Genes make up the blueprint for our bodies, governing factors such as growth, development and functioning. Almost every cell in the human body contains a copy of the blueprint, stored inside a special sac called the nucleus. The estimated 30,000 genes are beaded along tightly bundled strands of a chemical substance called deoxyribonucleic acid (DNA). DNA profiling is a way of establishing identity and is used in a variety of ways, such as finding out whether twins are fraternal or identical. DNA samples are usually obtained from blood.

While DNA contains material common to all humans, some portions are unique to each individual. These portions, or regions, contain two genetic types (alleles) that are inherited from the person’s mother and father. A person’s DNA profile is made by investigating a number of these regions. In a paternity test, for example, the mother’s DNA profile is compared with the child’s to find which half was passed on by the mother. The other half of the child’s DNA is then compared with the alleged father’s DNA profile. If they don’t match, the ‘father’ is excluded, which means he isn’t the father of that child. If the DNA profiles match, the ‘father’ is not excluded - which means there is a high probability (more than 99 per cent) that he is the father. DNA tests such as this can’t offer 100 per cent proof.

Some of the advantages of DNA profiling include:

* DNA tests can be applied to any human sample that contains cells with nuclei, such as saliva, semen, urine and hair.

* DNA is hardy, and resists degeneration even after contamination with chemicals or bacteria.

* The ability of DNA profiling to exclude a suspect means the police are able to confidently drop that line of enquiry and continue their investigation down other avenues. Between 1989 and 1996, the FBI used genetic testing in about 10,000 sexual assault cases; in 2,000 of those cases, the prime suspect was discovered to have not committed the crime. Without genetic testing, it can be assumed that some of these men would have been convicted. In fact, many prison inmates have appealed their conviction after spending years in jail, and have been discovered to be innocent.

Contrary to public belief, DNA profiling isn’t infallible. Critics point out various problems and limitations, including:

* New DNA profiling technologies can give incorrect results, due to errors such as cross-contamination of samples.

* Older DNA profiling technologies are more prone to errors, which could give false-negative or false-positive results.

* DNA profiles can only offer statistical probability (for example, one in a million), rather than absolute certainty.