An N−PA− individual with childhood SCZ would have to inherit the N− allele from one parent and the A− allele from the other.
In addition, partial inhibition of trait N could be the genetic basis of pseudo-narcissistic types (p. 333), or non-sanguine types having some expression of trait N (our N*PA type, for example).
Thus, the NPA model leads to the notion that one could divide SCZ into two main categories:
1. Those cases where the cause of SCZ is solely due to alleles at the N and A loci, and
2. Those cases where there are, in addition, other loci of SCZ susceptibility.
How significant is Group 1 is anybody's guess. However, the Hardy-Weinberg computation indicated that in some habitancies the frequency of non-viability is fairly high -- on the order of 5%, or 1 out of 20 pregnancies... but it is unclear how many of these end up as live births.
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