The Big Question: Where do we draw the limits to the genetic screening of embryos?

What is all the fuss about?

Preimplantation Genetic Diagnosis (PGD) is a way of screening embryos created through "test tube" fertility treatment before they are implanted in the womb. The fertilised embryo is tested at around three days old and screened for a known gene mutation that can cause an inherited and fatal condition, such as cystic fibrosis or Huntingdon's disease. The process allows doctors to select only those embryos without the gene mutation, thus ensuring a baby will not be born with that condition. The technique is strictly regulated by the UK regulatory body, the Human Fertilisation and Embryology Authority (HFEA). It is used around 200 times a year for patients with a family history of a certain condition. PGD can also be used to create "saviour siblings" for families where a child has a certain life-threatening illness and needs a bone marrow or stem-cell transplant from a genetic match. In these cases, embryos are screened to ensure they do not have the same condition and can act as a donor.

The watchdog has extended embryo screening. Why should we care?

It represents a marked shift in the range of conditions for which PGD is considered ethically acceptable. Until now, PGD has only been used for a limited number of conditions that a baby will either be born with - such as cystic fibrosis - or has an extremely high (more than 90 cent chance) of developing in childhood. Last year, the HFEA approved screening for genes known to cause a rare form of eye cancer called retinoblastima and an inherited type of bowel cancer - familial adenomatous polyposis. Both cancers occur early in life and have what is known as "high penetrance" - meaning that children born with the gene mutation have a 90 per cent chance of developing the disease.

The board of the HFEA has now agreed to approve the use of PGD for testing for the BRCA1 and BRCA2 gene mutations, which are linked to breast and ovarian cancer, as well as a fault in the HNPCC gene known to cause colon cancer.

The controversy arises because these diseases have "low penetrance". A baby girl born with BRCA1 or BRCA2 mutations has an 80 per cent chance of developing breast cancer at some point in her life - but the disease is much more likely to occur later in life rather than in the woman's twenties or thirties.

The HNPCC gene fault carries an 80 per cent risk of colon cancer while women with BRCA1 mutations have only a 40 per cent chance of developing ovarian cancer. All the diseases are also highly treatable.

What is the case for extending PGD?

Doctors who advocate its use believe that "selecting out" embryos before they are implanted is far less stressful - and less ethically problematic - than finding out during prenatal screening that an unborn child has a serious condition such as Down's syndrome, and then having to make a decision about abortion.

Women who are carriers of BRCA1 or BRCA2 mutations point out that they have to go through life knowing they have a very high chance of developing a devastating disease. Some opt for the removal of their breasts in early life simply to avoid such a waiting game, and say that PGD would allow them to prevent their children from having to go through the same ordeals.

A less emotive reason put forward by some medics is that screening out "sick" embryos will help to cut the number of people developing such cancers - and therefore reduce the costs of treating and caring for them - in the future.

So why the opposition?

Critics say that PGD is eugenics by the back door, where only embryos considered to be "normal" are allowed to develop. Some disabled people believe embryo screening is based on misguided assumptions about the quality of life of people with a condition such as cystic fibrosis. If we go down this road, they ask, will embryos eventually be discarded because they have an asthma gene or susceptibility to a similar condition that is potentially fatal and debilitating but by no means certain to limit a person's life?

Josephine Quintavalle, of the lobby group Comment on Reproductive Ethics, argues: "PGD is nothing more than a weapon of destruction, aimed at the ruthless elimination of any embryo which does not conform to eugenic standards of perfection."

There are also fears that PGD could be used to select the sex of a baby, their hair colour or potential IQ levels.

Opponents are also concerned at the psychological stress that could be placed on "saviour siblings" created by PGD if their sick brother or sister is not cured by the treatment they receive. And even if an embryo is selected because it does not have BRCA1 or another specific gene mutation, it may well carry another one - not yet discovered by scientists - that means the resulting child will still die early from another disease. In the end, they say, the bald truth is that PGD is not a guarantee of a long and healthy life.

What happens in other countries?

Britain has one of the most tightly-controlled fertility laws in the world, and some couples have sought treatment abroad because PGD is more freely available. Fertility clinics in India and China are known to offer PGD for couples wanting to choose the sex of their unborn child.

In 2002 a lesbian couple in the US, who were both deaf, used PGD to ensure they had a deaf rather than a hearing child. An American fertility clinic is currently conducting a controversial trial on sex selection, allowing couples to choose the gender of their child through PGD and then exploring the reasons behind the decision and its effects. Other European countries and Australia also offer PGD, but mainly for inherited diseases with a high risk of mortality.

What does the future hold?

PGD is currently only licensed for a handful of conditions, but scientists have the potential to test for around 200 gene mutations linked to a whole range of conditions. The HFEA is already looking at whether to approve the technique for diseases that have a penetrance of as low as 30 per cent. Some couples want the right to use PGD to ensure they do not have a child with autism. An even more sophisticated embryo-screening technique is being developed by British researchers. The procedure, known as multiple displacement amplification (MDA), could allow scientists to screen for more than 6,000 individual gene mutations.

The HFEA insists it frames its policies to reflect changing public attitudes, and errs on the side of caution rather than liberalisation. And research suggests that the public feels the same way - in a recent consultation on whether PGD should be allowed for sex selection, 80 per cent of people said they were opposed.

Are we on a slippery slope to eugenics?

Yes...

* PGD has already been used in questionable circumstances, such as choosing a deaf child, despite the supposed checks and balances

* Creating 'designer babies' smacks of eugenics and sends the wrong messages about disabilities and quality of life

* Scientists are 'playing God' - and we still don't know the long-term effects of testing embryos at such an early stage

No...

* This is a humane way of helping couples living with the reality or even just the threat of inherited, fatal disease

* PGD is restricted to a handful of conditions, and only a few clinics are licensed to perform the procedure

* Many people may know they have an increased chance of a certain disease but will not opt for PGD - it is simply giving them the choice