Many invertebrates, such as insects, can produce offspring asexually, cloning themselves to produce genetically identical offspring. But vertebrates, animals with a backbone, generally reproduce sexually.
In 2006, scientists discovered that two komodo dragons, the world's largest lizard species, had produced eggs that developed without being fertilised by sperm - a process called parthenogenesis. Then, in 2007, other scientists found that captive female hammerhead sharks could also reproduce without having sex. In 2008 and 2009 the captive- born female Boa constrictor produced two litters of live offspring at the time as being housed with four male snakes. DNA fingerprinting revealed that the offspring had a number of genetic differences from any of their potential fathers, which ruled out all the males as fathers.All the offspring had very unusual sex chromosomes. Sex chromosomes are packages of DNA that drive the development of sexual characteristics; they essentially make animals genetically male or genetically female. Humans for examples have X and Y sex chromosomes; female have two X chromosomes and males have a combination of X and Y chromosomes. In place of X and Y, snakes and many other reptiles have Z and W chromosomes. In all snakes, ZZ produces males and ZW produces females. Bizarrely, all the snakes on these litters were WW - essentially they are half clones of their mother. This female mother has given birth to sexually produced babies in the past. However, why she does not utilise male sperm is at present unknown. ( Biologist Dr. Warren Booth, BBC Earth News, 03/11/2010).