The classical genetically work that Mendel did no doubt established the science of Genetics. There is also no doubt that Mendelian principle of inheritance hold good for majority of the characters in almost all organisms.
But except for the basic principles of inheritance, the whole of modern Genetics is an exception to Mendelism. It (Modern Genetics) seems to defy if not question the universality of Mendelian inheritance. We will briefly analyze a few of these aspects:
(a) Mendel believed that one gene governs one character i.e. One gene one character hypothesis. There are many exceptions to this. Several genes control the development of one character as in comb pattern in fowls, flower color in Indian corn, coat color in mice etc.
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(b) Mendel believed that the inheritance of one gene is not influenced by the other and genes assort themselves independently.
Linkage of genes, where characters present in one individual always are inherited together (maize) shows that Mendel’s laws are not universal.
c) In governing a character a gene will not be influenced by the other according to Mendel. There are several instances of qualitative (flower color in some plants, plumage colors in fowls etc) and quantitative (skin color in man, aleurone color in wheat etc) interactions of genes; again an exception to Mendelism.
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d) Mendel opined that there are only two alleles for every gene and the characters are always contrasting. This is not true as proved by the existence of multiple alleles and by the intergradations of characters as in quantitative inheritance.
e) Characters are either always dominant or recessive – Mendel. This need not be so. There are instances of incomplete dominance or a non allelic dominant gene suppressing another dominant gene (Epistasis).
f) Mendelism is based on chromosomal or nuclear inheritance. But ‘there are several instances of inheritance of characters through cytoplasm.
The facts mentioned above reveal, that everyone. Of the Mendelian hypothesis has been questioned as to it’s/applicability to the entire biological world which was once believed to be true. Indeed in modern Genetics Mendelism is something of an exception than of a rule.
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The pattern of inheritance is not as simple as Mendel thought it to be. Mendel however was lucky in choosing pea plant and the characters. Every one of the characters he chose (the genes responsible for them) was present on a different chromosome and there was no linkage.
If he were to choose linked characters, he could not have enunciated the law of independent assortment. Having said this, it must be conceded, that these discoveries of modern Genetics have not made Mendelism either obsolete or irrelevant.
The rules of inheritance propounded by Mendel though not universally applicable, still hold good for quite a majority of traits, and they (rules of inheritance) remain forever, the corner stone on which is founded the science of Genetics.