Some diseases follow a clear hereditary scheme because they depend on one gene with a clear distinction between only two alleles: the healthy allele and the “sick” one, one dominant and one recessive. Such hereditary pattern is so logical that already in the 1850 Gregor Mendel, a monk, was able to discern the rules of transmission of hereditary characteristics by simply observing some physical features of pea plants (from there the name “Mendelian inheritance). In fact, besides some diseases, many other human physical traits are thought to follow the Mendelian scheme. One of the most popularized example is the rolled tongue, often used in the biology classes to introduce the concept of dominant allele: most of us can roll the tongue but few cannot.
This funny feature got the attention of a prominent geneticist in the ‘40, Alfred Sturtevant, which stated that the ability to roll one’s tongue was based on a dominant allele. Later one of his colleagues noted that some identical twins, who share exactly the same DNA, did not show the same tongue-behavior: one twin could roll it while the other couldn’t. This is incompatible with Mendelian inheritance. Such observation does not exclude the genetics influence on the ability of tongue-rolling – it likely has a strong genetic base but some other factors can mask the trait. So, if your parents don’t roll their tongues, but you can don’t worry — chances are you’re still their kid.
The mendelian inheritance needs to be taken with more care for other traits, for example, the hitchhiker’s thumb or the shape of the earlobes don’t fit a simple two-allele description. Thumbs, for example, do not divide into two categories, hitchhiker’s and non-hitchhiker’s; the angle or degree of flexion of the thumb may vary between a list of values, not just two. This complicates the prediction of the inheritance of some traits but also explains the great variety that exists between humans.