Here's why:
* Dominant alleles express their trait even when paired with a different allele (heterozygous).
* Recessive alleles only express their trait when paired with another copy of the same recessive allele (homozygous recessive).
Example:
Let's say brown eyes (B) are dominant to blue eyes (b).
* Heterozygous (Bb): A person with one brown eye allele (B) and one blue eye allele (b) will have brown eyes because the brown allele is dominant.
* Homozygous recessive (bb): A person with two blue eye alleles (b) will have blue eyes because there is no dominant brown allele to mask it.
Therefore, an allele that produces a trait in the heterozygous condition is the dominant allele.
