So let's suppose we have homologous chromosome pair number one, homologous chromosome pair number two and homologous chromosome pair number three. Now, in pair number one we have the A traits. So this can either be a upper case A or lowercase A. And this can also be either a upper case A or lowercase A. Now the second allele pair contains the B gene and likewise, we can either have uppercase B or lowercase B on each one of these individual chromosomes. And finally, we have allele pairs number three, homologous pair number three that contains the final gene, gene three or gene C that also controls and effects and has an additive effect on the skin color trait in humans.