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26 identical and 26 fraternal twin pairs were subjected to 1,2 ml/kg alcohol loading under standardized conditions; furthermore, 13 non-twin persons were repeatedly subjected to the same procedure in order to test the intraindividual variability. The alcohol effect was pursued by repeated EEG recordings. The EEGs of identical twins proved to react identically to alcohol loading, whereas the EEGs of fraternal twins became more dissimilar during the course of the experiment. The identical reaction of the brain wave pattern in monozygotic twins cannot be attributed to more similar blood alcohol concentrations; instead, this finding is a pharmacogenetic phenomenon of the central nervous system.

The genetic information for neoplastic transformation is inherited as a normal part of the genome in all individuals of Xiphophorus. Neoplasia, however, was found only in hybrids between members of different populations and local races. It can be classified in (a) a large group that is triggered by mutagens, (b) a large group that is triggered by mutagens, (b) a large group triggered by promoters, (c) a small group that develops, "spontaneously", and (d) a small group that is inherited according to Mendelian Laws. The process leading to susceptibility for neoplasia is represented by the disintegration of coadapted gene systems that normally protect the fish from neoplasia. Hybridization is the most effective process that leads to disintegration of the protection gene-systems. Environmental mutagens and promoters (i.e. carcinogens) may complete disintegration and thus may trigger neoplasia. The phenomenon of introducing susceptibility to neoplasia by means of hybridization has been observed in a large variety of plants and animals (Table 2). While we have no data on the relation between hybridization and cancer in human beings comparable to those in plants and animals, we put the question whether the many facts on tumor incidence in humans, that do not agree with the concept of the primacy of environmental factors in carcinogenesis may be explained by interpopulational and interracial hybridization in preceding generations. Based on our studies on Xiphophorus we suppose that environmental factors represent only the peak of an iceberg in the multistep process of the causation of neoplasia. The most important steps leading to neoplasia, i.e. those that bring out susceptibility, are supposed to be hidden in our ancestry.