From genetics to epigenetics.

In the post human-genome area, the challenge is to derive details of heritable variation in relation to how human variation reflects adaptation to the different environments. Heterozygote advantage represents a superior genetic adaptation presumably explaining the presence of the allele at frequencies above those to be expected from a simple replacement of a homozygous lethal allele by mutation alone (Saugstad 1977a, 1975b, 1972). Mean birthweight of unaffected offspring of parents heterozygous for the phenylketonuria (PKU) allele averaged significantly above mean weight of all Norwegian births, rendering unaffected offspring more viable at birth and thus improving the chance for survival of the allele. A successful adaptation requires natural selection acting on that part of the body that makes a difference in survival. Skin colour variation is such a successful adaptation, for the North as opposed to the dark skins of the equator. Human Evolution in Africa and subsequent adaptations have enabled human survival all over the world with highly different light intensity (Jablonski & Chaplin 2000). That continuous variables, height, pubertal age and brain development, are multifactorially inherited and affected by epigenetic factors, was nicely demonstrated in the increase in height in Norway 1860-1960 with at the same time a reduction in pubertal age by 4yrs which may have affected the final stage in brain development. This created an increased need for brain food, N-3, to secure optimal brain function. Body growth is not brain growth. Given that the consumption of brain food (N-3) has declined to 20% only of the level 100yrs ago, what disorders are to be expected with an N-3 dietary deficit: in pregnancy, infancy and later in life? In this paper I discuss the significance of prepubertal selective pruning of excitatory synapses compared to delayed pruning and suggest relationships with brain disorders.