Biology - cells, reproduction, and heredity
What the exam tests
Cell structure, mitosis and meiosis, reproduction, DNA and inheritance patterns. Expect Punnett squares and explanations of chromosome behaviour.
Core rules
- Plant cells have a cell wall, chloroplasts and a large vacuole; animal cells do not.
- Mitosis produces two genetically identical diploid cells for growth and repair.
- Meiosis produces four haploid gametes with half the chromosome number and introduces variation by independent assortment and crossing over.
- Fertilisation restores the diploid number. In humans there are 23 pairs; sex chromosomes are XX female and XY male.
- Genes occur in pairs; a dominant allele is expressed even with one copy, recessive only with two.
- Aa × Aa gives genotype ratio 1 AA : 2 Aa : 1 aa and phenotype ratio 3 dominant : 1 recessive.
Worked example
Two heterozygous tall pea plants (Tt) are crossed. Gametes are T or t. The Punnett square gives TT, Tt, Tt, tt. So 3/4 show tall and 1/4 short. If 400 offspring are produced, about 300 are expected tall. The recessive short phenotype appears only in tt.
Traps
- Meiosis halves chromosomes; mitosis keeps the number constant.
- Dominant does not mean more common in the population.
- A recessive allele is not “weaker”; it is simply not expressed when a dominant allele is present.
- Body cells are diploid; gametes are haploid.