Genetics and Evolution
Genetics and Evolution
This domain tests inheritance, Mendel’s laws, chromosomes and sex determination, variation, natural selection and evidence for evolution.
Core rules
- Genes are segments of DNA on chromosomes; alleles are alternative forms of a gene.
- Dominant allele is expressed in the heterozygote; recessive is expressed only when homozygous.
- Mendel’s law of segregation: allele pairs separate during gamete formation. Law of independent assortment: different gene pairs assort independently when on different chromosomes.
- Genotype is genetic makeup; phenotype is observable trait.
- Human sex determination: XX female, XY male; the father’s sperm determines the sex of the child.
- Evolution is change in inherited characteristics of populations over generations; natural selection favours traits that improve survival and reproduction.
- Evidence includes fossils, homologous organs, vestigial organs and molecular similarities.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt. Gametes are T and t from each parent. Punnett square gives TT, Tt, Tt, tt. Phenotypic ratio = 3 tall : 1 dwarf. Genotypic ratio = 1 TT : 2 Tt : 1 tt. The dwarf trait reappears because it is recessive and was hidden in the heterozygous parents.
Traps
Dominant does not mean more common or better. Acquired traits are not inherited. A 3:1 ratio is statistical, not guaranteed in every small family. Evolution acts on populations, not individuals, and is not a ladder of progress.