A species of ground beetle has 12 chromosomes, arranged in six pairs, in each body cell. A scientist compares two models for producing gametes over two generations. The chromosome number shown for each gamete applies to both male and female gametes. A zygote develops into an adult in the next generation.
| Model | Generation producing the gametes | Chromosomes in each body cell | Chromosomes in each gamete |
|---|---|---|---|
| A | 1 | 12 | 6 |
| A | 2 | 12 | 6 |
| B | 1 | 12 | 12 |
| B | 2 | 24 | 24 |
Explain why model A, but not model B, would keep the chromosome number at 12 from one generation to the next. Use the data to include the chromosome number expected in the zygote formed by generation 2 in each model.
Two offspring, R and S, are produced by the same female parent and the same male parent. The table shows the origins of the chromosomes in the gametes that formed each offspring.
| Offspring | Chromosomes in the egg that the female parent inherited from her mother | Chromosomes in the egg that the female parent inherited from her father | Chromosomes in the sperm that the male parent inherited from his mother | Chromosomes in the sperm that the male parent inherited from his father |
|---|---|---|---|---|
| R | 2 | 4 | 5 | 1 |
| S | 4 | 2 | 3 | 3 |
Explain why R and S have different genetic make-ups. Use the data in your answer.
92 exam-style questions on Edexcel GCSE Biology 3.1 Reproduction and meiosis, covering 3.1.1 Advantages and disadvantages of reproduction and 3.1.2 Role of meiotic cell division. Each one has a worked solution and a mark scheme showing where the marks go.