Thomas' DNA Some time after Thomas was born, his parents became worried that he was not progressing as fast as other children. He was subsequently recognised to have a severe learning disability and a paediatrician suggested genetic testing. This was carried out on a DNA prepared from a small blood sample. Analysis by a geneticist showed that a fragment of DNA on his X chromosome was considerably longer than usual. This genetic condition is called Fragile X syndrome and is associated with severe learning difficulties. The problem usually has its effects in males (who only have one X chromosome), but is usually carried by females (who have two). It was a relief to Thomas' family to know exactly what was causing his condition. Equally important, testing was good news for his sister Sarah, since it showed that she did not have the abnormality and was in no danger of passing it on to her children when she started her own family.

Over 60,000 genes contribute to the make-up of every one of us and very soon the Human Genome Project will have revealed their full sequence. There are many minor differences between individuals, but some changes have profound medical consequences. In the wrong place, an alteration of only one part in three billion can lead to such serious disorders as muscular dystrophy or cancer. Inside our cells, genes and their attendant proteins are organised into large pieces called chromosomes. Alterations in the number or arrangement of chromosomes can also have serious effects, as in Down's syndrome, where there is an extra chromosome 21.

These days geneticists specialise: molecular geneticists study changes in the gene sequence, while cytogeneticists examine the structure of the chromosomes. Information about genetic changes can be very useful to families. It may confirm a diagnosis, detect individuals who carry a particular disease, show that other family members are at no risk or predict the outcome of a pregnancy. As well as identifying the changes, geneticists play a vital role in interpreting the findings and explaining them to the people concerned.

Find out more about a career in Genetics