Heredity

Heredity is the passing of traits from parents to offspring through genes. Variation is the differences between individuals — created mainly during sexual reproduction. Variation is the raw material that helps a species survive changing conditions.

Mendel crossed pea plants and found that one form of a trait (the dominant, T) masks the other (the recessive, t). Crossing two hybrids (Tt × Tt) gives a 3:1 ratio of tall to short. Build the cross on a Punnett square:

Try the interactive at the top of the page — fill a Punnett square and watch the 3 : 1 ratio appear.

When two traits are followed together (e.g. seed shape and colour), they are inherited independently. A dihybrid cross of two double-hybrids gives the famous 9 : 3 : 3 : 1 ratio in F₂ — proving the two traits sort separately.

Genes sit on chromosomes (made of DNA). Different versions of a gene are alleles (T and t). The combination an organism carries is its genotype (e.g. Tt); the trait you actually see is its phenotype (e.g. tall). One dominant allele is enough to show the dominant trait.

Humans have a pair of sex chromosomes: females are XX, males are XY. The mother always passes an X; the father passes X or Y — so the father determines the sex of the child.