Fertilization occurs when a sperm and egg fuse together to form a single cell called a zygote. During fertilization, the alleles from the mother and father combine to form a new organism. These combinations of alleles are known as genotypes and are responsible for the physical traits of the organism. Some of these traits are determined by dominant alleles, while others are determined by recessive alleles. The genotype of the zygote is unique, as it is a combination of both the mother and father’s alleles.