Mendelian genetics of humans exophenotype deals only with properties in the domain of qualitative variations and/or that the quantitative properties that can be described in arbitrary categories, such as Mendel’s variety of low and high pea stalks. Therefore, this area classical genetics covers only those properties that behave according to the model monogenic inheritance, i.e. simple gene inheritance.
According to the model of Mendelian inheritance, if a child gets at least one of the parents dominant allele, the child will have a dominant phenotypic variant of the observed properties. Only those who get a recessive allele from both parents exhibit the recessive phenotypes. Those who get a dominant allele from one parent and recessive from the other will be the dominant variant features.
Among the infinitely large number of potentially describable features, very few can be introduced as a purely Mendelian trait, because most of the phenotypic expression of the exophenotypic traits is the showpiece incomplete dominance, codominance and quantitative contributions from smaller or larger number of genes (see: Polygenic inheritance, Oligogenic inheritance).
Theoretically, the recessive phenotype can skip any number of generations, remaining "dormant" in heterozygous "carriers", until they have children with someone who has one or both recessive alleles, which will transfer to his or her child. If a second partner has a recessive homozygote, the chance for expression of the recessive phenotype is 50%, and if, as mentioned holder, and the heterozygote, in their offspring, will appear 25% of individuals with recessive phenotype.
The studies of the morphological - anatomical, undoubtedly inherited properties, faces particular difficulties because of the incomplete gene expressiveness or penetrance which control them, regardless of their affiliation to any of the described models of inheritance. Most of those traits that exhibit a high heritability degree were listed in McKusick's Mendelian Inheritance in Man. Many of them are still included in today's OMIM's edition.
Before the discovery of DNA identification, many of these features were used worldwide as the genetic markers in medicolegal practice, including the cases of disputed paternity.