Heredity and Evolution
Heredity and Evolution tests inheritance of traits, Mendelian ratios, sex determination and the evidence for evolution. Genetic cross problems are common.
Core ideas
- Genes are units of inheritance; alleles are alternative forms. Dominant alleles express in the heterozygote; recessive alleles express only when homozygous.
- Genotype is the allele combination; phenotype is the visible trait.
- Mendel's monohybrid cross of tall TT with short tt gives all Tt tall in F1 and a 3:1 phenotypic ratio in F2.
- Sex in humans is determined by sex chromosomes: XX female, XY male. The father's sperm determines the sex of the child.
- Variation arises from sexual reproduction and mutation. Natural selection favours traits that improve survival and reproduction.
- Evidence for evolution includes fossils, homologous organs and similarities in DNA.
Worked example
Cross a heterozygous tall pea plant Tt with a short plant tt. Punnett square: Tt x tt gives Tt, Tt, tt, tt. So 50% are Tt tall and 50% are tt short. Reasoning: the short parent contributes only t; half the offspring receive T from the tall parent and show the dominant tall phenotype.
Traps that cost marks
A 3:1 ratio is phenotypic in F2 of a monohybrid cross, not genotypic; the genotypic ratio is 1:2:1. Acquired traits are not inherited through genes. Do not say the mother determines the child's sex. Dominant does not mean more common.