there are dominant and recessive alleles. when one allele from each parent combine, there are a couple different possibilities for traits. for example, whenever a parent gives off a dominant allele, you will automatically have that trait because it would have combined with another dominant allele, or it would have overpowered the other recessive alelle. you cam find these different combos by using a punnet square. but also, some traits, such as eyecolor, are determined by incomplete dominance, when the colors of your parents in their greenness combine in to a new color. or, you can have codominance when you have one of each eyecolor of your parents.