Variations in taste threshold for PTC in populations of Tibet and Ladakh

The genetics of phenylthiocarbamide perception

The ability to taste the bitter compound phenylthiocarbamide (PTC) and related chemicals is bimodal, and all human populations tested to date contain some people who can and some people who cannot taste PTC. Why this trait has been maintained in the population is uncertain but this polymorphism may influence food selection, nutritional status or thyroid metabolism. The gene product that gives rise to this phenotype is unknown, and its characterization would provide insight into the mechanism of bitter taste perception. Although this trait is often considered a simple Mendelian trait, i.e. one gene two alleles, a recent linkage study found a major locus on chromosome 5p15 and evidence for an additional locus on chromosome 7. The development of methods to identify these genes will provide a good stepping-stone between single-gene disorders and polygenic traits.

Mitochondrial DNA analysis in Tibet: implications for the origin of the Tibetan population and its adaptation to high altitude

Mitochondrial DNAs (mtDNAs) of 54 Tibetans residing at altitudes ranging from 3,000-4,500 m were amplified by polymerase chain reaction (PCR), examined by high-resolution restriction endonuclease analysis, and compared with those previously described in 10 other Asian and Siberian populations. This comparison revealed that more than 50% of Asian mtDNAs belong to a unique mtDNA lineage which is found only among Mongoloids, suggesting that this lineage most likely originated in Asia at an early stage of the human colonization of that continent. Within the Tibetan mtDNAs, sets of additional linked polymorphic sites defined seven minor lineages of related mtDNA haplotypes (haplogroups). The frequency and distribution of these haplogroups in modern Asian populations are supportive of previous genetic evidence that Tibetans, although located in southern Asia, share common ancestral origins with northern Mongoloid populations. This analysis of Tibetan mtDNAs also suggests that mtDNA mutations are unlikely to play a major role in the adaptation of Tibetans to high altitudes.

PTC-tasting ability in populations living in Kirghizia with special reference to hypersensitivity: its relation to sex and age

The dimorphism in sensitivity to PTC, with a dilution scale up to 29, was studied in 320 male and 320 female Kirghiz students, 45 male and 200 female Russian students in Frunze, and in 734 Kirghiz schoolchildren in a high-altitude area of South Kirghizia. Gene t frequency was 0.44 for the Kirghiz students, 0.56 for the Russian students, and 0.54 for the Kirghiz schoolchildren. In all the groups studied it was established that such factors as sex and age have no modifying effect on sensitivity to PTC and the pattern of the distribution of sensitivity. In all the groups the frequency of 'tasters' exceeded that of 'nontasters.' Among the Kirghiz students there was a very small percentage (1.2%) of hypersensitive subjects (dil. No. 18), while 14.1% of the Kirghiz children of one Kyzyl-Dzhar school were hypersensitive. Genealogical studies showed that all the hypersensitive subjects belonged to one small stock and lived in the same village. Family studies of probands suggest the existence of a new (third) allele (T2) which determines hypersensitivity to PTC.