The gene instructions of any two people differ by about one DNA base pair in every thousand, or 0.1% of the entire manual. Human Genome Project scientists based most of their studies on DNA from six different individuals. This has provided a 'reference manual' that will be over 99% accurate for everyone. Other scientists are looking for all the different versions of every human gene, to uncover the many variations.
What are alleles?
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's disease, heart disease or asthma. Other alleles may even affect your behaviour, for example your appetite, or how anxious you are.