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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.

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    $\begingroup$ Welcome to Biology.SE. The last question Further is there a distinction between the words Law,Principle and Rule [..] is a question of philosophy, not of biology (have a look at philosophy.SE). $\endgroup$ – Remi.b Jan 11 '17 at 8:08
  • $\begingroup$ We commonly tell about main 2 findings of mendel... (1) Phenomenon of segregation, and (2) phenomenon of independent assortment (second one later came out as as an incomplete-truth)... however normally there were more principles behind these findings. okay I'll write an answer when I'll get time. $\endgroup$ – Always Confused May 11 '17 at 7:31
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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.

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  • $\begingroup$ thank you for clearing things a bit , but going back to my basic question do these principles hold as the principles of inheritance or not ? Moreover do you perceive that there maybe a difference in what Mendel's law and principles ? also is the principle of unit character a repetition of the principle of paired factor or are they separate for they all seem to be overlapping to me $\endgroup$ – Anamika Ghosh Jan 11 '17 at 10:35
  • $\begingroup$ Answering your 3 questions in order. 1) Sure, they relate to inheritence. It is a human decision to call them one way or the other then. 2) The difference between principle and law is a question of philosophy, not biology. 3) No, they are different. Maybe try rereading my answer and if it does not help, you can try to specify what is unclear to you. $\endgroup$ – Remi.b Jan 12 '17 at 13:50

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