Genetic basis of the phenylthiocarbamide polymorphism

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Abstract
This study of the inheritance of the phenylthiocarbamide (PTC) taste acuity in 56 unrelated Berkeley families makes use of two categories of genetic analyses: population-based, which require the presence of genetic equilibrium and family-based, which do not. The first group of tests give a less good fit to a Mendelian hypothesis than does the second, not only for the data of this investigation but also for the data of others. The hypothesis of random mating was therefore examined and a weak but statistically significant negative correlation between taster and nontaster spouses was found. The patterns of deviations of the observed data from expectations, while not statistically significant, are consistent with patterns predicted under negative assortative mating. Interestingly, the same patterns exist in data collected in England, Canada, Norway and India. Results of the second group of tests show an outstanding fit of the Berkeley data to expectations based on a single locus, two allele hypothesis with dominance of the taster allele. This is in contrast to the much poorer fit to the hypothesis of the Indian and Norwegian data. An explanation may lie in the method used to classify tasters and nontasters. In the Berkeley data, the antimode of the biomodal distribution of taste sensitivity thresholds was determined separately for age and sex groups and the age-sex specific criterion was then used to distinguish taster and nontaster phenotypes. Presumably misclassification is less frequent in the Berkeley data than in the other data.