We have seen that in some cases genes manifest themselves with very distinct phenotypes.
For example, genes for pea seed color manifest themselves with well-defined phenotypes, with yellow or green seeds found. This well-defined gene manifestation we call discontinuous gene variationbecause there are no intermediate phenotypes.
There is inheritance of characteristics, however, whose gene manifestation (also called expressiveness) does not determine such defined phenotypes, but rather a gradation of phenotypes. This gradation of gene expressiveness, ranging from a phenotype that shows slight expression of the trait to its full expression, we call the reaction norm or variable expressiveness. For example, brachydactyly (short finger) gene carriers may have phenotypes ranging from slightly shorter fingers to complete lack of them.
Different degrees of brachydactyly by variable expression of genotype.
Some genes whenever they are present, we say they are highly penetrating. Others have a incomplete penetrance, that is, only a portion of the genotype carriers has the corresponding phenotype.
Note that the concept of penetrance is related to the expressiveness of the gene in a set of individuals, being presented in percentage terms. Thus, for example, we can say that the penetrance for the Huntington's disease gene is 100%, which means that 100% of carriers of this gene have (express) the corresponding phenotype.