Question 1
Which statement explains why brothers and sisters are not genetically identical?
- They are made by mitosis
- Fertilisation is random, so any sperm can fuse with any egg
- Each parent only passes on one chromosome
- They have different numbers of chromosomes
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Answer: B (1)
Question 2
State two uses of mitosis in a human.
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- growth (1)
- repair (1)
- allow replacing damaged cells
- ignore asexual reproduction
Question 3
Compare mitosis and meiosis, in terms of the number of cells produced, the number of chromosomes in each human cell, and genetic variation.
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- mitosis produces two cells and meiosis produces four cells (1)
- mitosis: each cell has 46 chromosomes / diploid; meiosis: 23 chromosomes / haploid (1)
- mitosis produces cells that are genetically identical (1)
- meiosis produces cells that are genetically different (1)
Question 4
Two identical twins were brought up in different places. One twin is 5 kg heavier than the other. (a) Suggest why the twins have the same eye colour. (1) (b) Suggest why the twins have different body masses. (2)
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- they have the same alleles / genes / are genetically identical (1)
- difference caused by the environment (1)
- for example different diet / different amount of exercise (1)
Question 5
State what is meant by the term mutation.
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- a (rare,) random change (1)
- in genetic material / DNA that can be inherited (1)
- allow change in the base sequence of DNA for the second marking point
Question 6
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels mean(a) Name one factor that can increase the rate of mutation. (1) (b) Explain how a mutation in a gene can change the phenotype. (3) (c) State what effect most mutations have on the phenotype. (1)
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- ionising radiation / gamma rays / X-rays / ultraviolet rays / chemicals in tobacco (1)
- mutation changes the base sequence of the gene (1)
- this changes the sequence of amino acids in the protein (1)
- the protein has a different shape / does not work, changing the phenotype (1)
- no effect (1)
- ignore harmful for (c)
Question 7
Some bacteria are resistant to an antibiotic. Explain how the number of resistant bacteria can increase in a population when the antibiotic is used, and explain why this makes infections difficult to control.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, logical explanation that links variation, selection, survival, inheritance and the increase in the resistant population, and explains why infections become difficult to control. |
| 2 | 3–4 | An explanation with some of the key steps in order, but with a missing link, or the effect on infection control is not explained. |
| 1 | 1–2 | A few relevant points, such as that some bacteria survive the antibiotic. Little or no use of the idea of natural selection. |
Indicative content
- a mutation produces an allele that gives resistance to the antibiotic (variation)
- the antibiotic kills non-resistant bacteria; the resistant bacteria survive (selection pressure)
- the resistant bacteria reproduce, with less competition, and pass on the resistance allele
- over time the proportion of resistant bacteria increases
- the antibiotic no longer works against the infection, so it is difficult to control / can spread to other people