An "unwelcome" development say scientists

But the technique has been criticised as unethical by some scientists and would be illegal in many countries, including the United Kingdom.

Genetic fingerprint tests on two one-year-old children confirm that they contain a small quantity of additional genes not inherited from either parent.

The additional genes were taken from a healthy donor and used to overcome the infertility problems of the mother.

Germline modification

The additional genes that the children carry have altered their germline, or their collection of genes that they will pass on to their offspring.

Altering the germline is something that the vast majority of scientists deem unethical given the limitations of our knowledge.

It is illegal to do so in many countries and the US Government will not provide funds for any experiment that intentionally or unintentionally alters inherited genes.

There is no evidence that this technique is worth doing

Lord Winston

The children were born following a technique called ooplasmic transfer. This involves taking some of the contents of a donor cell and injecting it into the egg cell of a woman with infertility problems.

The researchers, at the Institute for Reproductive Medicine and Science of St Barnabas in New Jersey, US, believed that some women were infertile because of defects in their mitochondria.

These are tiny structures containing genes that float around inside the cell away from the cell's nucleus, where the vast majority of the genes reside. There can be as many as 100,000 of them floating in the cells cytoplasm.

Two mothers

They are essential to cellular energy production and scientists suspect they have many other important, but as yet unappreciated, roles.

Mitochondrial DNA is passed down from generation to generation along the maternal line.

The US researchers wanted to supplement a woman's defective mitochondria with healthy ones from a donor.

Having just tested the children born as a result of this procedure, the scientists have confirmed that the children's cells contain mitochondria, and hence genes, from two women as well as their fathers.

Writing in the journal Human Reproduction, the researchers say that this "is the first case of human germline genetic modification resulting in normal healthy children".

'Great reservations'

British experts have severely criticised the development.

Infertility pioneer Lord Winston of the Hammersmith Hospital in London told BBC News Online that he had great reservations about it.

"Regarding the treatment of the infertile, there is no evidence that this technique is worth doing," he said. "I am very surprised that it was even carried out at this stage. It would certainly not be allowed in Britain.

"There is no evidence that this is a possible valuable treatment for infertility," he added.

Lord Winston said that, although the number of additional genes involved was tiny, it was in principle the wrong thing to do.

The Human Fertilisation and Embryology Authority (HFEA), the body that monitors and regulates UK reproductive medical activities, told BBC News Online that it was aware of the technique but had decided not to allow it in the UK because of its uncertainties and the possible alteration of the human germline.

'Back door'

The HFEA said it was an unwelcome development that "adds additional concern" to their worries. US researchers have also criticised the production of genetically altered children.

Eric Juengst, of Case Western Reserve University, said: "It should trouble those committed to transparent public conversation about the prospect of using 'reprogenetic' technologies to shape future children."

The US Government Recombinant DNA Advisory Committee told BBC News Online that the researchers had carried out this work without government money.

The committee said that in no circumstances would it consider any request for government funds that would result in modification of the human germline.

Professor Joe Cummins, of the University of Western Ontario in Canada, told BBC News Online: "Now is not the time to bring in human germline gene therapy through the back door."