So, what do we mean by dominant, and what do we mean by recessive? So, remember, according to the law of dominance, or Mendel's law of Dominance, the dominant gene codes for protein that inhibits the expression of that recessive gene. And so the recessive gene is never actually shown in the phenotype of that individual, and we'll see what that means in just a moment. So, an individual or organism is said to be heterozygous for a given trait if one of the alleles is dominant while the other one is recessive. Now, usually, an uppercase letter is used to designate the dominant gene, and the lowercase letter is used to designate the recessive gene. Let's suppose we're dealing with these two genes, the alleles that are shown in purple.