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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.
A pair of female mongoloid twins was studied when they were ten years old and, again, some thirty years later. A careful comparison was made of the similarities and differences between them in features or traits which have proved to be useful in determining the zygosity of like-sexed twins. Though some of these characters can be, and frequently are, considerably modified in the presence of mongolism and so lose part of their value for that purpose, the differences noted in these twins, both as children and as middle-aged adults, are thought to outweigh their similarities and so warrant the conclusion that they are, indeed, dizygotic.
Albumin excretion marks early glomerular injury in hypertension. This study investigated heritability of albumin excretion in twin pairs and its genetic determination by adrenergic pathway polymorphism. Genetic associations used single nucleotide polymorphisms at adrenergic pathway loci spanning catecholamine biosynthesis, storage, catabolism, receptor action, and postreceptor signal transduction. We studied 134 single nucleotide polymorphisms at 46 loci for a total of >51,000 genotypes. Albumin excretion heritability was 45.2+/-7.4% (P=2x10(-7)), and the phenotype aggregated significantly with adrenergic, renal, metabolic, and hemodynamic traits. In the adrenergic system, excretions of both norepinephrine and epinephrine correlated with albumin. In the kidney, albumin excretion correlated with glomerular and tubular traits (Na(+) and K(+) excretion; fractional excretion of Na(+) and Li(+)). Albumin excretion shared genetic determination (genetic covariance) with epinephrine excretion, and environmental determination with glomerular filtration rate and electrolyte intake/excretion. Albumin excretion associated with polymorphisms at multiple points in the adrenergic pathway: catecholamine biosynthesis (tyrosine hydroxylase), catabolism (monoamine oxidase A), storage/release (chromogranin A), receptor target (dopamine D1 receptor), and postreceptor signal transduction (sorting nexin 13 and rho kinase). Epistasis (gene-by-gene interaction) occurred between alleles at rho kinase, tyrosine hydroxylase, chromogranin A, and sorting nexin 13. Dopamine D1 receptor polymorphism showed pleiotropic effects on both albumin and dopamine excretion. These studies establish new roles for heredity and environment in albumin excretion. Urinary excretions of albumin and catecholamines are highly heritable, and their parallel suggests adrenergic mediation of early glomerular permeability alterations. Albumin excretion is influenced by multiple adrenergic pathway genes and is, thus, polygenic. Such functional links between adrenergic activity and glomerular injury suggest novel approaches to its prediction, prevention, diagnosis, and treatment.
An experiment was carried out on 40 pairs of adult male twins to investigate the extent of genetical determination in the cortical auditory evoked potential. Tonal stimuli of 1 sec duration, at an intensity of 95 dB and a frequency of 100 c/sec, were used. The inter-stimulus interval was 33 sec and the bipolar evoked potential was measured between the CZ and T3 scalp positions. The reliability of the seven latency and amplitude measures was also calculated and this was taken into account in the subsequent genetic analysis. The biometrical gentical approach, which gives maximal information particularly on small samples, was used to analyse the data. A strong genetic influence was found on all the amplitude scores. The environment made no significant contribution to these. For the latencies there was some evidence for a genetic effect; however, this was not very strong and can well be interpreted as being due to between family environmental effects.
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Ina a sample of 48 healthy adult male twin pairs (24 MZ, 24 DZ) the activities of DBH (serum), COMT (red blood cells), and MAO (platelets) were determined. The twins had undergone a detailed psychodiagnostic test procedure before. Interindividual variability of enzyme activities is almost exclusively genetically determined. No correlation between enzyme activities within one subject is found. Correlations between enzyme activities and MMPI test scores were calculated. As in a comparable investigation by Murphy et al. (1977), negative correlations between MAO activity and MMPI scores prevailed.
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Susceptibility of human skin fibroblasts to SV40 virus infection has been suggested as a marker of cancer risk. To evaluate the role of heritable factors in the regulation of SV40 T-antigen, fibroblasts from 9 pairs of identical twins and 129 members of cancer-prone families, including 16 with cancer, were tested in a 3-day immunofluorescence assay. In the twin study, the variance of T-antigen values was significantly less in identical than in fraternal or non-twin sibs, suggesting a heritable component in the regulation of SV40 infection. In the families, T-antigen values of parents and children were compared to models of Mendelian inheritance. At least three modes of inheritance--autosomal dominant, recessive, and X-linker--were observed. The distribution of offspring values compared to those of their parents suggested that interaction of multiple genetic factors influences the T-antigen value in individual patients. With the exception of Fanconi's anemia, the values for patients with cancer or predisposing syndromes were not uniformly elevated. The utility of this assay as a marker of cancer risk appears limited because of the complexity of factors that influence T-antigen expression in individual cases.