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The future of genetic testing

Updated Thursday 3rd August 2006

Dr Robert Saunders, a Lecturer in Molecular Biology at the Open University, explains the science involved in genetic testing and asks what comes next

Robert Saunders Copyrighted image Icon Copyright: Robert Saunders

What is the science involved in assessing the DNA of an embryo?

"Well, because every cell of the embryo, and in fact every cell of a person, contains the same DNA, one can just remove one or more cells of an embryo, or of a person, extract the DNA from those cells and use molecular biological techniques to look at the DNA. Many of these techniques are very sensitive and one can screen several genes from one small DNA sample. The process is really quite quick; a matter of days would be required for full analysis of a DNA test.

How are the tests done?

The tests can be performed on a small number of cells removed from an early stage embryo pre-implantation, in the case of in vitro fertilisation, or with cells removed from embryonic tissues in a pregnant woman, or indeed on parental DNA extracted from blood cells.

If an embryo was being genetically tested, does the scientist freeze it afterwards?

Typically with in vitro fertilisation, the embryo's allowed to develop through several cell divisions before you try and implant it in the woman. As I understand it, several embryos are produced but they're not all implanted in the same procedure, and they very often are stored frozen. So you would have the ability to make a reasonably large number of fertilised embryos; remove some material for testing, and freeze the embryos until such time that you decide which ones to try implanting.

What is seen under the microscope?

You don't use a microscope for DNA testing directly. You would use a microscope perhaps if you were looking at cells removed from an in vitro fertilised embryo. DNA is a molecule so it's too small to see under a microscope.

What sort of equipment do you use to look at DNA?

You use a wide range of laboratory equipment required for propagating DNA in the test tube. We look at DNA by techniques such a gel electrophoresis which enables us to see the length of DNA molecules amplified. You're effectively sieving the molecules, you're sorting the molecules by size, and you can then see the DNA on the gel by staining it with dyes that are specific for staining DNA, and from that you can work out a DNA fragment's length. In DNA sequencing, you determine the base sequence of the DNA of the fragments that you're examining, to look for differences.

So in terms of the drama, Making Astronauts, would the same science be involved?

You would probably be looking at DNA of the parents, in this case the father and surrogate mother, and also the DNA of the fertilised embryo - in this case it's an in vitro fertilised embryo. One could either look at it pre- or post-implantation. I think the surrogate mother would be rather annoyed if you were doing it post-implantation, so I suspect most of this would be done pre-implantation. You would be specifically amplifying sections of DNA corresponding to the genes that you are interested in by using Polymerase Chain Reaction (a highly efficient and sensitive method for obtaining quantities of specific DNAs from minute amounts of starting material) and assessing the DNA sequence for a specific variance.

Is it possible to choose a baby with certain characteristics, avoiding the embryos which might have the cancer gene?

Yes, it is definitely possible to choose an in vitro fertilised embryo that doesn't have a specific cancer susceptibility gene. Assuming you have the technical knowledge of which gene to look for. To say "the cancer gene", is a bit misleading as there are many genes which when mutated or altered or made defective will cause cancer or enhance one’s susceptibility to particular forms of cancer. But these are not the only causes of cancer.

Is there a set of genes which suggest what your sexual orientation might be?

Some years ago, research was reported which seemed to indicate that there was a genetic explanation for homosexuality. However, these experiments weren't able to be replicated and as far as I'm aware at the moment, there is no known genetic cause for homosexuality.

I would say that that tendencies or behavioural characteristics are very complex, and are very much dependent upon the environment in which an individual has grown up in. This is the whole nature versus nurture argument. And I think that the environmental component to behavioural characteristics is very high. I would think it very unlikely that genetic tests for specific behavioural characteristics within the normal range of the human population would be likely.

Do you think it might be possible in the future that the set of chromosomes in the father’s sperm could ever be joined with the set of chromosomes in his gay partner's sperm, so that there is no need for the chromosome set from the female?

To perform such an ‘experiment’ if you like, first one would have to remove the nucleus from the egg cell. That's perfectly practical; we can do these things by microscopic manipulation of cells. The next thing one would have to do would be to make at least one of the sperm nuclei behave as if it was an egg cell nucleus, and that actually would be technically very demanding. I don't see any near future prospect of this

Do you believe that it would be possible to eradicate genetic disorders in the future?

Now, you're on extremely thorny moral ground here. You may have heard of the Eugenics Movement in the 1930s. In particular, it was very big in the United States of America and then again in Nazi Germany, for example. In those days, the only way you could identify individuals who had genetic disorders that you wished to eradicate were in those individuals showing the defective characteristic. Because many of these things are recessive you can be a carrier without actually showing a physical or physiological defect. All these efforts were not only ethically undesirable but were scientifically impossible. Nowadays, we now have the ability to identify carriers, as well as those who are actually suffering from a genetic condition. So in principle, one could eradicate genetic disorders. I think it's unlikely to ever happen, for political and ethical reasons, not to mention practical reasons.

How long do you think it'll be before the situation as dramatised in Making Astronauts becomes commonplace?

I don’t believe that it's ever going to be particularly practical and there several reasons for this. Firstly, the process of in vitro fertilisation and successful implantation of embryos into the surrogate mother has a relatively low success rate. Secondly, the more characteristics you're screening for, the lower the proportion of embryos will come up to your criteria. So it's actually going to be very difficult to carry out this procedure for a large number of characteristics. Thirdly, several of the characteristics here such as intelligence or dexterity are polygenic characteristics, which means that there are many genes responsible for the genetic component of those things. Another aspect is: how do you measure intelligence? I.Q. tests only measure the ability to do I.Q. tests. So there's a lot of uncertainty at all levels here.

Basically, I don’t think that it is worthwhile. My personal opinion in the case of things such as intelligence is that I would rather just take the luck of the draw and try to improve the situation by bringing up the child in a stimulating and enjoyable environment.

Is there the possibility with genetic engineering that you could end up doing more damage than good?

If you genetically tested an embryo before implantation, and you damage the embryo, it would be an unsuccessful implantation. I don't think genetic testing would cause a disaster, I don't think that's very likely. The alternative to testing an embryo pre-implantation is that you test embryonic tissue from an on-going pregnancy and then decide what to do with a fetus that's perhaps going to develop with a particular medical condition. But there you have an ethical consideration of what the parents would like to do. Not everybody is in agreement with abortion, for example, on ethical or religious grounds. Doing it at the pre-implantation stage, may or may not get around these kind of issues. There's a lot of ethical issues here that come up."

 

 

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