The condition of low birth weight is compared in twins and in singletons in terms of birth weight distributions and with respect to factors such as the incidence of sillbirths, length of gestation, maternal age, parity, and legitimacy. In the light of demographic, biological, and developmental considerations, it is concluded that low birth weight in twins is a different condition from low birth weight in singletons and should be dealt with independently, especially in view of the different implications for child growth and survival.

A statistical model and analysis for genetic and environmental effects in twin-family data are presented. The model is used to derive expressions for phenotypic correlations of 22 essential pair relationships in twin-family units. The analysis proceeds in two steps. First, differential effects of sex, generation, and sex-zygosity of twin-family units and correlations due to cluster sampling are eliminated from correlation data. Then, estimates and tests of model parameters are calculated from the adjusted data. The theory and methods were developed for a Swedish twin-family study of many behaviors possibly related to the smoking habit. There, it is important to screen for behaviors that clearly are under genetic control and to assess relative influences of various biological and social environments on the development of all behaviors. Height data from the Swedish study are used to illustrate concepts and methods presented in this paper.

Patterns of NOR activity in 640 metaphase spreads from twelve monozygotic (MZ) and eight dizygotic (DZ) twin pairs were studied to evaluate the heritability of this chromosomal heteromorphism. NORs were stained by a modification of the Ag-AS technique and counterstained with quinacrine mustard dihydrochloride to facilitate chromosome identification and assess their value in zygosity determination. In this study, all karyotypes were read blind with respect to zygosity and pair membership.

A discriminant function analysis of pair score differences in MZ and DZ twins revealed that, in our sample, the probability of accurately determining zygosity with NOR scores was 0.93 and with QFQ scores was 0.99. We conclude that NOR and QFQ scores are highly heritable and of great value in zygosity determination.

Data were collected from 687 metaphase spreads on the frequency with which an acrocentric chromosome was found in a satellite association. A significant correlation was found between this frequency and the degree of Ag-AS stain of the NOR. This study, therefore, confirms previous results showing that a high degree of NOR activity is found in those chromosomes most often involved in satellite associations.

Taste threshold for phenylthiocarbamide (PTC) was measured in 393 offspring from the families of 85 monozygotic (MZ) twin pairs. PTC scores were bimodally distributed with modes at one and eight and the antimode at five. Because of the non-normality of the distribution, a jackknife procedure was used to obtain 95% confidence intervals for the estimates of genetic, maternal, and environmental parameters. Analyses which assumed no epistasis and which included additive genetic effects revealed that 37.9% of the observed variation in PTC threshold was due to additive genetic effects, 16.6% was due to dominance effects, 14.2% was due to maternal effects, 13.7% was due to a common sibship environment, and 17.6% was due to random environmental effects, yielding a broad sense heritability of 0.55 for the threshold ability to taste PTC. Analyses which did not include additive genetic effects revealed 26.6% of the observed variance was due to dominance effects, 23.6% to maternal effects, and 49.8% to environmental effects at the 0.67 confidence levels, but that environmental factors accounted for 72.4% and dominance effects for 23.6% of the observed variation at the 95% level.

The inheritance of total ridge count (TRC) was studied in 967 individuals from the families of 111 pairs of MZ twins. The sample included data on 47 male half-sibships with 227 offspring and 64 female half-sibships with 306 offspring. The males in this sample had a mean ridge count of 135 ± 2 and the females a mean ridge count of 124 ± 2. The distribution of scores for females showed evidence for significant skewness. For this reason, prior to the analysis, the data were corrected for sex and adjusted to normality using a power transformation. Nested analyses of variance were performed on the ridge counts from male and female half-sibships separately to derive estimates of among, between, and within-variance components. These estimates were then used in a nonlinear least squares program to estimate genetic and environmental parameters and to determine the goodness of fit of various models. A model which included additive genetic and dominance effects could not be rejected (P = 0.67) but did not fit the data as well as a simple additive genetic-random environmental model (P = 0.81). The addition of maternal effects to the latter model also provided a satisfactory fit (P = 0.68). However, there was no improvement in the goodness of fit over the simple model, and the estimated magnitude of the maternal effect was not significantly different from zero.

The variability of magnesium and zinc concentrations in plasma and erythrocytes was investigated by twin and family studies. Twins were sampled in two distinct ways and in two different West-European regions. In one of the samples, a distinction was made between twins living together and twins living apart. Two series of families were studied, one in a homogeneous environment, the other in a more diverse environment. Samples were compared by variance analysis. The results show 1) that genetic variability is significant for red blood cell (RBC) magnesium and zinc, minor for plasma magnesium and absent for plasma zinc; 2) that the family environment affects the extent of resemblance between twins and between siblings more for plasma levels than for RBC levels of magnesium and zinc. Furthermore, the intercorrelation analysis suggests that the genetic regulation systems of RBC magnesium and zinc are different, whereas some of the environmental regulation systems of plasma magnesium and zinc are the same. Biological interpretations are brought forward and discussed.

This article is a review of the technique of matching as it applies to epidemiological investgations of the etiology of disease. It is written for researchers who wish to use twins as the matched pairs in studies of this type.