Estimation of the twin concordance rate for a disease often requires two stages. First, the disease is ascertained in a population or in a population sample, and such twins as are found with the disease become probands. Second, twin pairs with only one proband are further investigated and additional concordant pairs are thus discovered. A mathematical model is presented that allows for continuous variation in completeness of ascertainment in both stages, for correlation within pairs in the primary ascertainment process, and for correlation within pairs in occurrence of the disease. The concordance rate can be estimated by the proband method if secondary ascertainment is complete; other measures of concordance are accurate only if primary ascertainment is complete. A parameter analogous to the concordance rate but related to correlation in primary ascertainment can be estimated from the same data.

Following previous formulations of a model of qualitative analysis of twin population data independent of zygosity, a new Bayesian approach has been developed. The present model can be applied to any qualitative genetic trait in twin population data, provided no specific source of variation be introduced by the twin condition, and allows not only estimation of the frequencies of mono- and dizygosity as well as the gene frequencies, but also verification of the trait's mode of inheritance.

The use of epidemiological studies of twins as a means of determining the existence of genetic factors in various conditions is well known. To be of value, the zygosity of the twin pairs must be identified with as great a degree of confidence as possible. A method of determining twin zygosity, using concordance in general likeness, hair colour, hair type, eye colour, ear form, tongue roll and phenylthiocarbamide (PTC) taste sensitivity, was developed for use in a survey of 244 pairs of like-sexed twins aged 5 to 15 years. The results obtained and the problems raised when applying this method are discussed. The method was simple to apply to a large group of twin pairs, and monozygotic pairs were identified with a degree of confidence greater than 95%.

Twin methodology for the analysis of continuously distributed traits is presented as a guideline for initial steps in the analysis of twin data. Tests for heterogeneity of twin means and variances are reviewed along with estimates of genetic variance.

Analysis of twin data appears on the surface to be a rather simple task, but investigators may well be confused by the wealth of choices available for hypothesis testing. This paper is designed to help the less experienced investigator, with perhaps a modest amount of data, begin analysis with maximum attention to testing all testable assumptions of the twin model, and using recent examples of applications of the methods.

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.

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.

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.

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.

The Budapest Twin Register recorded 88.6% of the 1739 multiple births that occurred between 1970 and 1974. Of these, 31.2% were male-female pairs. Based on the examination of the placenta (19.8% monochorial) and of genetic markers, 406 pairs proved to be monozygotic and 645 dizygotic. Sex ratio of twins at birth showed a female preponderance (1000 F: 1009 M) in comparison to singletons (1000 F: 1071 M). Average birth weight of the twins was 2101 g. The frequency of stillbirths was about four times higher in twins than in singletons, twice as high in monozygotic than in dizygotic twins, and 2.4 times higher among second than first-born twins.