How are heterozygous alleles defined?

Heterozygous alleles are defined as possessing two different versions of a gene, one inherited from each parent. This means that one allele can be dominant while the other is recessive, but what distinguishes heterozygous from homozygous (where both alleles are the same) is the presence of these differing alleles. For example, if one parent contributes an allele for brown eyes and the other contributes an allele for blue eyes, the resulting genotype is heterozygous because the two alleles are different.

Understanding that heterozygosity refers to the genetic variation present in such alleles is crucial in genetics as it has implications for traits expressed in an organism. The presence of different alleles can lead to different phenotypic expressions, especially when one allele is dominant over the other.