It is amazing to think that billions of living organisms on Earth are discrete individuals. Even organisms which are genetically identical to their parents and siblings change over time due to environmental factors and mutations to produce tiny differences which make them unique.
For many species, sexual reproduction where the male and female parent each contribute fifty percent of molecular DNA, ensure that the offspring are genetically different from their parents and their siblings. There are exceptions where siblings have the same DNA; the most familiar is identical twins. This occurs when one fertilised egg splits into two cells. Identical twins are genetic duplicates, but even before birth they begin to develop individually and by the time they are born they have different fingerprints and are no longer exact replicas of each other. Over time environmental factors and slight physical changes and mutations cause increasing divergence between twins.
Artificial reproductive cloning i.e. the creation of an individual which is genetically identical to its parent, involves transferring genetic material from the nucleus of a donor cell to an egg which has had its nucleus removed. The reconstructed egg is then stimulated to continue to cell division until it is transferred to a female host where it continues to develop until birth. Dolly the sheep was the first viable and most famous animal clone. Although cloned animals have identical genetic DNA as their biological parent, they inherit mitochondrial DNA from the enucleated egg, so are not exact replicas of their genetic parent. Cloned animals have high rates of abnormalities, death, deformity and disability. Poor Dolly lived half the normal lifespan of a sheep and was suffering from premature arthritis and cancer at the time of her death.
In nature, asexual reproduction can produce viable clones. In some species of plants a piece of the parent plant such as a stem, rhizome, runner, bulb, sucker, tuber or corm can break away and form a new individual plant which has the same genetics as the parent plant.
Species including bacteria, protozoa, sea stars, coral, sea anenomies, sponges and invertebrates utilise a variety of asexual reproductive mechanisms such as budding, binary fission, fragmentation and regeneration to produce genetically identical offspring.
One of the more complex methods of asexual reproduction is parthenogenesis or virgin birth, whereby the mother's chromosomes divide and split, then pair with a copy of itself, producing offspring which only has maternal DNA, but which is not necessarily a clone of the mother. Some species can switch between sexual reproduction and parthenogenesis. 70 species of vertebrates including lizards, sharks, fish, and amphibians can reproduce through parthenogenesis. Interestingly, in some species males have become redundant and the population is totally female.
Komodo Dragons can reproduce sexually and through parthenogenesis, however because of their sex determination system baby Komodo Dragons produced by parthenogenesis are always male. Female Komodo Dragons must have a male and female parent.