Multiple alleles is a condition where more than two variants of the gene occupy the same locus. Mutation is change in the sequence of DNA segment. Mutation in the sequence of amino acids can result in formation of a new allele if the changes are simple and not on many bases. Multiple alleles combine in different combination to produce different phenotypes. Human blood group is a classic example of multiple alleles having three alleles whose combination produces four possible phenotypes: $\mathrm{A},\mathrm{ }\mathrm{B},\mathrm{ }\mathrm{A}\mathrm{B},\mathrm{ }\&\mathrm{ }\mathrm{O}$ blood group.

When traits are controlled by three or more genes, they are called polygenic traits and their inheritance is called as polygenic inheritance. Human skin colour is a classic example following polygenic inheritance as the skin colour is regulated by three pairs of gene.

Gene is a segment of DNA responsible for expression of proteins which in true expresses the character or feature represented by the gene. When this gene is regulating the expression of more than one feature in an organism such phenomenon is called pleiotropy. Phenylketonuria is an inborn error of metabolism where the individual has reduced metabolism of the amino acid phenylalanine. It is a recessive autosomal disorder and an example of pleiotropy.