DNA screening has become routine for couples at risk

Over the past couple of decades genetic testing has gone through a period of dramatic development from an era when it was a truly experimental technique to one where it is almost routine for couples at risk of passing on inherited defects in their DNA.

There are more than 4,000 single-gene disorders that have been located or "mapped" to a precise position on one of the 23 pairs of human chromosomes. Many of the mutations leading to genetic diseases are now known in exact detail in terms of aberrant DNA sequences.

It is now technically feasible to test embryos or foetuses for scores of the more common genetic disorders - and many less common ones too.

But it was not always like this. It was in the early 1980s that scientists first came close to identifying the precise location of the gene for Huntington's disease, a terrible inherited disorder that strikes in middle age.

It led to the realisation that it might soon be possible to develop a test for it. Medical ethicists were quick to point to the immense problems that came with such a medical development.

The disorder epitomised the ethical quagmire we were getting into. First and foremost, there was and still is no cure for Huntington's. A test that could tell you that you carried the mutation would merely let you know that you were about to succumb to a quite horrendous death.

Then there were the ethical implications of a prenatal test and the offer of terminating an affected foetus. Leaving aside the morality of abortion, such a test raised different problems. A foetus carrying the Huntington's mutation must have inherited it from its parents, who may or may not know that they too were carriers and hence soon-to-be victims.

For these reasons, many people with a family history of Huntington's have declined to take the test. This is why Huntington's represents the sharp end of the difficult ethics of gene testing.

But for many other gene disorders, the issues are more clearcut, the benefits are more obvious and hence the take-up has been greater. It has mostly been by parents who already have one child affected by an inherited disorder.

Prenatal testing and termination is how most parents achieve the birth of an unaffected child. Not all however, find this acceptable, and for others it is not an option for medical reasons.

This is why pre-implantation genetic diagnosis - testing an IVF embryo before implantation into the womb - has given fresh hope. The latest development - pre-implantation genetic haplotyping - makes it easier, more accurate and more powerful.

It also for the first time gives women with "sex-linked" disorders the chance of choosing unaffected sons, and even healthy, non-carrier daughters who are free of the gene mutation.