How are Mendelian inheritance, polygenic inheritance and pleiotropy different from each other?
Mendelian inheritance refers to the expression of monogenic traits, i.e. gene expression is controlled by one gene. In a pair of alleles, an expression of the recessive gene is always masked by the expression of a dominant gene. The ability of a gene to have multiple phenotypic effects because it influences several characters simultaneously is called pleiotropy. It is due to the effect of the gene on two or more inter-related metabolic pathways which contributes to the formation of different phenotypes.
Polygenic inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have a cumulative effect with each dominant allele expressing a part or unit of the trait, the full being shown only when all the dominant alleles are present.