Biology - Cells, Reproduction, Heredity, Evolution
This domain tests cell structure, how organisms reproduce, how traits pass from parents to offspring, and how populations change over time. Expect diagram-free descriptions, Punnett squares, and explanations of variation and natural selection.
Cells and cell division
Plant cells have a cell wall, chloroplasts, and a large vacuole; animal cells do not. Both have a nucleus, cytoplasm, and cell membrane. Mitosis produces genetically identical somatic cells and maintains chromosome number. Meiosis produces gametes with half the chromosome number; fertilisation restores the full number. Crossing over in meiosis increases variation.
Reproduction and heredity
Asexual reproduction produces clones; sexual reproduction combines gametes and creates variation. Genes are DNA segments on chromosomes. In complete dominance, the dominant allele is expressed even when one copy is present. Genotype is the allele pair; phenotype is the observable trait. A Punnett square predicts offspring ratios from parental gametes.
Evolution
Individuals vary; some variants survive and reproduce better in a given environment. Natural selection increases favourable traits in the population over generations. Fossils, homologous structures, and DNA similarities provide evidence. Evolution acts on populations, not on individuals during their lifetime.
Worked example
Cross Aa × Aa. Each parent makes gametes A and a. The square gives AA, Aa, Aa, aa. Genotype ratio 1 : 2 : 1; phenotype ratio 3 dominant : 1 recessive. The chance of a recessive offspring is 1/4.
Traps
Mitosis keeps the chromosome number; meiosis halves it. “Dominant” means expressed, not more common. Acquired traits are not inherited. In humans, meiosis in males gives four sperm; in females, one egg and polar bodies. Do not claim that an individual “evolves” during its life.