Epigenetics

For generations the argument about "nature vs. nurture" has swirled back and forth, often with significant consequences. In Victorian Britain and the USA the idea of eugenics resulted in the sterilization of hundreds of thousands of "inferior" girls to remove their genes from the gene pool. Under Nazism it was thought that the "perfect Aryan" could be produced by racist selection (i.e. murdering all others) and under Stalin, Lysenko's theories that man could be "moulded" into "Soviet man" were considered consistent with Communism, and people who disagreed, who studied genetics, were sent to the Gulag and murdered.

But, throughout these murderous competitions it was clear that both nature (i.e. genes) and nurture (i.e. environment) were important for human development. The question was how much of each. We now know from academic studies of twins (identical and non-identical) brought up apart, that the influence of genetics is dominant. In other words, even though brought up in different environments, twins still develop almost identically. But, we also know that twins brought up in the same home environment can also show differences, in personality etc. (e.g. one can be straight and one gay). A rough estimate of the relative value of nature vs. nurture is 70% to 30%. So both sides were right to a degree.

Now the question arises, if as we know that genes are predominant in determining all physical features of a person and their personality and behavior (the total called the phenotype), then how is it that the environment can play a role? This is what is called epigenetics, namely the way in which the environment, including long-term and chance events, can alter the expression of the genes. It must be remembered that the genes are not static, and although all cells in a body contain the same complement of genes in their DNA (the genotype), different combinations or casettes of genes are expressed in specific cells, so that even though kidney, liver and eye cells all have the same DNA sequences in one person, they function completely differently. This is called selective gene expression, namely certain genes are repressed, usually by the presence of specific proteins called repressors, that bind to that specific gene or sequence of DNA, while other genes are actively expressed, thus producing their gene product, often a protein or enzyme. It is clear that in some cases the degree of expression can be altered by affecting the status of the repression or expression of a given gene. For example, it is known that in early foetal development a surge of testosterone is produced by specific genes that determine that the foetus develops as a male. If for some reason durng the pregnancy, this burst of testosterone is delayed or weakened, the sex of the foetus may be affected. Then of course there are subtle effects other than physcial sexual factors, such as gender effects of early development that can change the personality of the child. These epigenetic effects are now considered to be responsible for the so-called nurture or environmental effects on human development.