marks)
(b) When comparing variation in size between two groups of organisms, it is often considered more useful to compare standard deviations…
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(a)(i) Continuous variation. Evidence: the graph shows a bell-shaped curve / normal distribution, with a range of intermediate values and no distinct categories. (1 mark)
(a)(ii) Two from:
1. Independent assortment of chromosomes.
2. Crossing over between homologous chromosomes.
3. Random fusion of gametes (fertilisation). (2 marks, 1 each)
(b) Range only considers the extreme values (maximum and minimum) and is affect…