biotechnological applications & the human gene pool

One of the 2007 Scholarship exam questions sort of links to an earlier post I wrote, on xenotransplantation. It says

Human disorders are increasingly being diagnosed and treated using biotechnological applications such as:

•        Genetic testing, including testing of adults through to pre-birth diagnosis (for example: pre-implantation genetic diagnosis (PIGD) of embryos, amniocentesis or chorionic villus testing)

•        Gene therapy

•        Stem cell research

•        Xeno-transplantation.

Discuss how the use of named biotechnological applications may impact on the gene pool and the future biological evolution of Homo sapiens.

Quite a meaty one, isn’t it? But – as for all these questions – what you have to do is clearly set out for you. And the question doesn’t ask for more knowledge than you might reasonably have gained over the course of your year 13 bio studies; it simply asks to you demonstrate that you can integrate what you might have learned in studying several different standards, into a coherent, well-argued whole.

So, let’s look at some of these, starting with genetic testing – you’re given a couple of examples of this, & you’d  need to show that you understand what these techniques are (you should not be describing how they’ve done in any sort of detail!),  & how they could impact on the gene pool & on human evolution (given that at its most basic level, evolution is a change in the frequency of alleles in a population’s gene pool).

Pre-implantation genetic diagnosis is done, as the name suggests, before a ‘test-tube’ embryo is implanted into the mother’s womb. One or a few cells are removed & tested for a range of genetically-determined conditions. (This can safely be done at an early stage of embryo development because chordate embryos show what’s called indeterminate development. This means that while the embryo is composed of only a few hundred cells, those cells all have the potential to differentiate into just about any type of tissue – their fate has not yet been determined.) If, as a result of these tests, the parents decide not to have an embryo implanted, then the harmful alleles that embryo carries will be removed from the gene pool.

There are equity issues to do with this that you could well mention – not everyone has access to assisted reproductive technology, depending on where they live & whether there’s a cost to the individuals concerned. So this could potentially influence the gene pool in a particular community, or part of the community. These same issues also apply to the other diagnostic techniques mentioned in the quesiton, chorionic villus sampling & amniocentesis, and to adult testing. This last is where adults can be tested to see whether they are carriers for a particular heritable disease, & making a decision about whether or not to have children once the test results have been explained to them.

Gene therapy is where doctors attempt to provide an individual with a working form of the mutant allele. It was tried in the 1990s for children with severe combine mmune deficiency (SCID), for example. For gene therapy to be successful, the ‘normal’ allele has to be delivered to the target tissue and incorporated into the genome of sufficient cells (we’re talking millions) for it to have an effect when expressed. Because we’re not able to ensure that it’s added to the cell’s DNA in the ‘right’ place, there is the potential for problems if the ‘new’ allele is inserted in a place that disrupts the functioning of other genes – as appeared to happen in the SCID trials.

Anyway, back to the question! Because gene therapy doesn’t target the ‘germ line’ (ie the alleles don’t enter the cells that produce eggs & sperm), its outcomes aren’t heritable & so it has no direct impact on the population’s gene pool. (Theoretically, you could add the ‘good’ allele to a very early embryo, so that it was incorporated into all cells & so be passed on to future offspring.) However, if the therapy is successful, the affected individuals may go on to have children of their own, in which case the frequency of the harmful allele may actually increase over time, increasing genetic diversity.

What about stem cells & xenotransplantation? Well, both of them could certainly enable affected individuals to survive & have children of their own, adding their alleles to the gene pool & so increasing genetic diversity overall. But in both cases, the illnesses they are actually or potentially used to treat often don’t have a genetic underpinning, so there would be no direct impact on the population gene pool – other than through affected individuals surviving to reproduce & pass on their affected alleles.

Something else to think about: at least some genetic disorders – sickle cell anaemia, cystic fibrosis, for example – may actually confer a survival advantage in some circumstances. Someone heterozygous for sickle-cell disease is less severely affected by malaria, while the CF allele may help protect against cholera. You could argue that efforts to clean up the human gene pool now might conceivably be disadvantageous in a different future situation…