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| Non-Mendelian Inheritances |
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| The rediscovery of Mendelism and the subsequent idea of chromosomal basis of inheritance, paved way for several significant discoveries in the field of heredity. Many other patterns of inheritance which cannot be explained on the basis of Mendel's laws alone, were discovered in plant and animals. Such patterns of inheritance are described as non-Mendelian inheritance. |
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| It is now clearly established that the expression of specific traits is due to the expression of any one gene. It can be sometimes altered by the influence of other genes. Such a phenomenon where certain genes bring about a modification of the normal phenotypic expression of a given gene, is known as gene interaction. Gene interactions are of two types intragenic and intergenic. |
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 Intragenic interactions, also known as inter-allelic interactions occur between the two alleles of the same gene. Such an interaction results in a phenotype which is different from the typical dominant recessive phenotype. The common examples of intragenic interactions are incomplete dominance and co-dominance. |
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 Intergenic interactions, also known as non-allelic interactions occur between alleles of different genes located on the same or different chromosomes. Such an interaction also results in a change in the phenotypic expression. The familiar examples are complementary genes, supplementary genes, epistasis, plieotropy and lethal genes. |
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