Variations on a theme

The identification of different versions of our genes, called alleles, is a crucial part of modern genetics research. For example, some of these different alleles influence your height and the colour of your skin. Some can make you react differently to medicines such as codeine. Some can increase or decrease your susceptibility to common illnesses such as Alzheimer disease, heart disease or asthma. Other alleles may even affect your behaviour, for example your appetite, or how anxious you are.