What characterizes heterozygous genotypes?

Heterozygous genotypes are characterized by having two different alleles for a specific gene. This usually involves one dominant allele and one recessive allele. In this state, the dominant allele typically masks the expression of the recessive allele, which means that the phenotype (the observable traits) of the organism reflects the traits associated with the dominant allele.

In contrast, having two identical alleles refers to a homozygous genotype, which does not meet the definition of heterozygosity. The option mentioning three alleles is incorrect since diploid organisms, which include most multicellular organisms, can only possess two alleles for each gene. Lastly, the reference to alleles that do not express a phenotype does not align with the nature of heterozygous genotypes, where typically, at least one of the alleles is dominant, which actively influences phenotype expression. Thus, the correct characterization of heterozygous genotypes is the presence of two different alleles, one being dominant and the other recessive.