A method for the estimation of genetic correlations based upon analysis of genetic marker phenotypes is presented. At a given marker locus, the probability of observing a pair of individuals with a specific combination of phenotypes can be expressed as a function of the gene frequencies at that locus and the genetic correlation (R) between that pair. The likelihood of obtaining a sample of n such pairs with their phenotypes at m marker loci can be expressed as a product of nm such functions. From the likelihood function, maximum likelihood estimates of R can be obtained, and hypotheses about R may be tested. A sample of Swedish twin families (61 dizygotic twin pairs, 268 husband-wife pairs, and 164 sib pairs) were analyzed by this method using information from 21 markers. It was found that for the twin pairs, R = 0.458, which was significantly different from the R calculated for sib pairs (R = 0.5 58) but not significantly different from the expected 0.5. For the husband-wife pairs, it was found that R = 0.086, which did differ significantly from the expected value of 0, indicating the presence of nonrandom mating in this population.

Our starting point was Franz Alexander's well-known hypothesis about the hypertensive personality. According to Alexander (1939), hypertensive patients are characterized by a neurotic conflict between passive dependent tendencies and over-compensatory, competitive, hostile impulses. Hypertensive patients seem to be anti-aggressive, submissive, polite, and reserved. At the same time, they show no direct neurotic symptoms such as anxiety, obsessive ideas, or conversion symptoms. Hypertension may be looked upon as a substitute for symptom neurosis. According to Alexander, hypertensive patients are unable to express freely aggressive impulses, and at the same time unable to repress or absorb the impulses through symptom formation. An accumulation of psychic energy takes place, which manifests itself in constant vasomotoric stimulation. In the long run, this development may be able to change the balance of the baroceptors and finally destroy the circulatory system.

A sample of MZ twins is particularly suitable for studying the influence of inhibited aggression on blood pressure. In addition to the methodological advantages of controlling the genetic and external environmental factors of childhood, one frequently finds a casting of roles with one of the twins dominant and aggressive, and the other as a submissive and timid partner.

The problem of the equilibrium stability of a phenotypic selection process, with a single pair of autosomal alleles, has been numerically studied, on the basis of previously established reproduction differential equations.