A clone is any cell or individual which is identical to another.

In biology, cloning is the process of producing one or more genetically identical individuals. With whole individuals, it usually means the deliberate production of an identical copy. This was first achieved in mammals with the famous Dolly the sheep. Human identical twins are natural clones. So are the offspring of asexual reproduction, and any parthenogenetic reproduction which does not involve meiosis.

Cloning is natural to some animals, but rare in mammals. An exception is the nine-banded armadillo, which normally gives birth to identical quadruplets.

In genetics and cell biology, cloning refers especially to the DNA sequence, and by implication all the other macromolecules.

Clones in cell lines occur, but there are some obvious provisos. Changes to the DNA in any shape or form means the daughter cells are not identical with the mother cells. Typically during development genes are switched on and off, and the daughter cells gradually become differentiated into mature tissue cells. These are not identical with the original stem cells, so they are clones only in the sense of being derived from the same mother cell.

The laboratory copying of a molecule to produce exact copies is also called cloning.

Not quite so identical 
Although clones start off identical, they may not stay that way. Identical twins always have slightly different phenotypes.

Although monozygotic twins are genetically almost identical, a 2012 study of 92 pairs of monozygotic twins found that monozygotic twins acquire several hundred genetic differences early in foetal development. This is caused by mutations (or copy errors) taking place in the DNA of each twin after the splitting of the embryo. It is estimated that, on average, a set of monozygotic twins will have about 360 genetic differences that occurred early in foetal development. However, these changes may have little practical effect. In practice, identical twins look and act in a very similar way.

Another cause of difference between monozygotic twins is epigenetic modification. These are caused by differing environmental influences throughout their lives, which affects which genes are switched on or off. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.

Species cloned 
 Carp: (1963) In China, embryologist Tong Dizhou produced the world's first cloned fish by inserting the DNA from a cell of a male carp into an egg from a female carp. He published the findings in a Chinese science journal.
 Mice: (1986) A mouse was the first mammal successfully cloned from an early embryonic cell. Soviet scientists Chaylakhyan, Veprencev, Sviridova, and Nikitin had the mouse "Masha" cloned. Research was published in the journal "Biofizika" volume ХХХII, issue 5 of 1987.
 Sheep: (1996) From early embryonic cells by Steen Willadsen. Megan and Morag[19] cloned from differentiated embryonic cells in June 1995 and Dolly the sheep from a somatic cell in 1997.
 Monkey: (2000) Tetra, from embryo splitting.
 Gaur: (2001) was the first endangered species cloned.
 Cattle: Alpha and Beta males 2001 and 2005, Brazil. 
 Cat: CopyCat "CC" (female, late 2001), Little Nicky, 2004, was the first cat cloned for commercial reasons.
 Dog: (2005) Snuppy, a male Afghan hound was the first cloned dog.
 Rat: (2003) Ralph, the first cloned rat.
 Mule: (2003) Idaho Gem, a john mule, was the first horse-family clone.
 Horse: (2003) Prometea, a Haflinger female, was the first horse clone.
 Water Buffalo: (2009) Samrupa was the first cloned water buffalo. It was born at India's Karnal National Diary Research Institute but died five days later of a lung infection.
 Camel: (2009) The first cloned camel.
 Crab-eating macaque: (2018) The first time scientists cloned any ape or monkey using cells from a donor older than an embryo.

References 

Genetics
Cell biology
Developmental biology
Biological reproduction