I believe this illustrates Mendel's principle of Dominance. In Dominance an organism with a dominant allele for a particular form of a trait will always exhibit that form of the trait, for example Bb; the big B for the dominant trait. Homozygous term is used to refer to an organism that has two identical alleles for a particular trait BB. Heterozygous on the other hand; is a term used to refer to an organism that has two different alleles for the same trait Bb.