An allele is one of the possible forms of a gene. Most genes have two alleles, a dominant allele and a recessive allele. If an organism is heterozygous for that trait, or possesses one of each allele, then the dominant trait is expressed. A recessive allele is only expressed if an organism is homozygous for that trait, or possesses two recessive alleles. Alleles were first defined by Gregor Mendel in the law of segregation. genetics alleles genes. When studying heredity one of the most common things to mess up is the terms gene and allele.