When sex cells are produced (by meiosis), allele pairs separate leaving each cell with a single allele for each trait.
When the two alleles of a pair are different, one is dominant and the other is recessive.
Examples: The gene for seed color in pea plants exists in two forms. There is one form or allele for yellow seed color (Y) and another for green seed color (y). In this example, the allele for yellow seed color is dominant and the allele for green seed color is recessive. When the alleles of a pair are different (heterozygous), the dominant allele trait is expressed and the recessive allele trait is masked. Seeds with the genotype of (YY) or(Yy) are yellow, while seeds that are (yy) are green.