Chromosomes are made from DNA. Genes are short sections of DNA. DNA carries genetic code that determines characteristics of a living organism.

Genetic diagrams show how characteristics are inherited. Alleles can be recessive, dominant or codominant genes.

Sex is controlled in humans by the X and Y chromosomes. Genetic screening involves testing people or groups of people for the presence of a particular allele or other genetic abnormality.

Organisms of the same species vary in many ways. There are two types of variation – continuous and discontinuous. Natural selection explains how evolution occurs.