A different or alternate form of the same is gene is called and "allele". Sometimes they can cause different observable changes in the phenotypic traits. However, they normally cause little or no variation in traits.
Different forms of one gene are called alleles. Alleles of the same gene occupy the same locus of a specific chromosome. A gene can have two or more than two alleles. Irrespective of the number of alleles of a gene, all the alleles of the same gene regulate the same feature.
For example, ABO blood type in human is regulated by three alleles: IA, IB and i. All these alleles regulate ABO blood type only.
