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Genes affect our chances of having several common illnesses, like heart disease, asthma and diabetes but so do many other factors, such as diet and lifestyle.

Many genetic and non-genetic factors affect our health, but scientists don't yet know what they all are, or how they interact with each other.

Occasionally an error occurs during the division: for example, the egg or sperm might be missing a chromosome (22 chromosomes) or have an extra one (24 chromosomes), so at conception the baby has either too few (45) or too many (47) chromosomes. A well-known example of this type of genetic disorder is Down syndrome, where a person has 47 chromosomes rather than 46.

Over 1,000 known disorders are caused by chromosome abnormalities. A chromosome disorder means there is a change in either the structure or the number of chromosomes. This can happen in three main ways: The altered chromosome is passed from the parent to the child The abnormality happens when either the sperm or egg (germ cells) is created Soon after conception.

Around 6,000 known genetic disorders are caused by inheriting an altered gene.