Genetics & Inheritance
Reproduction and Inheritance · 6 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 17% of your exam marks.
Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.
Variation between individuals comes from two main sources: genes and environment. Most genetic variation in sexually-reproducing species comes from three particular events.
Mitosis vs meiosis
The body uses two completely different kinds of cell division:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Production of gametes |
| Number of daughter cells | 2 | 4 |
| Chromosome number in daughter cells | Same as parent (diploid) | Half of parent (haploid) |
| Genetic similarity of daughter cells to parent | Identical (clones) | Different (mix of parents' alleles) |
| Where it happens | Almost everywhere in the body | Only in the testes (men) and ovaries (women) |
How meiosis generates variation
Meiosis produces gametes that each carry only half the chromosomes (just one of each homologous pair). Critically, which chromosome from each pair ends up in which gamete is random. With 23 pairs of chromosomes, that's 2^23 ≈ 8 million different combinations of chromosomes per gamete, so a single person can produce a huge number of genetically different gametes.
Random fertilisation
When a sperm and an egg meet at fertilisation, the choice of which sperm fertilises which egg is also random. Combined with the meiosis-generated variation between gametes, this means that every zygote (and so every offspring) has a unique genetic combination.
This is why brothers and sisters from the same parents are different from one another (except for identical twins, who develop from a single zygote that has split, and so share the same genes).