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In diploid organisms, individuals inherit two alleles for each gene, one from each parent. A heterozygous state occurs when these two alleles are different. Typically, one allele, termed dominant, masks the expression of the other, recessive, allele. However, various scenarios exist where the presence of a single dominant allele does not completely dictate the observed characteristic. For instance, in incomplete dominance, the heterozygote displays a blended phenotype, intermediate between the homozygous dominant and homozygous recessive phenotypes. A classic example is flower color in snapdragons, where a red-flowered plant crossed with a white-flowered plant produces pink-flowered offspring.

Understanding the expression of traits in heterozygotes is crucial for predicting the outcomes of genetic crosses and for comprehending the complexity of gene interactions. Historically, Gregor Mendel’s work with pea plants established the foundation for understanding dominant and recessive inheritance. However, subsequent research revealed more nuanced patterns, highlighting the importance of considering gene interactions beyond simple dominance. These insights have significant implications for fields like medical genetics, where understanding how heterozygosity contributes to disease susceptibility is paramount.

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