I know of 3 laws of Mendelian inheritance : 1.law of segregation 2.law of dominance 3.law of independent assortment But, I've also come across a couple other "rules" of Mendelian inheritance such as principle of unit character/factor and principle of paired factor. I cannot understand what these other rules actually mean and whether they are a part of the Mendel's law of inheritance. Further is there a distinction between the words Law,Principle and Rule or they mean the same thing.
Principle of unit character/factor
To understand this concept, it is good to fall back into the era in which Mendel lived, an era where we knew so little about the nature of genetic information. Consider a famous (and highly racist) argument by Jenkins against the hypothesis of natural selection as formulated by Darwin.
Jenkins supposed that a white man was wrecked on a desert island inhabited by Africans. The shipwrecked European would kill many black men in struggle for existence. He would have many wives and children. Many of his subjects would live and die as bachelors. In the F1, there will be some intelligent young men who are mulattoes (mixed race) more intelligent than negroes. Eventually instead of the beneficial white characteristic rising in frequency through natural selection acting on intermediates, Jenkins said that the European alleles and genes will be diluted out.
Putting aside the obviously racist and false assumption that whites are superior to blacks, the argument is not silly. Why would not the beneficial characteristic of an individual just dilute into the majority unadapted population? The answer is in the long forgotten Mendel's work. Individual characteristics are not 'fluid'. Characteristic of parent don't just average to give the characteristic of the offspring. There are actually particular unit of heredity (a gene) which has some seemingly atomic property in the sense that its effect will be diluted by crossing individuals carrying a given allele (an allele is a variant of a gene) with individuals carrying different alleles.
Principle of paired factor
Here is the underlying discovery of diploidy. Of course, by far not all living beings are diploids (or diplontic) unlike Mendel may have thought but he still discovered that diploidy is a thing.
Mendel discovered that it is the combination of two 'alleles' (the terms 'gene' and 'allele' did not exist at that time yet) that would determine the 'phenotype' (this term did not exist yet either) of an individual.