Heredity, Evolution and Animal Classification
Heredity, Evolution and Animal Classification tests Mendelian inheritance, sex determination, evidence for evolution and the classification hierarchy. Expect genetic crosses, terminology and grouping of organisms.
Core rules
- Genes occur in pairs; alleles are alternative forms. Dominant alleles are expressed in the heterozygote; recessive alleles appear only when homozygous.
- Genotype is the allele combination; phenotype is the observable trait. A monohybrid cross of Tt x Tt gives genotypic ratio 1 TT : 2 Tt : 1 tt and phenotypic ratio 3 tall : 1 dwarf.
- In humans, females are XX and males XY; the father's sperm determines the sex of the child.
- Evolution is change in inherited characteristics over generations. Evidence includes fossils, homologous structures and analogous structures. Natural selection favours useful variations.
- Classification hierarchy: kingdom, phylum, class, order, family, genus, species. Related organisms share more levels.
Worked example
Cross two heterozygous tall pea plants, Tt x Tt. Each parent can give T or t. The Punnett square gives TT, Tt, Tt and tt. TT and Tt are tall because T is dominant; tt is dwarf. Thus the phenotypic ratio is 3 tall : 1 dwarf, while the genotypic ratio is 1:2:1. The reasoning depends on separating alleles during gamete formation.
Traps
Dominant does not mean more common in a population. Genotype and phenotype are not interchangeable. Acquired characters are not inherited in the Mendelian sense. Homologous structures share origin, while analogous structures share function. Do not reverse the classification hierarchy.