Normal language development was studied in 310 pairs of 4-year-old twins born in the United Kingdom in 1994. Twins were assessed individually in their homes on a diverse battery of language and nonverbal measures. Rotated factor analyses indicated the presence of a general Language factor (L) as well as a general Nonverbal (NV) factor. Moderate genetic influence was found for both L and NV abilities. Bivariate genetic analysis estimated a genetic correlation of .63 between L and NV abilities, implying that over half of the genetic influence on L overlaps with genetic influence on NV. These results suggest that at age 4, genetic influences on individual differences in language overlap substantially with genetic influences on individual differences in other cognitive abilities, although perhaps less so than later in development.

Research on risk factors and pathogenesis of otitis media has emphasized the environment, but previous twin studies suggest a strong genetic component. In those studies, no attempt was made to differentiate the role of initial acute infection from the chronic airway blockage that frequently accompanies persistent effusion. The authors estimated genetic and environmental determination of both of these aspects of otitis media histories at three time points. A large and representative subset of a total population cohort of all twins born in England and Wales in 1994, was studied prospectively at ages 2, 3, and 4 years (monozygotic, n = 715; dizygotic, n = 658). For the total symptom score, the estimated heritabilities at ages 2, 3, and 4 years were 0.49, 0.66, and 0.71, respectively. All correlations for the total score found in monozygotic twins were about 0.9, but correlations were lower for dizygotic twins (p < 0.65). Shared environmental influences declined in importance over time (0.41, 0.22, and 0.16 at ages 2, 3, and 4 years, respectively). No gender differences were found in parameter estimates. Symptoms of acute infections showed lower heritability (0.57 on average) and higher shared environment (0.18) than did chronic airway blockage (0.72 heritability and 0.10 shared environment). The strong genetic component, which this study confirms, is not unitary but distributes differently across the phenotypic aspects of otitis media and probably across pathogenetic stage.

OBJECTIVE: Although many twin and adoption studies document genetic in¯uence on individual differences in weight, much less is known about genetic in¯uences on overweight, about the genetic links between weight and overweight, or about the origins of weight and overweight in childhood, an age that might provide a good target for prevention of obesity. We tested the hypothesis that, in early childhood, overweight is as heritable as weight and that weight and overweight are linked genetically. DESIGN: Model-®tting analyses were used to compare monozygotic (MZ) and dizygotic (DZ) twins (same-sex and oppositesex) for weight and overweight. SUBJECTS: The sample included 3636 4-y-old twins born in the UK in 1994. MEASUREMENTS: Heights and weights reported by parents were used to assess weight corrected for height, which yields results similar to body mass index (BMI) but corrects more completely for genetic effects on height. RESULTS: At 4 y of age, genetic factors contributed substantially both to individual differences in weight throughout the distribution and to the mean weight difference between overweight children and the rest of the population. Unlike results later in life, weight and overweight in 4-y-olds also suggest substantial shared family environmental in¯uence. Results are similar for boys and girls. CONCLUSIONS: Overweight is the quantitative extreme of genetic and environmental factors responsible for normal variation in weight in childhood. Genes associated with overweight are likely to be associated with variation in weight throughout the distribution, as assumed by quantitative trait locus (QTL) theory. These ®ndings linking weight and overweight in childhood have far-reaching implications for molecular genetic attempts to identify speci®c genes responsible for genetic in¯uence, for investigating pathways between genes and behaviour, and for intervention and prevention.

Results-Univariate parameter estimates derived from model fitting were 68% heritability, 13% shared environment, and 19% non-shared environment. Conclusions-Our findings suggest that asthma is highly heritable in 4 year olds, whereas shared environmental influences are not statistically significant. (Arch Dis Child 2001;85:398-400)

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