In nature most families start when a male and female mate and have offspring that combines genetic characteristics of the parents (sexual reproduction). But nature is full of exceptions even for this basic rule. Some animals employ asexual reproduction, also called parthenogenesis, in which only one parent, usually a mother, is needed. It occurs throughout nature in many forms among reptiles, birds, and insects. Remarkably, mammals (including people) are the only creatures in which this virgin birth cannot occur due to a special way that our genes work.