The term heterozygosity refers to the condition whereby alleles of the same gene are different from each other. In the case in which the allelic forms are equal to each other there is a situation of homozygosity. The two conditions have direct influence on the visual manifestation of a genetic character, in relation to dominance or recessivity.
In particular, in genetics, we mean that situation in which alleles occupy the same loci on the corresponding homologous chromosomes.
In general, alleles can be dominant, usually indicated by capital letters; or recessive, indicated by lowercase letters. Therefore, the heterozygous diploid individual will be of type Aa. The phenotype of the heterozygous individual is determined by the dominant allele: it is therefore phenotypically equal to the dominant homozygote.
So we will have the genotype is Aa and the phenotype is A. The alleles are indicated with small letter (the recessive) and capital letter (the dominant), while the genotype is defined with the two alleles, therefore AA, Aa or aa. The phenotype is the one that manifests itself physically, therefore phenotype A for genotypes AA and Aa and phenotype a for genotype aa.
In this sense, being each individual composed of cells containing all the same genetic heritage, the heterozygosity refers both to the individual as a whole and to the cells that compose it.
Gregor Johann Mendel (Hynčice, 22 July 1822 – Brno, 6 January 1884) indicated heterozygous individuals with the term hybrid.