Biomedical subjects
Publications and source records attributed to J C Christian.
In a random sample of 104 pairs of middle-class Caucasian, handedness-discordant twins six years of age or older, a significant relationship has been found between birth order and handedness in monozygotic twins, there being an excess of left-handed among first-born twins (P less than 0.01). No such relation has been found in dizygotic twins.
The multiple relationships which exist within families of adult monozygotic (MZ) twins permit evaluation of genetic and environmental variance and provide a direct test of maternal effects. Systolic blood pressure (SBP) was measured in 610 members of the families of 76 MZ twin pairs. Age and sex effects on BP were removed via a Z-transformation. Correlations for SBP were .72 for MZ twins and .23 for full siblings. Regression of offspring on single parent was .26. The husband-wife correlation was significant, but it was no higher than the resemblance of a twin to the spouse of his co-twin. Comparisons of maternal and paternal half-siblings provide no evidence of maternal influences on SBP. To evaluate whether familial resemblance of BP is mediated by body size, the original systolic pressures were adjusted by stepwise multiple regression of age, sex, height, weight and skinfold thickness. Adjusting BP for effects of body size reduced regression of offspring on mid-parent from .40 to .23. The authors conclude that familial aggregation of systolic pressure reflects additive genetic variance mediated, in part, by body size and augmented by social homogamy arising from non-random mating.
Significantly larger variation between sibships within families of male MZ twins than between sibships within families of female MZ twins, indicative of maternal influences, was found for 10 of 41 dermatoglyphic fingertip variables. Of these, five were thumb-related with the effect primarily on the thumb radial and ridge count (larger of radial and ulnar count). These same variables were previously found to have unequal variances in MZ twins of known placental type, and the results indicate maternal influences in singletons as well as twins for these variables. Although the total ridge count (TRC), previously shown to differ in MZ twins of known placental type (paralleling the thumb radial and ridge counts) did not reach significance, the trend indicated that the observed thumb changes may be reflected in the TRC as well. Little finger pattern type and ulnar counts also showed less variability in families of female MZ twins, but the interpretation is complicated by the concomitant differences in mean squares within-sibships for these little finger variables.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Over the past 50 years a large number of methods have been proposed for estimating heritability from twin studies. The present paper describes the most commonly cited of these estimates as a first step in evaluating their usefulness. A critical review will then follow.
Factor analysis was employed using the ulnar ridge count, radial ridge count, ridge count (the larger of the radial or ulnar count as generally used for calculating total ridge count), and pattern type for each finger in 720 twins. Pattern type and ulnar count displayed parallel factor loadings while loadings for radial and ridge count also paralleled each other. This relationship did not hold for the index finger, indicating the importance of pattern direction and greater pattern diversity for this digit. Total ridge count was most closely associated with a factor of ring and little finger radial and ridge count and only secondarily with an index finger factor. When radial and ulnar counts were deleted to make comparisons with earlier studies, the result was factors having groupings of variables identical with previous reports. It appears that factor analysis results in consistent extraction of identical or very similar factors from different populations, and the use of radial and ulnar counts adds more information than when only the larger of the two counts is considered.
Explore the source record for details and available documents.
The data presented here indicate that different influences affect dermatoglyphic pattern development in MC-MZ and DC-MZ twins. Only five of 84 variables had significant mean differences but their clustering suggested a real difference in mean placement of the atd angle. Nineteen of 84 variables had significantly different within-pair mean squares for the two twin types. Larger numbers of twins will be required to obtain accurate estimates of the magnitude of the dermatoglyphic differences between MC-MZ and DC-MZ twins. Studies of dermatoglyphics in MC-MZ and DC-MZ twins are important to the understanding of factors which influence early embryonic development and when better documented may provide a mechanism for retrospectively diagnosing placental type of MZ twins.
The 7-year IQ scores (WISC) of 116 white and 143 black nonmalformed twins of known zygosity and placental type were ascertained from the NINCDS Collaborative Perinatal Project (NCPP). The type of chorion and zygosity had no significant effect on the mean IQ or among-pair variation. In white monozygotic twins, however, analysis of variance revealed a significantly greater within-pair mean square for dichorionic twins than monochorionic twins. On the other hand, the white dichorionic monozygotic (MZ) and dizygotic (DZ) within-pair mean squares were quite similar. These findings were not evident in blacks for either of the within-pair comparisons. In addition, estimates of genetic variance were dependent upon MZ chorion type in both races. These data suggest to us that (1) in white twin pairs dichorionic placentas are of greater influence than the similarity or dissimilarity of genomes with regard to intrapair IQ development, and (2) failure to consider chorion type may introduce a serious bias in the interpretation of genetic variance estimates of IQ variability.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Quantitative genetic studies have much potential in partitioning the causes of variation of quantitative traits such as risk factors for atherosclerosis. Only if specific causes of variation are identified can specific therapy be developed to modify risks. There have been extensive family studies of plasma cholesterol which reveal that the level of plasma cholesterol is correlated in family members. However, except for the relatively rare familial hypercholesterolemia the evidence is not convincing that correlations of relatives are due to genetic rather than environmental factors. Early twin studies were interpreted as supporting the hypothesis that levels of plasma cholesterol were strongly influenced by genetic factors. However a recent large study of twins cast doubt upon this hypothesis by finding no significant genetic variance of plasma cholesterol after correcting for differences in total variance of monozygotic and dizygotic twins.
The rationale for using an approximate t' test of the difference between the means for MZ and DZ twins is presented. This test avoids pooling the among-MZ and among-DZ twin-pair mean squares and has approximate degrees of freedom based upon the relative number of MZ and DZ twin pairs as well as the relative sizes of the among-MZ and among-DZ twin-pair mean squares. Sampling experiments simulating twin studies were used to show that the rate of Type I error for this t' test was appropriate while other tests could give either too many or too few Type I errors depending upon the relative sizes of the mean squares and the relative numbers of MZ and DZ twin pairs.