The Environmental Threat:
From Autism and ADHD to Alzheimer
More than 50 years ago the Edinburgh geneticist C.H. Waddington performed an experiment. He exposed eggs of tiny fruitflies (Drosophila) to a brief period of elevated temperature ([heat-shock]). Among the adults emerging from these treated embryos were, perhaps not so surprisingly, a number of flies with developmental abnormalities – such as anomalies of wing and body structure.
The surprise came when he bred these abnormal flies together, for the phenotype (the visible expression of the insult or deficiency) was soon expressed in offspring without any heat-shock. In other words, an [environmental] effect had somehow become [genetic.]1,2
The interpretation of this experiment is interesting. Waddington deduced that the flies carry a series of mild genetic impairments that, under normal conditions, give no discernible phenotype. Low-activity variants of developmental genes persist in the population because there is no selective pressure for their removal. But stress interferes with the activity of these genes just a little, enough to produce visible developmental abnormalities.
Then, when the abnormal male and female flies with subefficient gene variants are crossed, similar low-activity genes come together in the offspring. In these flies, now with a double dose of low activity, the same developmental abnormality appears, but without the stressor – uncovering what Waddington called an [occult] or hidden phenotype.
Other stresses have exactly the same effect. When newly-laid eggs were treated with ether, and the emerging adults were intercrossed, Waddington wrote: [individuals exhibiting the phenotype began to appear in samples of the selected stock which had not been subjected to the unusual environment.]3 The