Sex-limited expression of genetic or environmental factors occurs in two basic forms. First, the effects of a factor may be larger on one sex than on another, which is known as scalar sex limitation. Second, some factors may have an effect on one sex but not on the other, which is called nonscalar sex limitation. In the classical twin study, scalar sex-limited effects cause same-sex male and same-sex female twin correlations to differ. Nonscalar sex-limited effects would cause the correlations between opposite-sex pairs of relatives to be lower than would be expected from the correlations between relatives of the same sex. One approach to modeling such effects is to allow the genetic correlation between opposite-sex dizygotic twins to be less than one-half; another is to allow the common environment correlation for opposite-sex pairs to be less than unity. Extension of this approach to the multivariate case is not straightforward. Direct extension of the Cholesky decomposition such that each Cholesky factor is allowed to correlate less than one-half in opposite-sex pairs yields a model where the order of the variables can change the goodness-of-fit of the model. It is shown that similar problems exist with a variety of multivariate and longitudinal models, and in a variety of models of genotype × environment interaction. Several solutions to these problems are described.

When using maximum likelihood methods to estimate genetic and environmental components of (co)variance, it is common to test hypotheses using likelihood ratio tests, since such tests have desirable asymptotic properties. In particular, the standard likelihood ratio test statistic is assumed asymptotically to follow a χ2 distribution with degrees of freedom equal to the number of parameters tested. Using the relationship between least squares and maximum likelihood estimators for balanced designs, it is shown why the asymptotic distribution of the likelihood ratio test for variance components does not follow a χ2 distribution with degrees of freedom equal to the number of parameters tested when the null hypothesis is true. Instead, the distribution of the likelihood ratio test is a mixture of χ2 distributions with different degrees of freedom. Implications for testing variance components in twin designs and for quantitative trait loci mapping are discussed. The appropriate distribution of the likelihood ratio test statistic should be used in hypothesis testing and model selection.

The basis of most twin studies is the ascertainment of twins, often through twin registries, and determination of zygosity. The current rate of twin births in many industrialized countries implies that in the near future around 3% or more of individuals will be a twin. Hence, there are and will be a lot of twins around and many of those will not participate in twin studies. However, if large population-based samples are available that include appropriate identifiers, then twins can be detected and twin studies performed, even in the absence of zygosity information. We quantified the number of twin pairs that could be detected from a longitudinal survey in the Netherlands, which aims to answer questions about educational strategies and performance in primary education in the Netherlands. We detected 2865 twin pairs if we used a coded name identifier, date of birth, school, grade and year of survey, which is 2.01% of 284,945 pupils in five cohorts. Relaxing our selection criteria increased the number of apparent twin pairs identified, most of which are false positives due to chance matching of identification criteria. We show that the intraclass correlation on measured phenotypes can be used as a quality control measure for twin identification, and quantify the proportion of false negatives (true twin pairs not identified) due to missing data and data coding errors. We compared our estimated rate of twins in the sample to census data and estimate that with our most stringent selection criteria we detect more than 80% of all twin pairs in the sample. We conclude that the identification of twin pairs from large population-based samples is feasible, rapid and accurate if the appropriate identifiers are available, and that twin pairs from such sources are a valuable resource for studies to answer scientific question about twins versus nontwins and about genetic and environmental factors of twin resemblance.

In this article, we present the genomic DNA yield and the microsatellite and single nucleotide polymorphism (SNP) genotyping success rates of genomic DNA extracted from a large number of mouth swab samples. In total, the median yield and quality was determined in 714 individuals and the success rates in 378,480 genotypings of 915 individuals. The median yield of genomic DNA per mouth swab was 4.1 μg (range 0.1–42.2 μg) and was not reduced when mouth swabs were stored for at least 21 months prior to extraction. A maximum of 20 mouth swabs is collected per participant. Mouth swab samples showed in, respectively, 89% for 390 microsatellites and 99% for 24 SNPs a genotyping success rate higher than 75%. A very low success rate of genotyping (0%–10%) was obtained for 3.2% of the 915 mouth swab samples using microsatellite markers. Only 0.005% of the mouth swab samples showed a geno-typing success rate lower than 75% (range 58%–71%) using SNPs. Our results show that mouth swabs can be easily collected, stored by our conditions for months prior to DNA extraction and result in high yield and high-quality DNA appropriate for genotyping with high success rate including whole genome searches using microsatellites or SNPs.

Previous studies have looked at the structure of attention-deficit/hyperactivity disorder (ADHD) using latent class analysis (LCA) of Child Behavior Checklist (CBCL) or Diagnostic and Statistical Manual of Mental Disorders (DSM) symptom structure. These studies have identified distinct classes of children with inattentive, hyperactive, or combined subtypes and have used these classes to refine genetic analyses. The objective of the current report is to determine if the latent class structure of ADHD subtypes is consistent across informant using the Conners' Rating Scales (CRS). LCA was applied to CRS forms from mother, father, and teacher reports of 1837, 1329 and 1048 latency aged Dutch twins, respectively. The optimal solution for boys was a 5-class solution for mothers, a 3-class solution for fathers, and a 4-class solution for teachers. For girls, a 4-class solution for mothers and a 3-class for fathers and teachers was optimal. Children placed into a class by one informant had markedly increased odds ratio of being placed into the same or similar class by the other informants. Results from LCA using Dutch twins with the CRS show stability across informants suggesting that more stable phenotypes may be accessible for genotyping using a multi-informant approach.

This article applies methods of latent class analysis (LCA) to data on lifetime illicit drug use in order to determine whether qualitatively distinct classes of illicit drug users can be identified. Self-report data on lifetime illicit drug use (cannabis, stimulants, hallucinogens, sedatives, inhalants, cocaine, opioids and solvents) collected from a sample of 6265 Australian twins (average age 30 years) were analyzed using LCA. Rates of childhood sexual and physical abuse, lifetime alcohol and tobacco dependence, symptoms of illicit drug abuse/dependence and psychiatric comorbidity were compared across classes using multinomial logistic regression. LCA identified a 5-class model: Class 1 (68.5%) had low risks of the use of all drugs except cannabis; Class 2 (17.8%) had moderate risks of the use of all drugs; Class 3 (6.6%) had high rates of cocaine, other stimulant and hallucinogen use but lower risks for the use of sedatives or opioids. Conversely, Class 4 (3.0%) had relatively low risks of cocaine, other stimulant or hallucinogen use but high rates of sedative and opioid use. Finally, Class 5 (4.2%) had uniformly high probabilities for the use of all drugs. Rates of psychiatric comorbidity were highest in the polydrug class although the sedative/opioid class had elevated rates of depression/suicidal behaviors and exposure to childhood abuse. Aggregation of population-level data may obscure important subgroup differences in patterns of illicit drug use and psychiatric comorbidity. Further exploration of a ‘self-medicating’ subgroup is needed.

Numerous studies have reported association between variants in the dystrobrevin binding protein 1 (dysbindin) gene (DTNBP1) and schizophrenia. However, the pattern of results is complex and to date, no specific risk marker or haplotype has been consistently identified. The number of single nucleotide polymorphisms (SNPs) tested in these studies has ranged from 5 to 20. We attempted to replicate previous findings by testing 16 SNPs in samples of 41 Australian pedigrees, 194 Australian cases and 180 controls, and 197 Indian pedigrees. No globally significant evidence for association was observed in any sample, despite power calculations indicating sufficient power to replicate several previous findings. Possible explanations for our results include sample differences in background linkage dis-equilibrium and/or risk allele effect size, the presence of multiple risk alleles upon different haplotypes, or the presence of a single risk allele upon multiple haplotypes. Some previous associations may also represent false positives. Examination of Caucasian HapMap phase II genotype data spanning the DTNBP1 region indicates upwards of 40 SNPs are required to satisfactorily assess all nonredundant variation within DTNBP1 and its potential regulatory regions for association with schizophrenia. More comprehensive studies in multiple samples will be required to determine whether specific DTNBP1 variants function as risk factors for schizophrenia.

Twin designs, comparing correlations in monozygotic (MZ) versus dizygotic (DZ) twins, have an extensive history. One major confounder in such studies is that MZ twins may share postnatal environmental influences more so than do DZ twins. To avoid such confounding, twins separated at or soon after birth have been studied, but their scarcity often makes this approach impractical. Another method has been to measure the degree of contact twins have maintained over time, and adjust the observed correlations. Here, we remove confounding by utilizing the discrepancy between biological and self-perceived zygosity to separate environmental from genetic sources of twin similarity. We analyzed dietary patterns and physiologic traits in 350 female twin pairs of the 1988 Kaiser Permanente Twin Registry. Among twin pairs, 175 were MZ by self-report and genetic testing (MZC), 136 were DZ by self-report and genetic testing (DZC), 30 were MZ by genetic testing but not by self-report (MZW), and 9 were DZ by genetic testing but not by self-report (DZW) but were excluded due to small sample size. For healthy food patterns, MZC and MZW intraclass correlations were similar and greater than for DZC, yielding positive and significant heritability estimates. For unhealthy food patterns, the MZC, MZW and DZC correlations were similar with no significant heritability. For physiologic traits, MZC and MZW correlations were similar and higher than those for DZC, indicating significant heritability, except for insulin for which MZW and DZC were similar and which showed modest heritability. Twins of mistaken zygosity (TOMZ) provides a useful approach to robust determination of heritability.

The consensus from published studies is that plasma lipids are each influenced by genetic factors, and that this contributes to genetic variation in risk of cardiovascular disease. Heritability estimates for lipids and lipoproteins are in the range .48 to .87, when measured once per study participant. However, this ignores the confounding effects of biological variation measurement error and ageing, and a truer assessment of genetic effects on cardiovascular risk may be obtained from analysis of longitudinal twin or family data. We have analyzed information on plasma high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, and triglycerides, from 415 adult twins who provided blood on two to five occasions over 10 to 17 years. Multivariate modeling of genetic and environmental contributions to variation within and across occasions was used to assess the extent to which genetic and environmental factors have long-term effects on plasma lipids. Results indicated that more than one genetic factor influenced HDL and LDL components of cholesterol, and triglycerides over time in all studies. Nonshared environmental factors did not have significant long-term effects except for HDL. We conclude that when heritability of lipid risk factors is estimated on only one occasion, the existence of biological variation and measurement errors leads to underestimation of the importance of genetic factors as a cause of variation in long-term risk within the population. In addition our data suggest that different genes may affect the risk profile at different ages.

The aim of this study was to estimate the heritability of variation in testosterone levels in 12-year-old children, and to explore the overlap in genetic and environmental influences on circulating testosterone levels and androgen-dependent pubertal development. Midday salivary testosterone samples were collected on 2 consecutive days in a sample of 183 unselected twin pairs. Androgen-induced pubertal development was assessed using self-report Tanner scales of pubic hair development (boys and girls) and genital development (boys). A significant contribution of genetic effects to the variance in testosterone levels was found. Heritability was approximately 50% in both boys and girls. The remaining proportion of the variance in testosterone levels could be explained by nonshared environmental influences. The relatively high correlation between testosterone levels of opposite-sex dizygotic twins suggests that sex differences in genes influencing variation in testosterone levels have not yet developed in preand early puberty. Variance in pubertal development was explained by a large genetic component, moderate shared environmental influences, and a small nonshared environmental effect. Testosterone levels correlated moderately (r = .31) with pubertal development; the covariance between testosterone levels and pubertal development was entirely accounted for by genetic influences.

It is estimated that 1.6 billion people worldwide have myopia, a refractive error, and this number is expected to increase to approximately 2.5 billion by the year 2020. It is now well established that both the environment and genetics play a role in the development of myopia. However, the exact contribution of each of these components to myopia development has yet to be completely determined. Twin studies (classical twin model) are commonly used to determine the weighting of genetic and environmental components in disease. Over the last century, twin studies have investigated the heritability of refractive errors in different sample populations and have collectively supported a genetic basis to refractive errors. However, different sample populations and methods of data collection have produced a wide range of heritability estimates ranging from .5 to .9. This article will review those twin studies that have investigated refractive error, particularly myopia, as well as biometric measures linked to refractive error, to compare heritability estimates and methodology designs.

Diseases of the myoneural junction and muscle are disabling and some are life-threatening. Recent successes in the identification of the underlying genetic mechanisms have had profound implication for their diagnostics, treatment and classification. We define familial risks for siblings who were hospitalized for or deceased from diseases of the myoneural junction and muscle. A nationwide database on diseases of the myoneural junction and muscle was constructed by linking the Multigeneration Register on 0- to 69-year-old siblings to the Hospital Discharge Register and the Causes of Death Register from years 1987 to 2001. Standardized risk ratios (SIRs) were calculated for affected sibling pairs by comparing to those whose siblings had no diseases of myoneural junction and muscle. Among a total of 2307 patients, myasthenia gravis, muscular dystrophy and myotonic disorders were commonest diagnoses. The sibling risks for these disease were 22, 190 and 198, respectively, when a sibling was diagnosed with any disease of the myoneural junction and muscle. The concordant SIRs, both siblings presenting the same disease, were 42 for myasthenia gravis, 737 for muscular dystrophy, 2000 for congenital myopathy, 1211 for myotonic disorder, 909 for periodic paralysis and 209 for unspecified myopathy. Only a few discordant sibling pairs were noted. The very high overall SIRs for the diseases of the myoneural junction and muscle imply that the sporadic forms of these diseases are relatively rare and these diseases are overwhelmingly heritable.

Recent successes in identifying the underlying genetic mechanisms for neurological diseases, particularly for their Mendelian forms, have had profound implications for their diagnostics, treatment and classification. However, there has never been an attempt to compare familial risks in a systematic way among and between the main neurological diseases. Familial risks were here defined for siblings who were hospitalized because of a neurological disease in Sweden. A nationwide database for neurological diseases was constructed by linking the Multigeneration Register of 0- to 69-year-old siblings to the Hospital Discharge Register for the years 1987 to 2001. Standardized risk ratios were calculated for affected sibling pairs by comparing them to those whose siblings had no neurological disease. There were three main results. First, it was shown that all disease groups had a familial risk, with the exception of transient ischemic attacks, and the risks could be ranked from the highest (3451) for Huntington's disease to the lowest (2.1) for inflammatory diseases. Second, increased familial risks were shown for disease subtypes for which susceptibility genes or familial clustering have not been demonstrated previously, including multiple sclerosis, sleep apnea, nerve, nerve root and plexus disorders, and cerebral palsy. Third, based on the available sample size there was no convincing evidence for familial comorbidity between the disease groups, suggesting that the factors causing familial aggregation, probably usually heritable genes, are distinct for each subtype. The high familial risks for neurological disease imply heritable etiology and opportunities for identification of further susceptibility genes.

Trends in the civil registration of fetal death in multiple gestations that has occurred before, but expelled from the womb after, 24 weeks' gestation are examined using England and Wales 1993–2004 registration obtained from the Office for National Statistics. Count was made of fetal death registrations in which fetus papyraceous, fetal death before 24 weeks' gestation or fetocide before 24 weeks' gestation was recorded. There were 3700 fetal death registrations among 217,595 twin, triplet and higher order multiple births in England and Wales between 1993 and 2004. In 354 (9.6%) of these fetal deaths, death was recorded as having occurred before 24 weeks' gestation. There has been a threefold increase in such fetal death registrations. It is a legal requirement of parents to register a fetal death. The definition of a fetal death that meets formal registration criteria is that the fetus is expelled from the womb after 24 weeks' gestation. However, if the fetal death occurs before 24 weeks, there is confusion, nationally and internationally, whether or not registration is legally required. Fetal death in a multiple gestation has serious clinical implications for a surviving co-conceptus and failure to inform parents of an early death in a multiple gestation may have important repercussions. Legal definition for the registration of fetal death requires international agreement and application.

Temporal variations in the stillbirth rate among singletons, twins and triplets in Sweden between 1869 and 1967 were studied. Both among single and multiple births there were marked secular decreasing trends in the stillbirth rates. Based on our long time series since 1869, this study confirms that among twins and triplets the stillbirth rate was higher among same-sexed than among opposite-sexed sets. Comparisons between the stillbirth rates among twin births in urban and rural regions indicate higher stillbirth rates in rural areas. In addition, the stillbirth rates among twins of unmarried mothers were higher than those of twins of married mothers. These findings also hold for both same-sexed and opposite-sexed twin pairs. Analyses of the stillbirth rates for singletons and for different types of twins indicate that up to 1950 the risk of stillbirth among males was almost constantly between 15% to 20% higher than among females. After that the difference in the risk decreased. Comparisons with other populations were performed.

The combination of an onphalocele, an anterior thoracic wall defect and an anterior diaphragmatic defect constitutes classical Cantrell's pentalogy. We present a case of Cantrell's pentalogy diagnosed prenataly in twins with conventional and three-dimensional sonography.

One way to achieve the large sample sizes required for genetic studies of complex traits is to combine samples collected by different groups. It is not often clear, however, whether this practice is reasonable from a genetic perspective. To assess the comparability of samples from the Australian and the Netherlands twin studies, we estimated Fst (the proportion of total genetic variability attributable to genetic differences between cohorts) based on 359 short tandem repeat polymorphisms in 1068 individuals. Fst was estimated to be 0.30% between the Australian and the Netherlands cohorts, a smaller value than between many European groups. We conclude that it is reasonable to combine the Australian and the Netherlands samples for joint genetic analyses.

David Puts and Khytam Dawood's recent critique of my book, The Case of the Female Orgasm: Bias in the Science of Evolution, attempts to make plausible an adaptive account of female orgasm based on a hypothesized mechanism of uterine upsuck and sperm competition. Yet the authors fail to respond to the criticisms of such accounts that I detailed previously in my book. They raise a further concern about my definition of adaptation — a red herring — and manufacture a conceptual error regarding heritability that they then attribute to me. Most seriously, they fail to address the glaring failure of sperm competition accounts to accord with evidence from sexology. Specifically, the distribution curve of orgasm-with-intercourse — according to Dawood et al.'s own data, as well as others' — is relatively flat across the various classes. This curve needs to be tested against a well-formed multistrategy adaptive hypothesis; it cannot be explained by the adaptive account defended by Puts and Dawood in their critique.

The nature and frequency of quadruplet births are reviewed, followed by a close-up look at the 6-year-old monozygotic Mathias quadruplets. This essay is followed by reviews of new twin research on color–number association, male–female twins' finger-length ratios, and links between twinning and dietary practices. The article finishes by focusing on unusual twin athletes in football, basketball and golf, and on twin co-authors of a new book on the 1972 silver medal victory by the United States hockey team.