Comorbidity in hyperactive children: issues related to selection bias, gender, severity, and internalizing symptoms

Parental behavior ratings of hyperactive boys and girls aged 6-11 years seen for evaluation in a general outpatient child psychiatry clinic and in a specialty "attention deficit disorder" (ADD) program were analyzed. The pattern of comorbidity in the two groups showed no significant differences. Most of the associated internalizing and externalizing behavioral scales in hyperactive boys and in hyperactive girls were elevated and in clinical ranges except for the "somatic complaints" scale. Comorbidity in both hyperactive boys and girls was related to the severity of the hyperactivity itself.

Linkage between sexual orientation and chromosome Xq28 in males but not in females

We have extended our analysis of the role of the long arm of the X chromosome (Xq28) in sexual orientation by DNA linkage analyses of two newly ascertained series of families that contained either two gay brothers or two lesbian sisters as well as heterosexual siblings. Linkage between the Xq28 markers and sexual orientation was detected for the gay male families but not for the lesbian families or for families that failed to meet defined inclusion criteria for the study of sex-linked sexual orientation. Our results corroborate the previously reported linkage between Xq28 and male homosexuality in selected kinships and suggest that this region contains a locus that influences individual variations in sexual orientation in men but not in women.

Problem behavior in early and middle childhood: an initial behavior genetic analysis

Twin samples were used to estimate the relative importance of genetic and environmental influences on problem behavior in children, assessed by the Child Behavior Checklist (CBCL). For the younger cohort, mothers completed the CBCL/2-3, while for the older cohort the CBCL/4-18 was used. Nearly half of the older sample had also been assessed with the questionnaire for younger children at a prior date, providing tentative answers regarding continuity and change in the etiology of problem behavior. Results suggested that shared environmental influences may be more important in early childhood than in middle childhood, while the reverse holds for genetic influences.

A simple genetic basis for a complex psychological trait in laboratory mice

Psychological traits are commonly inferred from covariation in sets of behavioral measures that otherwise appear to have little in common. Emotionality in mice is such a trait, defined here by covariation in activity and defecation in a novel environment and emergence into the open arms of an elevated plus maze. Behavioral and quantitative trait analyses were conducted on four measures obtained from 879 mice from an F2 intercross. Three loci, on murine chromosomes 1, 12, and 15, were mapped that influence emotionality. This trait, inferred from studies of strain, sex, and individual differences in rodents, may be related to human susceptibility to anxiety or neuroticism.

Quantitative trait locus for reading disability: correction

In the news article "Can risky mergers save hospital-based research?" by Wade Roush (19 May, p. 968), the statement that University Hospitals of Cleveland rose from 20th in the rankings of teaching hospitals funded by the National Institutes of Health (NIH) in 1991 to 12th at present was incorrect. In fact, it was Case Western Reserve University (CWRU), with which University Hospitals of Cleveland is affiliated, that received $69 million in NIH grants in 1993, making it the 20th largest recipient of such grants among medical centers; the university then received $97 million in 1994, raising its rank to 12th. About $15 million of the increase, or 53%, was attributable to CWRU's 1992 affiliation with Henry Ford Hospital in Detroit. Other hospitals affiliated with Case Western include MetroHealth Medical Center, Mount Sinai Medical Center, St. Luke's Medical Center, and Cleveland Veterans' Affairs Medical Center.

Multipoint interval mapping of quantitative trait loci, using sib pairs

The sib-pair interval-mapping procedure of Fulker and Cardon is extended to take account of all available marker information on a chromosome simultaneously. The method provides a computationally fast multipoint analysis of sib-pair data, using a modified Haseman-Elston approach. It gives results very similar to those of the earlier interval-mapping procedure when marker information is relatively uniform and a coarse map is used. However, there is a substantial improvement over the original method when markers differ in information content and/or when a dense map is employed. The method is illustrated by using simulated sib-pair data.

Etiology of individual differences in reading performance: a test of sex limitation

To test the hypothesis that the etiology of individual differences in reading performance differs in males and females, reading performance data from twin pairs tested in the Colorado Learning Disabilities Research Center were fitted to structural equation models of sex limitation. The sample included 513 pairs of twins in which at least one member of each pair has a positive school history of reading problems [228 monozygotic (MZ), 176 same-sex dizygotic (DZ), and 109 opposite-sex DZ pairs] and 302 matched control pairs [148 MZ, 98 same-sex DZ, and 56 opposite-sex DZ pairs]. Estimates of the genetic correlation between performance in males and females were obtained by analysis of data from both same-sex and opposite-sex twin pairs (Neale and Cardon, 1992). The full model fit the data well (chi 2 = 17.74, df = 16, p = 0.340), and the resulting genetic parameter estimates were highly similar in males and females in both the proband and the control samples. The correlations between genetic influences in males and females do not differ among groups (change in chi 2 = 0.95, df = 1, p > or = 0.25), and the resulting pooled estimate is about .5. Thus, results of this analysis suggest that the etiology of individual differences in reading performance may differ to some extent in males and females.

Cognitive ability and academic achievement in the Colorado Adoption Project: a multivariate genetic analysis of parent-offspring and sibling data

To test the hypothesis that the etiology of covariation among measures of cognitive ability and academic achievement is due at least in part to shared genetic influences, data from 198 adoptive and 220 nonadoptive families participating in the Colorado Adoption Project were subjected to multivariate behavioral genetic analyses. Data on measures of cognitive ability (verbal comprehension and perceptual organization) and academic achievement (reading recognition and mathematics achievement) from related and unrelated sibling pairs tested at age 7, as well as from adoptive and nonadoptive parents, were analyzed. Phenotypic analyses confirmed previous findings of moderate correlations among measures of cognitive ability and achievement, averaging about .35. Although 54% of the covariation between reading and mathematics achievement was due to influences shared with verbal ability, a significant proportion of this covariation was independent of the cognitive ability measures. Heritabilities for the various measures were moderate, ranging from .21 to .37. Moreover, genetic influences accounted for 33-64% of their phenotypic covariation; for example, 33-60% of the observed correlations between verbal comprehension and the achievement measures, 64% of those between perceptual organization and the achievement measures, and 63% of that between reading recognition and mathematics achievement were due to shared genetic influences. Similar to the results of the phenotypic analysis, nearly half of the genetic covariance between reading and mathematics achievement was independent of cognitive ability. Their remaining covariance was due primarily to nonshared environmental influences.

Multivariate genetic analysis of Wechsler Intelligence Scale for Children--Revised (WISC-R) factors

Wechsler Intelligence Scale for Children--Revised (WISC-R) factor scores (Verbal Comprehension, Perceptual Organization, and Freedom from Distractibility) were obtained from 574 twin pairs in the Colorado Reading Project and subjected to multivariate genetic analysis. Variances were partitioned into components common to the three WISC-R factors and to those specific to each factor. Substantial commonality, both genetic and environmental, was found among the three factors. The full model fit the data well, and estimates of heritability and environmentality indicated that about half of the phenotypic variance for each factor is due to additive genetic effects. These results were compared to those obtained in a previous twin study of the three Wechsler Adult Intelligence Scale (WAIS) factors by Tambs et al. (1986).

Linkage analysis of a common oligogenic disease using selected sib pairs

Sib pairs drawn from the simulated common oligogenic disease families were selected for extreme quantitative trait scores and analyzed using interval mapping and multipoint methods. Linkage analyses of 112 selected sib pairs, in which one or more members had trait values exceeding the disease threshold, were compared with analyses of the total unselected sib-pair sample (771 pairs). Selected sample regression models yielded comparable significance levels to those obtained from the unselected sample at most loci on the six simulated chromosomes, demonstrating the efficiency of selected sib-pair analysis for quantitative characters. Two of the three disease QTLs were detected in both selected and unselected samples. Interval mapping and multipoint analyses yielded location estimates close to the simulated positions of the QTLs. The combined strategy of using interval mapping and multipoint methods with selected sib pairs appears to provide improved accuracy and sensitivity over more traditional sib-pair methods for detecting quantitative trait loci.

Quantitative trait locus for reading disability on chromosome 6

Interval mapping of data from two independent samples of sib pairs, at least one member of whom was reading disabled, revealed evidence for a quantitative trait locus (QTL) on chromosome 6. Results obtained from analyses of reading performance from 114 sib pairs genotyped for DNA markers localized the QTL to 6p21.3. Analyses of corresponding data from an independent sample of 50 dizygotic twin pairs provided evidence for linkage to the same region. In combination, the replicate samples yielded a chi 2 value of 16.73 (P = 0.0002). Examination of twin and kindred siblings with more extreme deficits in reading performance yielded even stronger evidence for a QTL (chi 2 = 27.35, P < 0.00001). The position of the QTL was narrowly defined with a 100:1 confidence interval to a 2-centimorgan region within the human leukocyte antigen complex.

The power of interval mapping of quantitative trait loci, using selected sib pairs

The interval-mapping procedure of Fulker and Cardon for analysis of a quantitative-trait loci (QTL) is extended for application to selected samples of sib pairs. Phenotypic selection of sib pairs, which is known to yield striking increases in power when a single marker is used, provides further increases in power when the interval-mapping approach is used. The greatest benefits of the combined approach are apparent with coarse maps, where QTLs of relatively modest (15%-20%) heritability can be detected with widely spaced markers (40-60 cM apart) in reasonably sized sibling samples. Useful information concerning QTL location is afforded by interval mapping in both selected and unselected samples.

Continuity and change in infant shyness from 14 to 20 months

Objective measures of shyness in the MacArthur Longitudinal Twin Study were obtained in two testing situations: the laboratory and the home. A longitudinal hierarchical model was fitted to the data, allowing estimation of the extent to which genetic, shared environmental, and unique environmental influences contributed to continuity and change of the shyness phenotype from 14 to 20 months of age. The sample consisted of 163 monozygotic and 138 same-sex dizygotic twin pairs. Models were fitted to raw data using a maximum-likelihood pedigree approach. Genetic, shared environmental, and unique environmental first-order factors, with specific variances, were modeled on each of four shyness ratings assessed in the laboratory and home at 14 and 20 months. Four second-order genetic, shared environmental, and unique environmental factors were also modeled. Results indicated that developmental change from 14 to 20 months and situational specificity between the laboratory and the home are mediated largely by shared and unique environmental influences. Genetic variation is largely responsible for both the stability in shyness from 14 to 20 months and the phenotypic correlations observed between the laboratory and the home settings.

A sib-pair approach to interval mapping of quantitative trait loci

An interval mapping procedure based on the sib-pair method of Haseman and Elston is developed, and simulation studies are carried out to explore its properties. The procedure is analogous to other interval mapping procedures used with experimental material, such as plants and animals, and yields very similar results in terms of the location and effect size of a quantitative trait locus (QTL). The procedure offers an advantage over the conventional Haseman and Elston approach, in terms of power, and provides useful information concerning the location of a QTL. Because of its simplicity, the method readily lends itself to the analysis of selected samples for increased power and the evaluation of multilocus models of complex phenotypes.

A model of developmental change in hierarchical phenotypes with application to specific cognitive abilities

A hierarchical longitudinal path model is described for analysis of twin and sibling data. The model combines multivariate and longitudinal methodologies for assessment of continuity and change in the relationships among characters over time. Additionally, the model permits assessment of shared and independent etiologies for groups of measures at single and multiple occasions. The procedure is illustrated by application to specific cognitive ability data from 103 adopted and 109 nonadopted sibling pairs at ages 3, 4, 7, and 9 years, and 50 pairs of monozygotic and dizygotic twins at ages 3 and 4 years. The results suggest that much of the observed continuity in general intelligence measures is attributable to genetic influences common to specific abilities and indicate differential etiologies for specific abilities at different occasions in childhood.

Genetic and environmental influences on early childhood behavior

The Child Behavior Checklist for Ages 2-3 (Adelbach et al., J. Abnorm. Child Psychol. 15;629-650; 1987) was completed by mothers of 229 pairs of twins (mean age = 33 months). Using the two broad-band groupings of Internalizing and Externalizing described by Achenbach et al. (1987), various models to estimate genetic and environmental parameters were fitted using LISREL 7. Model-fitting results showed that the genetic components to the observed phenotypical variation were small and not necessary in the model. Influences from the shared environment, however, could not be dropped from the model without a deterioration in fit. Parameter estimates were not significantly different in boys and girls.

Cognitive abilities of children at 7 and 12 years of age in the Colorado adoption project

To test the hypothesis that adopted children are at an elevated risk for learning disabilities, the achievement and WISC-R test scores of a nonclinical sample of adopted and nonadopted (control) children were compared at 7 years of age (n = 108 adopted males, 91 adopted females, 116 control males, 100 control females) and 12 years of age (n = 69 adopted males, 61 adopted females, 56 control males, 44 control females). Although the average Verbal IQ of the adopted children was significantly lower than that of the nonadopted children at both ages, these differences accounted for only about 2% to 4% of the variance. When scores on the individual subtests of the WISC-R were compared, the group difference was significant only for Similarities at age 7, and for Comprehension at age 12. Moreover, with regard to the achievement tests, the proportion of adopted children who scored more than 1.5 standard deviations below expected, based on IQ, was not significantly greater than that of controls. Also, the proportions of adopted and control children placed in special education classes were not significantly different. Thus, the results of this study provide little or no evidence for an increased risk of learning disabilities in "easily placed" adopted children.

Genetic and environmental effects on blood cells

In a sample of 105 concordant sex MZ and DZ twin pairs, the following characteristics were measured: red cell count, haemoglobin concentration, package cell volume, mean cell volume, mean cell haemoglobin, mean cell haemoglobin concentration, reticulocytes, platelets, white cell count and the six types of leucocytes, lymphocytes, monocytes, band and segmented neutrophils, eosinophils and basophils. The statistical model employed in the univariate twin analysis allows for three sources of variation: genetic (h2), shared environmental (c2) and specific environmental influences (e2). A genetic component was significant for red cell count, haemoglobin and mean cell haemoglobin (0.64, 0.60 and 0.46 respectively), with heritable variation suggested for package cell volume, mean cell volume, mean cell haemoglobin, lymphocytes and monocytes. Shared environmental variation was only present for neutrophils.

Genetic change and continuity from fourteen to twenty months: the MacArthur Longitudinal Twin Study

Genetic change as well as continuity was investigated within the domains of temperament, emotion, and cognition/language for 200 pairs of twins assessed at 14 and 20 months of age in the laboratory and home. The second year of life is marked by change rather than continuity: correlations from 14 to 20 months averaged about .30 for observational measures of temperament and emotion, about .40 for language measures, and about .50 for mental development. 2 types of genetic change were examined: changes in the magnitude of genetic influence (heritability) and genetic contributions to change from 14 to 20 months. In general, heritability estimates were similar at 14 and 20 months. Evidence for genetic influence on change from 14 to 20 months emerged for several measures, implying that heritability cannot be equated with stability. Analyses of continuity indicated that genetic factors are largely responsible for continuity from 14 to 20 months.

A genetic and environmental time series analysis of blood pressure in male twins

Systolic and diastolic blood pressures were measured on 254 monozygotic (MZ) and 260 dizygotic (DZ) male twin pairs, during middle age (average age 48 years) and at two later age points. Genetic and environmental components of covariation were modeled by time series. For both measures, shared environmental influences were absent and specific environmental influences were largely time-specific. Although heritability was about 0.5 at each time point, genetic variation present at middle age contributed only about 60% to that present 9 years later, the remaining 40% being new. Fifteen years later, at the third time point, no new genetic variation was evident, variation in individual differences being entirely attributable to genetic differences laid down at the two earlier ages.

A maximum-likelihood model-fitting approach to conducting a Hayman analysis of diallel tables with complete or missing data

A method is presented for conducting a Hayman analysis of non-replicated diallel tables using a maximum-likelihood (ML) model-fitting approach, rather than a traditional analysis of variance (ANOVA) approach. Hayman's linear model for a diallel analysis is used to generate a table of expected cell means. This table of expected cell means is fit to a table of observed cell means, and the fit is assessed using a chi-square value. Often data collected from diallel crosses fail to meet the underlying assumptions of ANOVA. The ML method makes no assumptions about equal cell sizes or homogeneity of variance. Thus, the ML method for diallel analysis provides some statistical advantages over ANOVA methods. The ML method also offers the advantage of having the ability to analyze diallels with missing cells. Using the ML method, incomplete diallel tables can be analyzed, and the partitioning of all the sources of variation in a diallel table is still accomplished from the remaining crosses. These advantages make the ML method an attractive approach for extracting the maximum amount of information from a diallel table.

Temperament, emotion, and cognition at fourteen months: the MacArthur Longitudinal Twin Study

200 pairs of twins were assessed at 14 months of age in the laboratory and home. Measures were obtained of temperament, emotion, and cognition/language. Comparisons between identical and fraternal twin correlations suggest that individual differences are due in part to heritable influences. For temperament, genetic influence was significant for behavioral observations of inhibition to the unfamiliar, tester ratings of activity, and parental ratings of temperament. For emotion, significant genetic influence was found for empathy and parental ratings of negative emotion. The estimate of heritability for parental report of expression of negative emotions was relatively high, whereas that for expression of positive emotions was low, a finding consistent with previous research. For cognition and language, genetic influence was significant for behavioral indices of spatial memory, categorization, and word comprehension. Shared rearing environment appears influential for parental reports of language and for positive emotions, but not for other measures of emotion or for temperament.

Confirmatory factor analysis of reading and mathematics performance: a twin study

Reading and mathematics performance data from a sample of 264 reading-disabled twin pairs and 182 matched control twin pairs were subjected to multivariate behavior genetic analysis. The factor structure of reading and math performance measures was found to be highly similar for both groups. Consistent with previous findings obtained using alternative methods, a significant heritable component to individual differences in reading performance was found both within the reading-disabled (h2 = 0.78) and control (h2 = 0.74) twin samples. In addition, a substantial genetic influence on mathematics performance was found (h2 = 0.51 and 0.60 in the reading-disabled and control samples, respectively), although shared environmental influences common to both members of a twin pair also contribute significantly to the variance in math scores of both groups (c2 = 0.44 and 0.37). Moreover, genetic influences accounted for 98% of the observed correlation between reading and math performance within the sample of reading-disabled twin pairs, and for 55% of the observed correlation in the control sample. Thus, individual differences in both reading and mathematics performance are highly heritable and appear to be caused by many of the same genetic influences.

Multiple regression analysis of twin data: a model-fitting approach

The multiple regression methodology proposed by DeFries and Fulker (DF; 1985, 1988) for the analysis of twin data is compared with maximum-likelihood estimation of genetic and environmental parameters from covariance structure. Expectations for the regression coefficients from submodels omitting the h2 and c2 terms are derived. Model comparisons similar to those conducted using maximum-likelihood estimation procedures are illustrated using multiple regression. Submodels of the augmented DF model are shown to yield parameter estimates highly similar to those obtained from the traditional latent variable model. While maximum-likelihood estimation of covariance structure may be the optimal statistical method of estimating genetic and environmental parameters, the model-fitting approach we propose is a useful extension to the highly flexible and conceptually simple DF methodology.