Another barrier to the age of the designer baby for all has been demolished. A child can now be selected before birth to become a "saviour sibling" - chosen from a group of embryos solely because his tissues match those of a sickly older brother or sister. The argument that invasive testing, involving the removal of one of eight cells from the three-day embryo, should only be undertaken if the potential new baby itself is at risk of serious inherited disease has been swept away.

Are we slithering down the slippery slope? Doctors involved in reproductive medicine deny it. What has happened is simply a logical piece of tidying up.

The cases of the Hashmis and the Whitakers make the point clearly. Why should Zain Hashmi be allowed a life-saving sibling, while Charlie Whitaker is not? It is medical hypocrisy not to permit both children a chance of life, said Simon Fishel of the Care fertility unit at the Park Hospital in Nottingham, who is treating the Hashmi family.

Look at what we do to embryos already - disposing of them and carrying out research on them is permitted in law, and so is the termination of pregnancy. Testing them for tissue type and selecting one embryo over another is a small price for the life of a child.

Ever since the birth of Louise Brown, the first test tube baby, there have been arguments, Dr Fishel said. "But abuses do not happen provided there is good regulation and strong licensing [of fertility clinics]. We will not go down the route of selecting for trivial or cosmetic characteristics."

But there is no suggestion that the scientific and technological advances, and with them the tough ethical dilemmas, are going to end here.

Mohammed Taranissi, the Whitakers' fertility doctor, who runs the Assisted Reproduction and Gynaecology Centre in London, said there was a further debate that we should be having now. It is possible to test embryos for the genes that will cause certain "late onset" diseases, such as a form of Alzheimer's which can occur in middle-age and some cancers. Doctors could identify and select embryos that would have a healthy childhood and youth, but are destined to die prematurely. "Is this something that we should do? That to me is a very important issue," said Mr Taranissi.

"This is going to be the future. I don't know myself what is right and what is wrong. It is something that needs to be put to a wider audience."

Already he knows of at least seven babies born in the United States after tests were made on them at embryo stage to ensure they would not be prey to late-onset diseases.

One woman, whose case was published in the Journal of the American Medical Association, had the early Alzheimer's gene, which meant that she would probably not live beyond her mid-30s or early 40s. The Reproductive Genetics Institute in Chicago retrieved eggs from her ovaries, fertilised them with her husband's sperm and tested them for the rare gene APOE-4 that gives those who have it a high probability of early Alzheimer's. They selected four that did not carry the gene, and she now has a baby daughter who she can be sure will not inherit the disease that has devastated her family.

Mr Taranissi has already been approached by somebody who wants such pre-implantation genetic screening for late-onset disease. He is inclined to help them, he said. What is the difference between that and aborting a pregnancy after a positive test for Down's syndrome? "That seems to be common practice," he said. "Everybody does it and nobody questions the ethics of that."

Simon Fishel goes further. Any fertile couple probably loses hundreds of embryos without even realising it in a lifetime, he says. They are conceived but fail to implant in the womb. "If you had two and one of them was to have serious Alzheimer's disease or breast cancer, what would you do?" he asked. "I think society should allow you that freedom."

Saudi Arabia has recently announced that because there is so much thalassaemia in the country - the potentially fatal blood disorder suffered by Zain Hashmi - all couples wanting a child should be tested and if they are both carriers, they should undergo fertility treatment so that their embryos can be screened for it. The concern is partly to do with the financial as well as human cost of treating the disease.

It could happen here. A primary care trust has offered to pay for a couple at Dr Fishel's clinic to undergo the tests to have a "saviour sibling" baby. The cost of the treatment of their existing sick child could run to more than £1m. The cost of bringing a new baby into the world in order to cure that child through a bone marrow transplant might be £20,000. "The savings are considerable," said Dr Fishel.

"In the future we ought to be contemplating whether the NHS should be funding this procedure."