A U.S. fertility clinic has announced that within six months it will begin offering couples the option to have tailor-made babies, selecting not only their offspring's gender but also cosmetic traits such as hair and eye color. The Fertility Institutes, an organization which has offices in New York City, Los Angeles and Mexico, says it can provide this service using a procedure called pre-implantation genetic diagnosis, or PGD, according to the New York Daily News.

Critics have denounced the idea as unethical tinkering with nature, among them Pope Benedict XVI, who called the practice an "obsessive search for the perfect child [that] tends to justify a different consideration of life and personal dignity," the Daily News reports. Others question whether child-customization is actually possible to pull off. Sean Tipton, a spokesperson for the American Society of Reproductive Medicine in Birmingham, Ala., told ScientificAmerican.com that peddling eye and hair color selection is like "Virgin Atlantic offering to sell [people] tickets … to the moon." He says he is not aware of any published research showing that PGD can be used to dictate eye and hair color. "The science is not there," Tipton insists.

PGD has long been used to screen for genetic diseases in embryos used for in vitro fertilization (IVF), and some parents have used the technology to create embryos that are genetically compatible stem cell and bone marrow donors for their ailing siblings, as illustrated in the case of Adam and Molly Nash and in the novel My Sister's Keeper by Jodi Picoult.

We asked Maria Lalioti, a geneticist and PGD specialist at Yale University School of Medicine, to explain the procedure and exactly how far it can go.

[An edited transcript of the interview follows.]

What is pre-implantation genetic diagnosis (PGD)?
PGD is technology that allows scientists to look for genetic diseases and chromosome problems in embryos implanted in women using in vitro fertilization [a procedure used by couples struggling with fertility wherein scientists combine a woman's egg with a man's sperm in a laboratory dish, then implant the resulting embryo in a woman's uterus]. The idea behind PGD to find out if something is wrong with an embryo before implanting it in a woman's uterus.

Some couples use PGD because genetic diseases run in their families and they want to make sure that their children are not affected—these diseases include cystic fibrosis [a disease in which sufferers produce profuse amounts of thick mucus that can clog the airways and cause fatal lung infections] and thalassemias [inherited blood anemia disorders]. Other couples use PGD to find out if they have chromosomal abnormalities blocking them from having successful pregnancies. For example, some women produce a large number of eggs with abnormal numbers of chromosomes that, when fertilized, produce embryos that cannot develop into healthy fetuses.

How long have fertility specialists been using this technique?
Since about the early 1990s.

How is PGD done?
First women go through IVF, which begins with taking hormones that stimulate egg production. Usually a woman's ovaries produce only one fertilizable egg per month, but fertility drugs stimulate the production of 10 to 15 eggs. Doctors remove these eggs using a needle, then bring them to the laboratory to be fertilized. This is done by taking a sperm, putting it inside a needle, and injecting it into the egg. They let the fertilized egg grow for two days, during which it divides into a ball of eight identical cells. An embryologist removes one cell from this mass of cells and tests it for Down's syndrome, cystic fibrosis or whatever disorder the couple is concerned may be in their DNA.

To test for a genetic disease, scientists take a section of DNA from the cell and use a technology called polymerase chain reaction to produce multiple copies of sections of genes that could contain an error or mutation that could cause a disease. Then they run the DNA samples through a sequencing machine that sorts the gene fragments according to size. If there is an abnormal-size fragment, this could indicate a genetic flaw.

Does extracting a cell put an embryo at risk?
It is not dangerous to remove one cell if it is done by someone trained to do the procedure. But if other cells are damaged in the process the embryo will stop growing.

What happens to the embryos?
Usually one or two of them are implanted into the woman's uterus with a very big plastic needle that is inserted through the vagina. [The hope is that at least one embryo will attach to the uterine wall and begin developing.]

What happens to the defective embryos?
It's up to the patients to decide what to do with the undesirable embryos. They may donate them to science or discard them.

Can PGD be used to choose the embryo's gender?
It can be used to select an embryo that will develop into the desired gender. Scientists use the same method of removing a cell from the eight-cell embryo, and then look inside the nucleus for segments of DNA that are known to lie on the X chromosome [females have two X's] or the Y chromosome [males have one X and one Y]. Basically, you put the cell on a glass slide and apply a probe, a piece of DNA labeled with a color that will stick to either the X or Y chromosome. [For example, a green probe may be chosen to latch onto the X chromosome and a blue probe onto the Y.] If scientists detect two green probes, that means there are two X chromosomes, and hence it will develop into a girl. If there is one green and one blue probe, then there is one X and one Y, and it's a boy in the making. We don't allow gender selection at this clinic, because our fertility experts are opposed to it for ethical reasons. But there are clinics that do it, and they are making a lot of money.

Can you use PGD to pre-determine eye and hair color?
I would think it's not possible with technology available today. You would have to be looking for multiple genes [there is no single gene that encodes, say, green eyes or blond hair], which is not easy to do using only one cell. Future technology, however, will most likely make this possible. Then, you would just need to know what genes determine the color of eyes and hair.

Do you think scientists should try to use PGD to select for traits such as eye and hair color, intelligence or height?
This is an ethical concern. If you do this, then you've given priority to something that it shouldn't have. Our main focus is to detect disease.