Woman with brown eyes, May 2000.
© Science Museum / Science & Society Picture Library
Chromosomes contain about 30,000 genes: instructions that determine body characteristics. Genes contain two sets of instructions: one set inherited from each parent. Most of these genes have several versions, called alleles. For example, different alleles of genes for eye colour give rise to blue or brown eyes. Although particular alleles may be common to many people, no two people have exactly the same combination of alleles, unles they are identical twins.