April 29, 2003
Preimplantation Genetic Diagnosis To Grow In Popularity

Preimplantation Genetic Diagnosis (PGD) is done to embryos in vitro to identify embryos which are free of specific genetic disorders. In Vitro Fertilization (IVF) followed by PGD was first performed in 1989 and is now widely available. In Sydney Australia 74 babies have been born who were the result of PGD screening.

A special investigation by The Daily Telegraph today reveals she is one of 74 babies born in Sydney since the genetic screening program began in the late 1990s.

The particular clinic featured in this article, Sydney IVF can currently screen for 37 different genetically caused disorders.

Dr Kylie De Bore, from Sydney IVF, said the service was now available to rural families who previously had to come to Sydney for it. "We can screen for 37 disorders, because families have come to us with that many diseases," she said. "It is not restricted to those."

As the cost of DNA testing declines it seems likely that many people will elect to have tests done on themselves and their mates before conceiving children. There are already many genetic diseases such as Thalassemia, Tay Sachs, Cystic Fibrosis, and Retinitis Pigmentosa which could be avoided thru use of PGD. As the list of known genetic disorders and methods to test for them grow the value of PGD will increase.

Most genetic variations that cause differences in offspring are not diseases. Genetic variations affect eye color, hair color, height, body build, proneness to depression, proneness to obesity, intelligence, personality, and a great many other characteristics about which most people have preferences. We all have characteristics that we have gotten from one of our pairs of chromosomes. When someone reproduces there is currently no way to control which of each pair of chromosomes gets passed along. If, say, someone knew that for their genetic complement only one of a particular pair of chromosomes coded for higher intelligence or a happier disposition one can easily imagine that person would elect to use PGD to make sure that their offspring received the chromosome that coded for the desired feature.

Most people do not have the genetic variations for the big genetic disorders. But everyone who has children has a set of preferences about what they want those children to be like. Therefore it stands to reason that the real big future increase in demand for PGD will come when it can be used to exert some degree of control over the passing along of genetic variations that are not genetic disorders. PGD will therefore become much more popular once the effects of a much larger number of genetic variations become known and testable.

The biggest problem with PGD that will limit its usefulness is that for every trait to be selected for at the same time the number of embryos goes up by a factor of 2 or 3 or 4 (depending on whether it is a dominant or recessive trait and whether each parent has 0, 1, or 2 copies of the desired genetic variation). The number of needed embryos quickly becomes too large for too many different traits.

Scientists hope to improve the technology to the point where it can screen for one, maybe even two, positive 'traits' - for example blue eyes and height. That would still rule out the ideas of the genetic visionaries like Stock, who think PGD could be the first step to 'designer babies' and the re-engineering of mankind, by allowing parents to select among their embryos for all sorts of desirable (to the parents) qualities. The reasons this cannot work are not technical so much as statistical, to do with the way genes are passed on through sex. To screen for two traits you need at least 16 embryos, for three, 64 embryos and so on. Since the maximum number of embryos an IVF procedure produces are typically between 16 and 20, you can do the sums.

What is needed is either the ability to select individual chromosomes by separating them out and choosing which ones to recombine or the ability to do gene surgery to chromosomes to introduce desired genetic variations into an embryo. The gene surgery style of gene therapy is the technique that will ultimately obsolesce the need to generate a large number of embryos with IVF in order to get one that has the desired characteristics. The ability to change the genetic sequences in chromosomes will allow a single IVF embryo to be reshaped to have any desired genetic variations.

Share |      Randall Parker, 2003 April 29 02:00 AM  Biotech Reproduction

Patrick said at April 29, 2003 7:29 PM:

Alternatively, a way to screen individual eggs and sperm for the genes desirable. That slashes the total number of eggs required (and sperm, but they are hardly the problem).

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