J Reprod Med-. 1997; 42:

Objective: Twin zygosity determination can be performed with anthropologic, serologic and genetic markers; however, these methods are more than occasionally inefficient, often expensive and sometimes inaccurate. We used microsatellites as DNA markers and developed a largly automated, rapid and efficient method of determining zygosity.
Study design: We used five highly polymorphic tandem repeat loci, coamplified by polymerase chain reaction (PCR) using fluorescence-labeled primers. Thirty-six samples were simultaniously analyzed by electrophoresis and laser detection. The PCR products were sized by automated fragment analysis.
Results: We typed 132 pairs of monozygotic (MZ) and dizygotic (DZ) twins. With five markers, the probability that any twin pair was MZ if all markers were concordant was 99%.
Conclusion: This method is a rapid and reliable approach to zygosity detection.

Heritability Analysis of Lipids and Three Gene Loci in Twins Link the Macrophage Scavenger Receptor to HDL Cholesterol Concentrations
Knoblauch H, Busjahn A, Zsuzsanna N, Münter S, Faulhaber H-D, Schuster H, Luft FC.
Arterioscler Thromb Vasc Biol 1997; 17:2054-2060 .

We studied 100 healthy monozygotic and 72 dizygotic twin pairs (mean age, 34 +/- 14 years) to test for genetic influences on blood lipids and to examine relevant gene loci. Total cholesterol (TC), LDL cholesterol (LDL-C), HDL cholesterol (HDL-C), and triglyceride (TG) levels were determined after a 12-hour fast. Zygosity was determined with the use of microsatellite markers. Heritability estimates were conducted by using the lisrel 8 program; a sib-pair analysis was conducted by using the sibpal program. Linear regression analyses were carried out between identical-by-descent status and squared within-pair differences of TC, LDL-C, HDL-C, and TG values. Heritability estimates of the lipid serum concentrations ranged from .58 to .66. A significant linkage relationship was found for HDL-C (P = .008) and TGs (P = .05) with D8S261 on chromosome 8p. However, no linkage was found between any of the lipid variables and the lipoprotein lipase gene locus (LPL GZ14/15 and D8S282). Because D8S261 is located approximately halfway between the LPL and macrophage scavenger receptor genes, we examined the nearby markers D8S549 and D8S1731. Linkage was found for HDL-C and D8S549 (P = .001) and for HDL-C and D8S1731 (P = .04). On the other hand, we found no linkage between the LDL receptor gene locus and LDL-C serum concentrations nor between the LPL gene locus and the various other lipid fractions. Our data suggest a significant influence of the macrophage scavenger receptor gene locus on HDL-C and weak influence on TG levels. We suggest that inherited variability in the macrophage scavenger receptor gene has an influence on serum lipid concentrations.

JOURNAL-OF-HYPERTENSION. OCT 1996; 14 (10) : 1195-1199.

Objectives To determine the genetic and environmental contributions to resting blood pressure, the level of blood pressure during the cold pressor test and the increase in blood pressure with the cold pressor test in an adult cohort of normotensive twins. Design and methods Ninety one monozygotic and 41 dizygotic normal twin pairs were recruited by advertisement. The mean age was 34 +/- 14 years (mean +/- SD), Systolic blood pressure (SEP), diastolic blood pressure (DBP) and heart rate were measured continuously at the finger (using a Finapres device) and verified at the upper arm oscillometrically (using a Dinamap device) every minute. The cold pressor test was conducted by immersing the nondominant hand into cold (< 4 degrees C) water for 2 min, Statistical analysis was performed by using the SPSS program; parameters of the quantitative genetic models were estimated by path analysis techniques using the LISREL 8 program.
Results Heritability estimates of additive genetic effects were statistically significant for SBP and DBP but not for heart rate during rest and during the cold pressor test. Furthermore, the path analysis indicated shared as well as specific genetic components both for the blood pressure level at rest and for that during the cold pressor test, However, the genetic influences on the blood pressure level at rest and on the increase in blood pressure during the cold pressor test (the blood pressure level during the cold pressor test minus that during rest) were entirely independent of one another.
Conclusions A significant genetic covariation exists for SEP and DBP during rest and during the cold pressor test, as well as a significant genetic variation that is specific to the cold pressor stress condition, These findings suggest that different genes or sets of genes contribute to blood pressure regulation during rest and to blood pressure reactivity to cold pressor stress.