#T02 Genes are generalists

The Jim twins
The Jim twins – identical, but separated at birth.

The fascination of twins

The 1980s were the golden age for twin studies. Identical twins share the same DNA, because they come from the same single fertilised egg.

Some pairs of twins were (sadly) separated at birth to be adopted into different families. Many did not know they were twins until they were adults.

Separated identical twins are a natural laboratory for studying the effects of genes and the environment on behaviour.

Thomas Bouchard studied such twins in the University of Minnesota. One pair, both called Jim, became international celebrities.

The Jims were remarkably similar, enjoying maths and carpentry at school, but not spelling. They both married  women named Linda and then Betty.

The Jims both had a child called James Allan.

They both worked in the security business, both drove a Chevrolet, and both chain smoked the same cigarettes. Their families took holidays on the same beach in Daytona at the same time of year.

The conclusion drawn was that their genes were somehow producing these behaviours and that there had to be many genes to produce such specific effects.

How many genes are there?

I remember being in seminars where these (and other similar) findings were being shared. This was “cutting-edge” science and we were caught up in the enthusiasm. One (now very) eminent geneticist speculated that there had to be at least 250 000 human genes.

The idea was that one (or more) genes somehow caused each of these characteristics. Jim and Jim were machines built by their genes.

We no longer think like that. The Human Genome Project reported that there were 30 000 genes; now we think it is nearer 20 000. There are fewer genes than there are human proteins, so the old idea that one gene produces one protein is also wrong.

Furthermore, the idea that there are genes “for” choosing a type of car or a wife by her name or a beach to holiday on is also redundant.

Genes shape personality but only in very general ways, probably through their effects on brain development and the actions of  nervous systems.

So, the second tool for clear thinking is:

#T02 “Genes are generalists: they only have an indirect effect on the development of characteristics.”