Possible genetic influences on glomerular filtration rate and electrolyte excretion were investigated in 55 (37 monozygotic, 18 dizygotic) young adult white twin pairs. Subjects were studied during a five-day hospitalization involving sodium loading and sodium depletion. No evidence of genetic variability was found in the control levels of serum or urine sodium and potassium. Following a saline infusion it was possible to detect genetic influence in electrolyte handling. Creatinine clearance, used as a measure of glomerular filtration rate, did not appear to be genetically mediated. The results indicate that genetic factors are important in sodium handling in normal individuals and that this is independent of glomerular filtration rate.

Birthweight was measured on 188 monochorionic monozygotic, 54 dichorionic monozygotic, 102 like-sexed dizygotic, and 94 unlike-sexed dizygotic liveborn twin pairs. Overall, males were found to be significantly heavier than females. These differences were not significant, however, when birthweights were compared within zygosity/chorion-type categories. Males were also characterized by a slightly greater overall total variance. Comparisons of intrapair variation of monochorionic and dichorionic monozygotic twins revealed significant differences between monochorionic pairs and dichorionic separate pairs and no significant differences between monochorionic pairs and dichorionic fused pairs. The results of this study suggest that placental proximity may have as important an influence on variation in birthweight as does the presence or absence of vascular anastomoses.

Two cases of holoacardius are presented. Both had a normal 46,XX female karyotype, identical to that of their respective cotwin. Data are presented supporting the concept that the placental vascular anastomoses are the primary agents in the formation of an acardiac. Cytogenetics, epidemiology and other theories of pathogenesis are discussed.

A follow-up of 78 twin pregnancies diagnosed in the first trimester indicated that early ovular resorption occurred in 63% of spontaneous and 64% of Clomiphene-induced gestations, so that singletons were born. No failure was observed in the 12 twin conceptions following administration of gonadotropins or Cyclofenil.

A criticism of twin studies has been that the difference between the behavioral similarities of identical and fraternal twins is largely created by parental influences based on their perception of the twins' zygosity. This issue is examined for differences in the IQ scores found within pairs classified by parents and bloodtyping. The systematic differences in IQ scores could be attributed to zygosity classified by bloodtyping rather than by parental belief. The available evidence indicates that the twin method is still appropriate for human behavior genetics.

Using a special method, the dermatoglyphics of the plantar interdigital areas of 86 monozygotic and 76 dizygotic Polish twin pairs were obtained. The frequencies of different types of zygodactylous triradii were estimated using a recently published new scheme of classification. The second, third, and fourth interdigital areas were grouped together. The findings indicate a high heritability which suggest that zygodactylous triradii might be used as a valuable genetic marker.

A series of 978 repeated ultrasonic measurements of the fetal biparietal diameter (BPD) assessed the growth of 119 twin pairs during the second half of pregnancy. The mean BPD values for each gestational week were close to the mean values for singletons. From the 32nd week, the BPD growth curves for twins deviated from that for singletons. The second twins' growth curve ran consistently below that of the first and fell away more from the 32nd week on. In 80% of the measurements, the first twin (defined as the twin in, or close to, the pelvic inlet) was the one with the largest BPD. The intrapair difference in BPD had no correlation to the intrapair difference in body length, head circumference, or birth weight. In 91 of the 119 twin pairs, zygosity was settled by sex, histological examination of placenta, or blood grouping. The longitudinally compiled BPD growth curves (678 BPD determinations) showed the curve for dizygotic twins consistently running above that of the monozygotic twins. The BPD differences between the first and the second twin remained also within mono- and dizygotic groups of twins.

The formula H′C = 1 − CDZ/CMZ is suggested as a better summary of twin concordance data than the familiar Holzinger concordance formula, HC = (CMZ − CDZ)/ (1 − CDZ). The new formula better estimates degree of genetic determination, G, as calculated from a threshold model, but never exceeds unity, as G sometimes does. For high concordance rates, if CMZ + CDZ > 1, HC, may be more useful than either G or H′C.

Analysis of variance was performed on the radial and ulnar finger ridge counts and ridge count diversity index in 360 twin sets from which estimates of genetic variance were obtained. Findings for radial and ulnar counts paralleled those previously obtained for finger pattern type and ridge count (larger of radial and ulnar count). In contrast, ridge count diversity showed no indication of unequal total variances, as previously found for the total ridge count. One must be cautious not to exclude genetic influences on traits, such as the thumb variables in this study, where there are unequal total variances between monozygotic and dizygotic twins and the more conservative estimate of genetic variance is not significant.

A total of 123 twin pregnancies were studied, 68 by records analysis only, and 55 by serial haematological assessment in detail. The incidence of low haemoglobin levels is found to be greater than in singleton pregnancy; evidence of iron and folic acid deficiency is common in sternal marrow aspirates but not in peripheral blood films. Routine haematinic prophylaxis in twin pregnancy is not advocated.

In a random sample of 104 pairs of middle-class Caucasian, handedness-discordant twins six years of age or older, a significant relationship has been found between birth order and handedness in monozygotic twins, there being an excess of left-handed among first-born twins (P < 0.01). No such relation has been found in dizygotic twins.

The hypothesis of a slower rate of glucose disappearance after intravenous glucose injection has been tested and proved in a sample of 54 women with twin pregnancies. Different standards for glucose tolerance being therefore required for these women, care should be taken before labelling them as diabetic.

Weinberg's differential method, whereby the frequency of dizygotic twins in a population is obtained by doubling the number of opposite-sexed pairs, might not be valid. Data are presented indicating a higher frequency of same-sexed than opposite-sexed pairs among samples of dizygotic twins ascertained at birth, thus favoring the hypothesis that, contrary to the method's fundamental assumption, the sexes of the two twin zygotes be not independent.