Biomedical subjects

D W Fulker

Publications and source records attributed to D W Fulker.

At least 55 records · Page 3Linked to original sources

PubMed1993

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.

Alleles↗

PubMed1993

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.

Blood Cells↗

PubMed1993

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.

Achievement↗

PubMed1993

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.

Child Development↗

PubMed1992

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.

Aptitude↗

PubMed1992

Differential heritability across levels of cognitive ability.

Differences in heritability and shared environmentality across levels of cognitive ability were assessed in a sample of 264 twin pairs tested at 1 year of age and in subsets tested at 2 and 3 years. Using an extension of the DF multiple regression methodology for analyzing twin data, no evidence was found for a linear or quadratic effect of level of cognitive ability on estimates of heritability or shared environmentality.

Aptitude↗

PubMed1992

Quantitative genetic analysis of IQ development in young children: multivariate multiple regression with orthogonal polynomials.

The study of psychological development has recently benefited from innovative analytic methods for estimating and examining the correlates of individual growth curves. These methods are more consistent with a conceptualization of development as an ongoing, continuous process, rather than as increases or decreases in a trait between two discrete time points. Recent developmental behavior genetic models have focused on continuity and change in the genetic and environmental influences underlying phenotypes. In contrast, we present a model for genetic and environmental influences on phenotypic development per se. In this model, we adapted multiple regression methods developed for twin designs (DeFries and Fulker, 1985) to a parent-offspring adoption design and to a multivariate framework in which repeated measurements are decomposed into orthogonal polynomial trends. We applied these analyses to the development of IQ during infancy and early childhood using parent-offspring data from adoptive and nonadoptive families in the Colorado Adoption Project. The results suggested familial environmental influences on children's mean IQ for ages 1-4 but environmental influences specific to fathers' cognitive ability on children's IQ development. We also discuss advantages and disadvantages of the multivariate multiple regression method for studying genetic and environmental influences on development.

Adoption↗

PubMed1992

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.

Adolescent↗

PubMed1992

Identifying children in the Colorado Adoption Project at risk for conduct disorder.

Clustering techniques were used to identify a subsample of young adopted and nonadopted children in the Colorado Adoption Project at risk for conduct disorder. Although data from both boys and girls were analyzed, a cluster of girls large enough for subsequent statistical analysis could not be identified; therefore, results are reported for boys only. Identifying measures were selected based on the DSM-III-R diagnostic criteria. Cluster analyses confirmed the existence of a small group of boys who appeared to be significantly at risk. Subsequent parental and teacher ratings of these children verified the stability over time of the classification. The poor conduct group was significantly associated with difficult temperament in infancy, with poor conduct on the part of parents when they were youths, and with high achievement orientation in the home environment.

Adoption↗

PubMed1992

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.

Cognition↗

PubMed1991

A LISREL 8 model with constrained parameters for twin and adoptive families.

Constrained optimization recently has been implemented in the LISREL software package, allowing formulation of parent-offspring transmission models in a simple and efficient manner. A reverse path model of parental transmission is described within the LISREL framework for application to twin and/or adoptive family data. The model incorporates genetic and environmental parameter constraints arising from assortative mating and cultural transmission. An illustration of the LISREL model is given using measures obtained from twins and parents involving fear of social criticism.

Adoption↗

PubMed1991

Parent-offspring and sibling adoption analyses of parental ratings of temperament in infancy and childhood.

A first step toward understanding the etiology of personality is to investigate the relative impact of genetic and environmental factors using twin and adoption designs. Twin studies of infants and young children indicate substantial genetic influence for parental ratings of temperament in the preschool years. Adoption studies, however, have not previously been reported during the early years of life. We present parent-offspring comparisons for temperament (emotionality, activity, sociability, and impulsivity) for adopted and nonadopted children yearly from 1 to 7 years of age and their biological, adoptive, and nonadoptive parents. Also presented are correlations for adoptive and nonadoptive siblings when each child was 1, 2, 3, and 4 years of age. In contrast with twin results, little evidence is found for genetic influence. The average correlation between biological parents and their adopted-away children for data averaged over the 7 years is only .03. Similarly, the average parent-offspring correlation in nonadoptive families (.08) is no greater than in adoptive families (.12). Results for nonadoptive and adoptive siblings also indicate little genetic influence. The difference between the twin and adoption results may be due to environmental effects or to nonadditive genetic variance.

Adoption↗

PubMed1991

Longitudinal prediction of specific cognitive abilities from infant novelty preference.

A test of visual novelty preference, the Fagan Test of Infant Intelligence, was administered to a group of 113 full-term infants from the Colorado Adoption Project at 5 and 7 months of age. The infants were followed longitudinally and the Bayley scales were administered at 12 and 24 months, the Sequenced Inventory of Communication Development at 24 and 36 months, and the Stanford-Binet and the Colorado Specific Cognitive Abilities Test at 36 months. 1 novelty preference score was obtained for each infant by averaging across the 2 test ages. Novelty preference correlated significantly with 36-month Binet IQ, the first unrotated principle component from the cognitive battery, and the 24-month Bayley MDI score. Novelty preference was also compared to specific abilities at all 3 follow-up ages; all of the specific abilities were significantly related to novelty preference, with the exceptions of 12- and 24-month Imitation and 36-month Perceptual Speed. Partial correlations suggest that novelty preference predicts language and memory independent of IQ. Overall, the results indicate that novelty preference during the first year of life not only predicts later IQ but may also reflect specific cognitive processes.

Attention↗

PubMed1990

A simple method of model fitting for adoption data.

Traditional models used with adoption data often make strong assumptions concerning the nature of genetic transmission and assortative mating. A simple model is presented which avoids these assumptions. The model is linearized and, thus, has the further advantage that it can be used with standard statistical packages such as LISREL or EQS. The model allows tests of the internal consistency of the data, in addition to tests of the relative strength of genetic and environmental transmission parameters. To illustrate the model, measures of general cognitive ability in parents and their 7-year-old children from the Colorado Adoption Project (CAP) were fit to the model using the LISREL program. This relatively simple model may be expanded to incorporate more complex designs involving multiple measures or siblings. Although the model will not always allow constraints on the parameter estimates in more complex models, it offers a quick, flexible method for initial exploration of adoption data.

Adoption↗

PubMed1990

Twin temperament during the transition from infancy to early childhood.

A multiple-regression model for the analysis of twin data in which one twin's score is predicted from that of its co-twin, the coefficient of relationship, and the interaction term provides direct estimates of heritability (h2) and the proportion of variance due to shared environmental influences (c2). Several multiple-regression models were fitted to parental ratings of infant and toddler temperament for 306 pairs of twins. High estimates of h2 were found for eight of the nine temperament scales, and three scales showed significant effects of c2. No differential heritability was found between males and females across the infant and toddler years.

Child, Preschool↗

PubMed1989

Testing structural equation models for twin data using LISREL.

Simple genetic models can be fitted to twin data using software packages such as LISREL (Jöreskog and Sörbom, 1986a). After discussion of data preparation and routine checks on possible violation of assumptions of the twin method, we illustrate univariate, bivariate, and multivariate genetic models which can be tested in cross-sectional twin data using LISREL. These include models for cohort or cohabitation effects, genotype x sex interaction, and certain types of genotype x environment interaction and genotype-environment correlation.

Computer Simulation↗

PubMed1989

Fitting genetic models with LISREL: hypothesis testing.

A brief introduction to the mathematical theory involved in model fitting is provided. The properties of maximum-likelihood estimates are described, and their advantages in fitting structural models are given. Identification of models is considered. Standard errors of parameter estimates are compared with the use of likelihood-ratio (L-R) statistics. For structural modeling, L-R tests are invariant to parameter transformation and give robust tests of significance. Some guidelines for fitting models to data collected from twins are given, with discussion of the relative merits of parsimony and data description.

Computer Simulation↗

PubMed1989

The effects of social homogamy and cultural inheritance on the covariances of twins and their parents: a LISREL model.

In general, models involving parent-offspring transmission are not possible to specify in LISREL due to the complex constraints implied by assortative mating and genotype-environmental correlation. In this short note we describe one simple model of resemblance among twins and their parents which can be accommodated in the LISREL specification due to the strong assumption of social homogamy. The specification is described fully and checked for identification, and its power to resolve different parameters is briefly examined.

Computer Simulation↗