Heterozygosity refers to an individual's Genotype. It occurs when an organism inherits a dominant and recessive allele for a particular gene from the parents. The phenotype of this heterozygous individual is determined by the dominant allele in the genotype while the recessive allele will show nothing in the phenotype. A heterozygous gene is typically denoted by the abbreviation Aa but this can be different depending on your source. The opposite to heterozygous is Homozygous.