Effects of Chorion Type on Genetic and Environmental Influences on Height, Weight, and Body Mass Index in South Korean Young Twins

Abstract

T he present study examined the effects of chorionicity of twins on variations of height, weight, and body mass index (BMI) during childhood in the classical twin design. Mothers of 81 pairs of monochorionic monozygotic (MCMZ), 47 pairs of dichorionic monozygotic (DCMZ), and 457 pairs of dizygotic (DZ) twins drawn from the South Korean Twin Registry reported their children's height and weight. Twins' age ranged from 1.9 to 8.7 yrs, with a mean of 4.0 yrs and SD of 1.7 yrs. We computed maximum likelihood twin correlations and performed model-fitting analyses. In correlational and modelfitting analyses, we treated age and sex as covariates to control their main effects. Maximum likelihood MCMZ, DCMZ, and DZ twin correlations were, respectively, .96, .92, and .74, for height, .88, .91, and .57 for weight, and .93, .92, and .61 for BMI. The pattern of these twin correlations suggested very modest chorion effects on body measures. Model-fitting analyses confirmed the observations from twin correlations. Whereas genetic and shared environmental influences were significant for all three body measures, chorion effects attained statistical significance only for height (4%), and those for weight and BMI were zero. These findings indicate that genetic and environmental estimates for height, weight, and BMI during childhood are biased little by the chorion type of MZ twins, supporting the validity of the equal prenatal environment assumption in the classical twin design.The classical twin method compares similarities between monozygotic (MZ) and dizygotic (DZ) twins. One of the crucial assumptions of the classical twin design is that MZ and DZ twins experience similar degrees of prenatal environment. Due to the variation in placental anatomy, however, MZ and DZ twins experience different environments during the prenatal period, and if these substantially influence the trait under study, the classical twin study will yield biased estimates of genetic and environmental factors.As the zygotes of DZ twins implant individually in the uterus, each embryo develops its own placenta and chorion. Unlike DZ twins, MZ twins vary in their placentation, according to the timing of division of the inner cell mass. If MZ twins are divided at, or before, the morula stage, that is, around the fourth day of gestation, then each twin will develop an individual chorion and amnion like DZ twins. These twins are known as dichorionic MZ (DCMZ) twins. If the division occurs between the fourth and the seventh day of the gestation, then these twins will share a common chorion, known as monochorionic MZ (MCMZ) twins. Finally, if the division takes place after the eighth day, then the two fetuses will share a common amnion as well as a common chorion. These twins are called monochorionic monoamnionic (MCMA) MZ twins.Approximately a third of MZ twins are DCMZ and two thirds are MCMZ. Only 2% to 3% of the MZ twins are MCMA twins (Bulmer, 1970). The sharing of a chorion and a placenta, and the presence of vascular anastomoses between the circulations of th...

Genetic Regulation of Pre-Pubertal Development of Body Mass Index: A Longitudinal Study of Japanese Twin Boys and Girls

“…Whether the proportions of genetic and environmental (Silventoinen et al 2010), genetic factors explained the major part of the variation of BMI over childhood. A rare example of a twin study in a nonCaucasian population is a Korean study of pre-pubertal twin children from 2 to 9 years of age (Hur and Shin 2008). The heritability estimate was slightly lower than in our study, i.e., 0.55 in the presence of common environmental factors, although overlap in the confidence intervals showed that the difference was not statistically significant.…”

The genetic and environmental influences on childhood obesity: a systematic review of twin and adoption studies

“…These results are based on the meta-analysis of the nine existing twin studies. [13][14][15][16][17][18][19][20][21] Background information of these cohorts is presented in Appendix Table A1 and the estimates of variance components with 95% CI for each study in Appendix Table A2.…”

“…In the case when studies reporting correlations in BMI have been presented several times in different papers, only one paper was included. On the basis of these criteria, we found nine twin studies [13][14][15][16][17][18][19][20][21] and five adoption studies; [22][23][24][25][26] however, no family studies reporting heritability estimates of childhood obesity were found. In addition, we found three twin studies that had pooled information on children at different ages.…”

“…[27][28][29] We did not limit our search to weight, but except one early twin study, 27 all of these studies had used BMI or relative weight as an indicator of obesity. Two twin studies represented Korean population, 16,19 one Taiwanese population 29 and one had pooled information from four East Asian populations, 30 but otherwise the studies were conducted in the Caucasian populations.…”

The South Korean Twin Registry

“…For physical traits, height, weight, birth weight and Sasang constitution types have been obtained either through parental report or self-report for most twins. In addition, pubertal timing (Hur, 2007c;Hur & Shin, 2008; and cold hands symptoms (Hur, Chae et al, 2012;Hur, Yu et al, 2018) have been collected from adolescent and young adult twins.…”