Gene variants of the same feature/characteristic found in the same loci of the homologous chromosome are called allele. The alleles for a genetic feature present in a diploid organism interact to produce the physical characteristics. If an allele for a gene expresses itself in homozygous and heterozygous condition, such allele is called as dominant allele. The allele which expresses itself only in homozygous condition but not in heterozygous condition is called recessive.

Example, gene for stem height in pea plant has two alleles - tall and short. Tall is dominant and short is recessive as short is expressed only in homozygous condition.