Complex inheritance and quantitative inheritance
Opposition to believe that Mendelian inheritance was of higher organisms and humans, was because few of our normal features were of either / or type, but rather a continuous variation (body height, intelligence, etc.). This was solved by Eugen Fischer in 1918. He noted that the sum of the effects of many (either / or) the effects of individual base pairs would provide a quantitative normally distributed trait.
Gene with such effect was long called polygene, and it was not uncommon to believe that this was anything other than the classical Mendelian genes. After the "one gene - one polypeptide 'hypothesis was put forward, put, however, British scientists started a systematic study of the standard protein variation in enzymes and found that at least every third enzyme has at least two common gene variants and that the enzyme activity varied somewhat according to the normal structures they had inherited. It has become increasingly clear that this is the normal variation in the genes for the enzymes, which in addition may be rare, severe enzyme defect variants, corresponding to what we previously called polygene. The most typical of polygenic or multifactorial inheritance is then just the quantitative variation in the property. These characteristics also like to modify environmental factors.
Body height is a good example, both parents' height and diet play a role. The fingerprint pattern is a purely polygenic trait, in which environment did not tend to modify: here, people who share 50% of the same genes (father-child or mother-child or sibling), on average, measured as a value exactly halfway between the selected output person's value and the population average.
Actually, this was already discovered by Charles Darwin's cousin Francis Galton, as in studies of families of very prominent people in the 1800's England, found that children deviate from halfway back to the population average in their intellectual achievements.