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.

As a part of a study of the dietary habits of monozygotic twins, a nutrition survey including a 24-hour dietary recall interview and a three-day dietary diary was conducted on the members of 15 male and 13 female pairs of identical Caucasian twins ranging from 25 to 61 years of age. In this study, overall mean caloric, protein, fat, and carbohydrate intake of males was significantly greater than females. Members of male twin pairs also tended to be more similar than female twins in mean intake of all major nutrient groups. In general, the observed differences in overall intake between males and females were maintained even after adjustment of body weight. Male twins were characterized by greater similarity in intake of sodium and potassium; however, there was no consistent difference in the degree of correlation in intakes of members of male and female twins for either iron, calcium or phosphorus.

Longitudinal study of 514 pairs of US white male twin veterans was made possible by compiling data from military records, mailed questionnaires, and physical examinations. Height and weight were used to calculate the Quetelet index for twins at different ages covering induction into the Armed Forces (generally 17–24 years), 25 years, 40–50 years at questionnaire response, and 42–56 at physical examination. Cross-sectional analyses of Quetelet Index at these 4 points in time showed consistent evidence of significant genetic variance. Additional twin analyses of change in Quetelet Index both from induction and from age 25 to approximately age 48 were done. Both analyses showed significant genetic variance for change in Quetelet Index during this time span of 25 or more years for most subjects. Heritability estimates were 0.6 for change in Quetelet Index from induction (mean age 20) to examination (mean age 48) and 0.7 for change in Quetelet Index from age 25 to examination. While studies of younger twins have shown significant genetic influence on growth (height and weight), this study of adults demonstrates significant genetic influence on change in weight even after maturation.

A high heritability estimate was obtained for serum arylesterase activity level in a series of 40 male twin pairs (23 MZ, 17 DZ) aged 33–39 years. The heritability estimated for Cholinesterase activity was strikingly lower. In unrelated individuals a positive correlation between arylesterase, Cholinesterase, and various lipoprotein parameters appeared to support the suggestion that these enzymes interact with various lipoproteins at the molecular level. The nature of these associations is not known. It is, however, tempting to speculate that some of the hereditary influence on lipoprotein levels in man may be mediated through the genetic control of one or both of these enzymes.

Twin data on bipolar and unipolar affective disorders are analyzed by multiple threshold models of inheritance. The two illness types are represented in the models on a continuum of genetic–environmental liability in which bipolar illness has a higher liability threshold than unipolar disorder. Autosomal single major locus model provides an acceptable fit to observed concordance rates in monozygotic twins. The multifactorial-polygenic model is rejected.

Among 93 cases of thalidomide embryopathy ascertained in Brazil, one pair of female dizygotic twins was found. One of them presented complete paraxial hemimelia in the right arm while the other showed a triphalangeal thumb at the left hand and had duodenal stenosis. Of the eight dizygotic twin pairs exposed to thalidomide in utero described thus far, only four were concordant for the type of malformation.

Plasma lipids were studied on 92 pairs of twins (66 MZ and 26 like-sexed DZ). The DZ twins had significantly greater total variance than the MZ twins for total plasma cholesterol but not for triglycerides or the high, low, and very low-density lipoprotein cholesterol fractions.