• Heredity is determined by a molecule called DNA. The structure and mechanisms of action of DNA are understood in detail.

• The DNA molecule can be divided into genes that encode for proteins. The code in the DNA is read off to produce a protein.

• DNA is carried on structures called chromosomes. All diploid organisms have a double set of chromosomes (one inherited from its father, the other from its mother) and therefore two sets of all its genes. An individual's particular combination of genes is called its genotype.

• When two individuals mate together, the proportions of genotypes in their offspring appear in pedictable Mendelian ratios. The exact ratios depend on the genotypes of their parents.

• Different genes are preserved over the generations under Mendelian heredity, and this enables natural selection to operate. Before Darwin it was wrongly thought that genes blended rather than being preserved; if genes did blend, natural selection would be much less powerful than under Mendelian heredity.