Scientists are rapidly approaching the most substantial technological threshold in human history: the ability to alter the genes we pass to our children. Genetic engineering is already being successfully carried out on nonhuman animals. Researchers have recently been able to connect the genes in the striped zebra fish to genes in humans that determine skin color. The controversy is surrounded upon if people should be able to select desired traits for their children, whether it is for medical purposes when screening for a disease or if parents simply want to alter cosmetic traits and other non-medical abilities. The term "designer babies" refers to the use of pre-implantation genetic diagnosis to select desired qualities of a child, and also has been used to describe the babies that would result if human gametes, zygotes, or embryos were to be genetically engineered. Human reproduction is very complex and involves many underlying factors in the process such as, the genetic constitution of the parents, the condition of the parents' egg and sperm, and the health and behavior of the impregnated mother. When you consider that and the enormous complexity of the human genome, containing billions of DNA pairs, it becomes rational that reproduction will never be fully conquered.