Below are examples of how Aztecas horses were created following Mexico's Crossbreeding Plan. This method is called selective crossbreeding whereby an attempt to develop the desired breed characteristics is achieved and intensified over time. (Crossbreeding is the mating of different breeds within a species.) Using Mexico's Crossbreeding Plan, after three generations of crossbreeding the Azteca "A" level is reached whereby the genetic characteristics have become intensified or fixed. This will allow subsequent third generation and more offspring selected for breeding to procreate more true to type. The Mexican Azteca Association requires a phenotype inspection process on all Aztecas to be used for breeding in an effort to assure only desired characteristics are passed on. The Azteca is a phenotype breed because it actually takes many generations for a breed to become pure. (Phenotype is the visible and measurable appearance of an individual which results from the interaction of genotype as well as environment. Genotype means the gene types an individual inherits from both parents.) Crossbreeding can be useful to produce heterosis or a sudden increase in vigor and fertility. Crossbreeding is one way a new breed can be developed.