Mendelian Principles


Mendel (1865) published a very comprehensive study of inheritance.  In that original paper, he set forth three important principles concerning the inheritance of traits (phenotypes) in hybrids between two pure-breeding varieties.   These principles are the:


1. Principle of Uniformity
2. Principle of Segregation
3. Principle of Independent Assortment(not active)


Mendel's paper


Mendel's First Principle: Uniformity


The Principle of Uniformity is seldom mentioned by most geneticists.  However, one of Mendel's most important contributions to genetics was the idea that the appearance of the hybrids are uniform (that is, all heterozygotes share a common phenotype). This is true if the experimenter has taken proper precautions. The constancy of the phenotype of the hybrids was not being observed by all experimenters.  This was probably due to the fact that they did not have pure-breeding lines of plants or they did not protect their plants from accidental pollination. These two precautions were emphasized by Mendel.

The Principle of Uniformity is essential to subsequent genetic analyses.


Quotes from Mendel 1865

"The striking regularity with which the same hybrid forms always reappeared whenever
fertilization took place between the same species induced further experiments to be undertaken..."
"If two plants which differ constantly in one or several characters be crossed, numerous experiments have demonstrated that the common characters are transmitted unchanged to the hybrids and their  progeny; but each pair of differentiating characters, on the other hand, unite in the hybrid to form a new character..." 


The Principle of Uniformity implies that in each of four lines, strains or generations (two Parental Lines and both the F1 and reciprocal F1 generations) that there should be a uniform phenotype.  Mendel also directly stated that variation in the phenotype should occur in new generations derived from the F1 and reciprocal F1 generations (e.g., the F2 and other hybrid generations)


Although Mendel discussed hybrid phenotypes which resembled almost exactly one of the parental phenotypes, he briefly mentioned that other hybrid phenotypes might be intermediate or even more extreme than the parental phenotypes.  (see discussion of dominance patterns.)

Mendelian Genetics Index