Neomendelism is the study of the phenomena that modify the transmission and manifestation of hereditary characters with respect to the schematic clarity of Mendel's laws.
The characters chosen by Mendel for his experiments were diallelic, segregated independently and presented the phenomenon of dominance. If Mendel had chosen other characters, he would probably have found and enunciated different laws.
INTERMEDIATE HERITAGE
If instead of the color of peas Mendel had studied that of Mirabilis jalapa, the "beauty of night", the first law of genetics would have been the law of intermediate inheritance. In this case, in fact, heterozygotes have an intermediate color among those of homozygotes. Crossing red varieties with white varieties all individuals with pink are obtained; crossing the latter together, the F2 has a 1: 2: 1 ratio, ie 25% red, 50% pink, 25% white. Already knowing the mechanism, we know that these are the proportions between the two types of homozygotes and heterozygotes.
From the point of view of the heterozygote phenotype, it can be assumed that each of the two alleles partially contributes to it, for example by synthesizing enzymes by red pigment and by white pigment respectively starting from a common precursor substance: the two pigments, mixed, give color intermediate.
ADDITIVE AND POLYMER CHARACTERS
If Mendel had studied the color of human skin, rather than that of peas, he would have had much difficulty formulating a simple law.
From many subsequent researches it appears that the color of our skin (apart from environmental influences, such as sun exposure) presents a continuous variability, due to the concurrence of at least 4 or maybe up to 9 different genes.
In the discontinuous variability (as in the case of the clear yellow or green alternative) the Mendelian laws find direct application, but in the continuous variability we need another statistical reasoning.
If several allelic pairs contribute to determine a character in the phenotype, in each pair we can suppose to have a favorable allele and an unfavorable one. Since we assume that each pair segregates independently, each individual may randomly have either allele for each pair. That all favorable alleles are found casually together in an individual will be extremely unlikely, as it is unlikely that throwing a coin 9 times in the air will have 9 times head. The same applies to the contrary, while the probability of intermediate situations will be maximum.
This can be expressed by saying that the combinations of n pairs of alternative factors are expressed by the formula (a + b) n, in which the coefficients of the single terms (ie the respective frequencies of the individual combinations of favorable and unfavorable factors), in the development of power of the binomial, are given by the corresponding line of the so-called Tartaglia triangle. It is a so-called bell-shaped distribution, bounded by the Gauss curve.
A monomer is defined as a character regulated by a single gene (that is, by two or more alleles that can alternatively occupy a certain locus, ie a certain trait of a given chromosome), as in Mendel's experiences, while polymery is spoken of when a character is regulated by multiple genes placed in different loci.
POLIALLELIA
A monomer character is not necessarily diallelic. If the alternative alleles for a single locus are more than two, they can variously interact in the respective heterozygotes. Such a case will be found for example for the three alleles in the blood group locus of the AB0 system, in which the homozygotes of the three alleles have the respective phenotype A, B and 0, but in heterozygotes A and B they are dominant over 0, while in the heterozygous AB there is co-existence. Of course in the case of polyalelias the mathematical formulation will be more complex and the number of genotypes and phenotypes will increase.
codominance
Two alleles are said to be codominant when each determines the respective phenotypic result in both homozygote and heterozygote. This is precisely the case of AB heterozygotes (to cite the example of blood groups). The concept can be represented thinking that each of the two alleles induces a separate enzymatic modification of a precursor substance: the two resulting structures do not interact, nor are they excluded, so both are manifested in the heterozygote phenotype. In reality, codominance and intermediate inheritance are two different manifestations of the same phenomenon, also called incomplete dominance.
pleiotropy
We must not confuse polymery (participation of several genes with the determination of the same phenotypic character) with pleiotropy, which consists of the multiplicity of phenotypic manifestations by a single gene.
In reality it can be considered that pleiotropy is due to the fact that the enzyme conditioned by a single gene controls a reaction that engages with numerous other reactions (coupled, upstream or downstream), which in turn manifest their respective modifications in the phenotype .
Edited by: Lorenzo Boscariol
