In conjunction with full-sib and parental observations, half-sib analysis permits an estimation of the genetic and environmental variance as well as a partitioning of the genetic variance into its additive, dominance and epistatic components. The offspring of identical twins are a unique class of human half-sibs who provide an unusual opportunity to resolve and measure several additional potentially important sources of human variation including maternal effects, the influences of common environmental factors and assortative mating.

The genetic model thus developed for the analysis of quantitative inheritance in man has been applied to the analysis of total ridge count and birth weight, confirming the existence of a major additive genetic effect on ridge count and a significant maternal effect on birth weight.

One of the most important lines of research on unselected large scale twin registries are mortality follow-ups. Death-specific concordance rates have been calculated using different measures of concordance. These estimates are of great importance but they only reflect an “all or-none” classification of the genetic effect. It is suggested that mortality studies could be extended to, and include studies on, death-discordant twins. Such studies combine the advantages from mortality follow-ups on total twin populations with the detailed clinical investigation on subsamples. By examining the surviving cotwins with respect to various traits and supposed risk factors, an opportunity has been provided to study the genetic determination on the development of disease. This approach has been used in a study of ischaemic heart disease (IHD) and its risk factors in a representative subsample from the Swedish Twin Registry.

Zygosity determination is generally carried out by different methods in small or large twin samples. The probability method based on sex and genetic markers is limited to relatively small samples, as a consequence of its cost. The empirical questionnaire method is applied in several large twin registers. Its margin of error is low enough for population studies, its cost is negligible, but its accuracy is insufficient when zygosity of twin pairs included in definite samples must be individually assessed. Efforts to bridge the distance between the two methods should be made, and they may take either direction: (1) find new, inexpensive genetic markers, or (2) increase the number and accuracy of empirical methods. The accuracy of a number of empirical methods applied to a twin sample of established zygosity has been compared. One modification of the “two peas in a pod” method, originally called “equivocalness method”, appears to warrant inclusion in questionnaire methodology. Also, compound probability as expressed by several empirical methods may reach an acceptable level of accuracy in zygosity assessment.

The anatomical characteristics of the arteries, veins and arteriovenous relationships, velamentous vessels, and vascular communications in 148 placentae from multiple pregnancies were studied. All features were found to show discordance of varying degrees, irrespective of the twins' zygosity. Singular involvement of one component of the twin placenta by hydramnios or congenital defects, incidence of vascular communications and the anatomical characteristics of the vessels in conjoined twins, acardiac monsters and triplets, and a chromosomal discordance in a MZ pair, lend additional support to the initial surmise of anatomical characteristics of fetal blood vessels of the placenta being determined by functional demands. It is suggested that the inequalities of prenatal environment be assessed by an examination of the fetal blood vessels of the placenta before drawing homologies in the twin concordance studies.

The National Heart and Lung Institute undertook a twin study on the etiology of coronary heart disease and genetic relations underlying differential levels of coronary risk. Between 1969 and 1974, 250 MZ and 264 DZ male twin pairs aged 42-56 were examined. The examination featured a medical and family history, a dietary interview, ECG, blood pressure, weight and height measurement, a variety of blood chemistry tests, including complete lipoprotein analyses, and lung function tests. Zygosity was determined through 22 red cell antigens. A detailed interview dealing with the twins' relationships to each other was also obtained. Each of the quantitative variables was tested for the presence of genetic variance using the method of Christian. This method first tests the equality of the total variances of MZ and DZ twins by a two-tailed F' test. When the hypothesis of equality of these two variances is rejected, use of the among-component estimate of genetic variance is indicated. The current report discusses the organization and methodology of the study while accompanying reports focus on the genetic variance in blood lipids, blood pressure, and coronary-prone behavior patterns.

Moderately alcohol-discordant male twin pairs, aged 45-65 years, have been examined with respect to ischemic heart disease (IHD) and associated factors. No conclusion can yet be drawn with regard to manifest or subclinical IHD, while significant disparities were found with regard to systolic and diastolic blood pressures, high cigarette consumption, hyperglycemia, and serum cholesterol; the greater number of findings of pathological values were found in the high alcohol-consumption as compared to the low alcohol-consumption cotwins. These findings offer a possible explanation for the increased number of reports showing a connection between high consumption of alcohol and occurrence of ischemic heart disease.

Myopia is the world's most common cause of defective vision. The role of controlled accommodation on the progress of myopia is unsettled. This cotwin control study is designed to provide a careful appraisal of the effectiveness of a clinically acceptable method of controlling accommodation. There are no previous reports of similar prospective studies among MZ twins. Expansion of such studies would be aided by collaboration among interested investigators.

The National Heart and Lung Institute Twin Study has examined 514 white adult male twin sets aged 42-56 with respect to blood pressure. The data were analyzed by a method of Christian et al. which eliminates possible biases in estimated genetic variances that could result from different total variances in MZ and DZ twins. Results of the test for the presence of genetic variance indicate that both systolic and diastolic blood pressure are to a considerable extent genetically controlled with an estimated heritability of 0.8 for systolic and 0.6 for diastolic pressure. Although these findings are at variance with some previous reports, it is thought that much of the discrepancy results from application of different analytic techniques, not in the data themselves. The application of these findings to our understanding of hypertension epidemiology and community hypertension control programs are discussed.

Fasting plasma triglycerides and cholesterol, as well as cholesterol in very low density, low density and high density lipoproteins, were measured in 250 MZ and 264 DZ adult male twin pairs. A new method was used to choose an estimate of genetic variance. This includes an estimate of genetic variance for use when the total variances of MZ and DZ twins are unequal. DZ twins had greater total variance for cholesterol. When genetic variance was estimated by subtracting the within-MZ-sets mean square from the within-DZ-sets mean square, all of the lipids had significant estimates of genetic variance; however, when genetic variance was estimated by a method designed to correct bias due to unequal total variances of MZ and DZ twins, only triglycerides had significant genetic variance. The heritability of plasma triglycerides was calculated to be 0.68.

Volunteer adult twins (aged 18-65), after a 12-hour fast, provide two 15 ml samples of blood for analysis by Auto Analyzer II, by agarose electrophoresis and by direct gas chromatography. Provisional diagnosis of zygosity is based on subjects' own opinions, supported by dermatoglyphic analysis and, in any difficult case, substantiated by an enzymatic series. Analysis of the AA-II and electrophoresis data on the first 2 × 45 subjects shows a significant positive correlation within MZ pairs for each of the 7 lipid measurements considered, while the estimate of correlation within DZ pairs is lower in every case. Heritability appears to be higher for cholesterol than for triglyceride fractions. The electrophoretic band for the sinking pre-beta lipoprotein fractions has behaved like a simple genetic marker. Interrelationships among different lipoprotein fractions are studied in terms both of phenotypic and genetic correlations, both of which confirm the expected positive association between amounts of triglyceride and of cholesterol in the supernatant (Sf > 20), and both also show negative correlation between the pre-beta and the alpha fractions. Another aspect of the analysis is the study of lipid differences between MZ cotwins in relation to corresponding intrapair differences in body weight and medical history; male, but not female pairs, show a measurable statistical dependence of cholesterol and triglyceride levels on « excess » body weight.

A previous study had shown that only MZ twins are easily exchanged by trained police dogs. This is certainly due to the superimposability of the biochemical basis of body scent. A different approach has now been tried, based on plasma protein fractionating procedures. A pilot study has shown that PolyAcrilamide Gel Electrophoresis (PAGE) results, as expected, in widely different patterns only in DZ twin pairs. Plasma specimens from a sample of 55 supposedly MZ twin pairs were analysed by PAGE. The substantial superimposability of MZ cotwin patterns was confirmed. Analysis of the different fractions indicates the extent of environmental variability. Application to zygosity determination is suggested.

A total of 10 pairs of twin adolescent boys, 14 MZ and 5 DZ, were used to determine the relative contribution of the genotype to the individual differences observed in surface density of muscle, subcutaneous fat, and bone. The cross-sectional areas of these tissue components were calculated from photographs of the upper arm, taken by means of an ultrasonic apparatus. The respective mean percentage intrapair difference in MZ and DZ twins was 6.9 and 9.8 for total area, 6.6 and 7.8 for muscle, 13.9 and 32.0 for fat, and 6.7 and 31.3 for bone tissue. Interindividual variations in bone only could be ascribed to genetic differences.

The distribution of A1A2BO, MN, and Rh-Hr blood groups, of salivary ABH substances, and of natural antibodies, was assessed in a sample of 47 MZ twin pairs. The resulting distribution provides indications as to the level of genetic control over the quantitative expression of these traits.

The present report describes the preliminary results of an extensive dermatoglyphic investigation among 491 pairs of MZ and DZ twins, and sibs of both sexes. Paired homolateral comparisons were performed for the determination of the degree of discordance for each dermatoglyphic trait and the results were presented in a series of histograms. Some of the outstanding points brought forth by this study are: (1) different dermatoglyphic traits at different digits or palmar areas have different degrees of discordance; (2) in MZ twins the frequency of discordance for each trait is constant regardless of sex or laterality, whereas in DZ twins and sibs both sex and lateral differences are observed; (3) the higher number of significant differences in concordance between the MZ twins vs. sibs than between MZ vs. DZ twins suggests that, at least in the female, the dermatoglyphics may be affected by changing intrauterine environments.

The present study involving MZ and DZ twin pairs, as well as paired normal sibs, was undertaken to investigate further the genetic involvement in the formation of normal palmar crease patterns, as well as simian and Sydney patterns. Two variations of normal creases were investigated: the thenar type R and the distal type I. Both types cluster in families. The R is more frequent in the females. For both types, MZ twins are more concordant than DZ twins or sibs. Sib studies suggest strong genetic involvement in the development of both the R and I crease types. Parents with simian and/or Sydney creases have more children with these traits than offspring of parents without these traits; however, concordance rate comparisons of MZ or DZ twins and sibs do not suggest strong genetic involvement as in the case of the R and I crease types.

Among 59 sets of MZ male twins and 48 sets of MZ female twins from Italy, 7 male sets (11.9%) and 7 female sets (14.6%) had absent c-triradius on one or more palms; among 214 individuals who had a MZ twin, 19 (8.9%) had absent c-triradius in one or both palms. Due to small numbers these percentages are comparable to the occurrence of absent c-triradius found in 8.2% of 3946 Caucasians. When one member of a set of MZ twins showed the trait in one or both palms, 35.7% (5/14) of the other members also showed the trait in one or both palms. When an individual showed the trait in one palm, 26.3 % (5/19) of the other palms also showed the trait. These two estimates of penetrance combined give an average value of 30.3%, (10/33) which is comparable to an estimate of 30% found in a study of 478 family units in which transmission was compatible with an autosomal dominant gene.

Cupping of the optic nerve head is regarded as one of a triad of diagnostic signs associated with glaucoma. This study was undertaken to assess the role of genetic factors in determining size of the physiologic cup of the normal optic nerve head, as measured by a horizontal cup/disc ratio. This is important to our ultimate understanding of the determinants of pathologic change. Horizontal cup/disc ratio was estimated in a sample of 37 MZ and 26 like-sex DZ twin pairs aged 15 years and older. Differences between MZ and DZ samples with respect to intrapair variance and intraclass correlation coefficient were highly significant. This finding of high heritability for measurements of cup diameter is in contrast to low heritability estimates found in a companion investigation on heritability of the effect of corticosteroids on intraocular pressure.

It is observed that neither the methodology of twin studies nor that of behavior genetics in general has been adequately explored in most investigations of human behavior. General distrust of genetic methodologies as well as the belief that genie disorders are unalterable appear to be salient factors in explaining the neglect of those areas by social scientists. A plea is made for putting aside biased attitudes that prevent utilization of the best available techniques for the study of human beings as distinguished from all other animals.

The article presents a brief review of the genetic aspects of intelligence as measured by IQ tests. The results from family and twin investigations seem to agree in as much as high heritabilities for IQ can be calculated. There are, however, a large number of insecurities and sources of error which invite to criticism. Although, at present, it is justified to assume that genetic factors contribute substantially to variations in IQs, it would be premature to claim that genetic and other causes of variation could be expressed in precise figures. It seems doubtful if this situation could be further improved by continued use of IQs in genetic investigations. It should be more profitable to use other entities, such as well-defined special abilities, which could be designed directly for genetic analyses.

A large-scale adoption study is underway at the University of Texas where IQ test scores are available for over 1200 women who gave up their children for adoption immediately following delivery. The adopted children and their adoptive families are now being located and tested and, if current trends continue, complete data should eventually be available on over 400 families. About half of these families will contain two or more adopted children or both adopted and natural children. At present, data are available for 89 families. The results are consistent with twin data and support a genetic hypothesis. The biological mother-adopted child correlation is 0.37 whereas the adoptive mother-adopted child and adoptive father-adopted child correlations are only 0.22 and 0.04 respectively. There are 59 pairs of unrelated children reared together in this sample and the IQ correlation for these children is only 0.13.