MPs have an important free vote in the House of Commons today on the divisive issue of mitochondrial donation, which would allow the creation of IVF babies with DNA from three different people.
The MPs have come under enormous pressure from scientists and charities to support the historic and controversial amendment to the Human Fertilisation and Embryology Act.
Britain will become the first country ever to allow the procedures if MPs vote yes. The amendment is aimed at preventing serious or deadly genetic disease being passed on to the child.
But at least 60 MPs are said to have indicated their opposition to the change, which has also been fiercely opposed by Church figures and those questioning whether the procedures could pave the way for further genetic modification in the future.
Mitochondrial donation could potentially help more than 2,400 women in the UK, according to new research.
All are said to be at risk of transmitting harmful DNA mutations in the mitochondria, tiny rod-like power plants in cells, onto their children and future generations.
The new treatments would allow a child to receive normal "nuclear" DNA from its mother and father, but also a small amount of healthy mitochondrial DNA (mDNA) from a woman donor.
Currently they are unlawful because any tampering with inherited "germline" DNA in eggs or sperm is banned.
Dr Gillian Lockwood, a reproductive ethicist, told the BBC that despite being widely referred to as 'three-parent babies', "in fact it is 2.001-parent IVF".
"Less than a tenth of one per cent of the genome is actually going to be affected. It is not part of what makes us genetically who we are," she said.
"It doesn't affect height, eye colour, intelligence, musicality. It simply allows the batteries to work properly."
There are the main arguments for and against the legal change from the Press Association.
FOR: Supporters of the amendment, such as IVF pioneer and broadcaster Lord Winston, argue that it would be immoral not to take advantage of a technique that could prevent devastating and potentially lethal diseases. They point out that even though no medical technique has zero risk, three in-depth reviews by experts have concluded that mitochondrial donation is "not unsafe".
They also stress that even if the change goes through, it does not follow that women will automatically be treated. It will be up to the fertility regulator, the Human Fertilisation and Embryology Authority (HFEA) to judge each application on its merits and decide whether or not it should be approved. Licences permitting mitochondrial donation will only be granted if the Authority is satisfied that women or their babies will not be put in harms way.
Mitochondrial DNA only accounts for 0.1 per cent of a person's total DNA, and affects metabolism but not individual characteristics such as facial features and eye colour, which are determined by nuclear DNA. Changing the law to allow mitochondrial donation in no way affects the firm ban on altering nuclear DNA or reproductive human cloning, it is claimed.
AGAINST: Critics, such as the group Human Genetics Alert and Roman Catholic Church in England and Wales, maintain that despite the scientific reviews, the decision to legalise mitochondrial donation is being taken too hastily.
One of the chief arguments against the vote being taken today is that the procedures have so far only been tested in the laboratory and on animals. No clinical trial has taken place to show conclusively that the treatments are safe in humans. The first women to be treated will therefore be human guinea pigs.
Opponents point out that potentially serious problems might only arise once the procedures are carried out for real. For instance, it might turn out that mDNA affects personal traits as well as metabolism in unforeseen ways.
Epigenetic effects are seen as another hazard. These are environmental influences that can affect the way genes work, either switching them on or off to produce permanent changes.
More generally, critics say mitochondrial donation is a step too far, and crosses an ethical boundary. It could mark the start of a slippery slope that leads to the creation of "designer babies" whose genes are tweaked to provide desirable characteristics. Rather than risk a future Brave New World of human genetic engineering and eugenics, a line should be drawn in the sand now, it is claimed.
Additional reporting by PA