1. Which process halves chromosome number for sexual reproduction?
- Meiosis
- Fertilisation
- Ordinary mitosis
Check the answer and reasoning
Meiosis
Meiosis produces haploid cells; fertilisation combines two haploid sets.
Biology / IGCSE
Inheritance, variation and selection
Genes are DNA regions with biological functions; alleles are alternative versions.
Learning objectives
- Distinguish genes, alleles and chromosomes.
- Use a simple genetic cross.
- Explain natural selection without purposeful mutation.
The big idea
Genes are DNA regions with biological functions; alleles are alternative versions. Genotype describes inherited variants and phenotype reflects genotype interacting with environment. Meiosis produces haploid gametes and fertilisation restores the diploid number.
Variation may be genetic, environmental or both. Natural selection changes allele frequencies when inherited differences affect reproductive success; organisms do not acquire needed mutations deliberately. Selective breeding is a human-directed choice of parents.
Worked example
For Aa × Aa with complete dominance, what fraction of offspring is expected to be aa?
Follow the solution
The equally likely combinations are AA, Aa, aA and aa; one of four is aa.
Investigate and explain
Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.
Related simplified PhET model. Explain what it represents and what the living system does differently.
Check your understanding
2. For Aa × Aa with complete dominance, what fraction of offspring is expected to be aa?
- 1/2
- 1/4
- 3/4
Check the answer and reasoning
1/4
The equally likely combinations are AA, Aa, aA and aa; one of four is aa.
3. Do antibiotics cause bacteria to deliberately develop the needed resistance?
- Yes
- Only after repeated use
- No
Check the answer and reasoning
No
Selection favours resistant variants already present or arising by mutation; variation is not generated with foresight.