A NUMBER of neurological conditions are hereditary. Some of them follow more or less a pattern of mendelian inheritance, while others are the result of an interaction between genetic and environmental factors. Some geneticists even believe that there is "no phenotypic trait independent of either hereditary or environmental agents." 59 Disease, then, is not due to a single cause but rather to a diversity of etiological and pathogenetic factors. This should be kept in mind in the following analysis of the elements of genetics.
The genotype of an individual is represented by genes which are aligned on the chromosomes. The correct chromosomal complement of man was discovered only a few years ago by Tijo and Levan (1956).61 All cells of man except the pregametes and gametes normally have a diploid set of 46 chromosomes, that is, 44 autosomal and 2 sex chromosomes. Meiosis reduces the chromosomal complement of ovo