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.

Twin concordance rates are usually reported without reference to the number of parents affected, apparently because the simple demonstration that monozygotic (MZ) twins are more concordant than are dizygotic (DZ) twins is the goal of most twin studies. Depending on the underlying mechanism, however, twin concordance rates can vary widely when conditioned on the number of parents affected. For the generalized single-locus model it is shown that conditional concordance rates in monozygotic twins, along with an estimate of the disorder's prevalence in the population, uniquely specify the underlying parameters of this important model. Knowledge of the exact parameter set is essential for competent genetic counseling.

Perinatal mortality in twins is found to vary widely in six developed countries for which national data are available. The influence of factors such as birth weight, maternal age, and parity is examined, but no consistent trend is found.

Systolic blood pressure, standardized for age and sex, was measured in 76 MZ twin pairs and their 341 children. Maximum likelihood estimation of genetical and environmental parameters from the independent parental and offspring ANOVAs indicated a complete absence of both maternal and shared environmental effects, together with a heritability of 63%. These results are shown to be reasonably consistent with those from previous studies.

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.

The case presented here may be the first report of symbrachydactyly in one child of monozygotic twins. Detailed testing of blood groups was performed to confirm zygosity. This case suggests that symbrachydactyly may be caused by intrauterine environmental factors.

There are several dynamic influences on anterior fontanelle development in infants; among them, brain growth, dural attachments, suture development, and osteogenesis. It thus seems reasonable to hypothesize that variation in anterior fontanelle development between infants, related and unrelated, might have a significant genetic component. Anterior fontanelle size was quantitated by the method of Popich and Smith for 94 monozygotic (MZ) and 187 dizygotic (DZ) four-month-old twin pairs. The general model for estimating genetic variance from quantitative twin data was applied to MZ and DZ twins and then separately by chorion type. Since there were significant mean differences between blacks and whites, races were analyzed separately. The within-pair mean square estimates of genetic variance (ĜWT) were highly significant for both blacks (P < 0.02) and whites (P < 0.002). Comparisons of means, total variances, and among-pair mean squares within races revealed no heterogeneity. There were also no significant chorion effects. Since the anterior fontanelle closes at around 1–1½ years of age, it was evaluated at age one in 95 MZ and 194 DZ twin pairs as a qualitative trait – ie, open vs closed, concordance vs discordance. There were no significant differences in proband concordance rates between MZ and DZ twin pairs for either blacks (P > 0.5) or whites (P > 0.10). Again, there were no significant chorion effects. These data suggest that anterior fontanelle developmental variation has a significant genetic variance component at four months of age but not at one year. This finding may be related to the rapid brain growth witnessed between birth and eight to nine months of age.

A sample of 119 pairs of twins, together with their 87 siblings and 100 pairs of parents, were examined with respect to susceptibility to the motion sickness syndrome. Children were found to exhibit resistance to the syndrome below two years of age, and females were more susceptible than males. The incidence appeared to be highest in families with both parents affected. Concordance rates were much higher in MZ (100%) than in DZ pairs (27.6%). Genetic effects are likely, but cultural factors should not be ignored.

The co-twin control model using paired-comparison analysis provides an extremely powerful and cost-effective experimental design for clinical research. This paper suggests the possibility of using sequential analysis to further increase the efficiency and specificity of cotwin control studies.