Some amphibian species are capable of changing their biological sex characteristics to facilitate reproduction. This type of morphology (changeability) is not present in reptiles. However, some reptiles, including several snakes and lizards, can undergo a process called parthenogenisis. While this phenomenon does not entail a reptilian animal having both genders' sex organs, it does, in effect, accomplish the same reproductive goal: to allow for fertilization when one of the sexes can not be found in order to complete it.
Parthenogenesis occurs within female reptiles when no male can be located for fertilization. In such an event, the female can actually produce offspring despite this lack. In all observed cases, including those of the garter and timber rattlesnake, all of the resulting offspring have been males. To substantiate that related females have not been fertilized by males, scientists inspect the DNA strands found in the offspring. Results then show that all the strands match the mother, indicating that no mate was involved in the process.
Normally in fertilization, the egg is affected by an agent introduced by the male; typically sperm. Since this sperm is not supplied during parthenogenesis, another agent must take on its role. When a cell first divides, its chromosomes are split in two without being duplicated, so each has only half of the required information. As the cells split again, parthenogenesis causes one of the new cells to re-enter the other rather than diverge, in effect, causing self-fertilization in place of the sperm. The newly fertilized egg now has a full genetic sequencing derived specifically from the mother alone.
Again, this process has only been discovered in lizard and snake species found within the reptilian family. As it pertains to snakes, it usually is only live-bearing species -- such as vipers and garter snakes -- that have displayed a capacity for parthenogenesis. However, it has also been observed in Burmese pythons on rare occasions as well. The lizards of the group Cnemidorphous are among the most recognized parthenogenic reptiles. Some species found within this genus are exclusively female, always reproducing asexually.