The capacity to roll up the edges of the tongue is one of the classic examples in school books on genetics of a genetically determined trait. However, like almost everything in biology, things are not that simple.
The explanation as to why two genetically identical people differ in this trait is in itself an interesting lesson in basic genetics.
The trait is determined by a gene, whose abbreviation is T, so that those who have the dominant homozygous form (TT) can roll up their tongue and those with the recessive gene (tt), cannot. The interesting thing happens with those who have both forms of the gene, the heterozygotes (Tt): some can and some cannot. But precisely in those places where studies were carried out, at least in the south and southeast of Brazil and in Europe, the dominant form of the gene is more common than the recessive (60%) form and around 75% of the heterozygotes are successful in rolling up their tongues. These numbers were obtained from various studies with families and pairs of both identical (univiteline) and fraternal twins. The specialist in human genetics, Paulo Otto, took part in one of these studies in the 1990s. The results are a fine example of the concept of incomplete penetrance, in which one gene does not completely determine a trait. This may happen because of genetic or environmental peculiarities, or because of the interaction between the two factors.
University of São Paulo (USP)