Birthweight was measured on 188 monochorionic monozygotic, 54 dichorionic monozygotic, 102 like-sexed dizygotic, and 94 unlike-sexed dizygotic liveborn twin pairs. Overall, males were found to be significantly heavier than females. These differences were not significant, however, when birthweights were compared within zygosity/chorion-type categories. Males were also characterized by a slightly greater overall total variance. Comparisons of intrapair variation of monochorionic and dichorionic monozygotic twins revealed significant differences between monochorionic pairs and dichorionic separate pairs and no significant differences between monochorionic pairs and dichorionic fused pairs. The results of this study suggest that placental proximity may have as important an influence on variation in birthweight as does the presence or absence of vascular anastomoses.Key words: Birthweight, Twin pregnancy, Placental effects, Length of gestation, Sex effects, Fetal hormones Variation in human birthweight is known to be influenced by such factors as length of gestation [ 3 , 6 ] , race [ 1 ] , sex [17], parity [9] and maternal smoking history [11]. In addition, "maternal factors" are recognized to have powerful and possibly overriding effects on the expression of this trait [10,13], but the relative importance of fetal genoThis is paper No.

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.

The sampling variances in twin and sibling studies of man for estimation of intraclass correlation coefficients and their relation to heritabilities are evaluated. For twins and full-sibs the higher the estimated heritability of a trait the smaller the sampling variance. With increase in the number of twin sets or of full-sibships studied, the sampling variances decrease by their proportions. Twin and full-sib studies with lower heritabilities (h² < 0.25) need at least 50 sibships to obtain a significant estimate. Using half-sibs to estimate heritabilities, increase of the offspring number from the same parent would be more efficient for reducing the sampling variance of the heritability compared with increases of half-sibship families. However, because of the limited number of offspring per family in modern man, the efficiency of the half-sib design could be increased by the large number of monozygotic twin families. It appears that with only 2 or 3 children per full-sibship family, approximately 100 half-sibships are needed for significant estimate of heritability.

A total of 71 pairs of like-sexed dizygotic twins were studied, comparing within-pair differences for plasma total, free and esterified cholesterol, and triglyceride with the number of HLA haplotypes the twins had in common. If associations are present between HLA and the blood lipids studied, the twins with no haplotypes in common would be expected to have the largest within-pair mean square, those with two in common the smallest, and those with one in common an intermediate value. No significant differences were found comparing within-pair mean squares for the variables studied.