Genetic variability is a desirable quality in a species. Without it, we would all be identical in every way except for random mutations throughout life. Although most of our cells divide into identical daughter cells, our sex cells -- called gametes -- do not. Because of a process called meiosis, each gamete contains only one copy of each chromosome. When these haploid gametes combine to form a diploid zygote, genetic material is passed from both parents to form a genetically different organism.
Fertilization is the name given to the fusion of male and female gametes. By themselves, female egg cells -- called ova -- and male sperm cells cannot grow and divide. In fact, the sperm is little more than genetic material with wheels. However, once a sperm cell breaks through the egg cell membrane, the genetic material combines and the new organism begins to grow and develop as a living thing.