A recent article in the Wall Street Journal depicts the frequency with which cloned horses are now used in equestrian sports. This use is especially common in polo ponies. The practice is banned in thoroughbred horses. The public seems to think that a cloned animal is identical to its donor. This belief is not true.
The popular image of cloning often portrays it as a biological photocopying process that produces an exact duplicate of an animal. In reality, cloning creates an animal that is genetically very similar to its donor, but not identical in every respect. A cloned animal is best understood as the equivalent of an identical twin born at a different time rather than a perfect copy – and even that representation requires modification. While the clone and donor share nearly all of their nuclear DNA, differences in development, environment, epigenetic programming, and life experience inevitably produce individual variation.
In the most common cloning technique, known as somatic cell nuclear transfer, the nucleus from a donor body cell is transferred into an egg cell whose own nucleus has been removed. The resulting embryo contains essentially the same nuclear genetic information as the donor. As a result, the clone usually inherits the donor’s sex, body plan, coat type, eye color, mature size range, metabolic characteristics, and many inherited disease susceptibilities. From a genetic standpoint, the similarity is extraordinarily high, approaching 100 percent.
Nevertheless, even the genetic similarity is not absolute. The clone typically receives its mitochondria – the structures that generate cellular energy – from the egg donor rather than the nuclear donor. Because mitochondria possess their own DNA, the clone’s mitochondrial genome may differ from that of the animal from which the nucleus was obtained. Although these differences are generally small, they demonstrate that a clone is not a complete genetic duplicate.
The greatest similarities between a clone and its donor are usually found in physical appearance. Healthy adult clones commonly resemble their donors so closely that an experienced observer can immediately recognize the relationship. Coat texture, body shape, facial structure, and general appearance are often remarkably alike. However, noticeable differences may still occur. Coat markings can vary, body proportions may differ slightly, and mature weight can diverge. These differences arise from developmental events occurring during embryonic growth and from environmental influences throughout life.
One of the most striking examples comes from cloned cats. The famous cloned cat CC (“Carbon Copy”) differed visibly from her donor despite sharing the same nuclear DNA. The variation arose because of differences in X chromosome inactivation and other developmental processes that affected coat-color patterns. Rainbow, the donor, was a calico, while CC was a ginger. Such examples illustrate that identical genes do not necessarily produce identical appearances.
Behavior and temperament show even greater variation than physical characteristics. Many cloned animals display behavioral tendencies similar to those of their donors, reflecting the influence of genetics on temperament. Activity level, trainability, sociability, and responses to stress may resemble those of the original animal. However, behavior is shaped not only by genes but also by experience. A clone does not inherit memories, training, social interactions, illnesses, injuries, or life history. Consequently, two genetically identical animals may develop distinct personalities. In this respect, cloned animals come close to resembling identical twins, who often share many traits yet remain clearly individual people or animals. But identical twins have the same mitochondrial DNA, while cloned animals do not.
The principal biological reason for differences between a clone and its donor lies in epigenetics. Epigenetic mechanisms regulate which genes are active and which remain silent without altering the DNA sequence itself. When an adult cell nucleus is transferred into an egg, the egg must erase many of the molecular marks associated with the donor cell’s previous identity and restore an embryonic pattern of gene activity. This reprogramming process is not always perfect.
One important epigenetic mechanism involves DNA methylation. Chemical groups attached to DNA can silence genes or alter their activity. If the cloning process fails to reset these patterns completely, certain genes may remain inappropriately active or inactive. Similar problems can occur with histone modifications, which influence how tightly DNA is packaged and therefore how accessible genes are for expression.
Genomic imprinting provides another source of variation. Certain genes normally retain a memory of whether they originated from the mother or the father. Errors in imprinting can affect fetal growth, placental development, and other physiological processes. These abnormalities are among the reasons cloning remains inefficient, with many embryos failing to develop normally.
Beyond epigenetics, post-transcriptional events can further increase differences between clone and donor. Once a gene has been transcribed into messenger RNA, numerous regulatory processes determine how much protein is ultimately produced. MicroRNAs may suppress specific messenger RNAs, alternative splicing may generate different protein products from the same gene, and differences in RNA stability can alter protein production. As a result, two animals with essentially identical DNA can produce different amounts of key proteins, leading to physiological and developmental differences.
Taken together, these factors mean that a cloned animal is neither a perfect copy nor merely an ordinary relative. Genetically, a healthy clone is almost identical to its donor. Physically, the resemblance is usually striking and often comparable to that seen between identical twins. Physiologically, most traits are highly similar, although differences in gene expression may produce measurable variation. Behaviorally, similarities may be substantial but are generally less predictable because experience plays such an important role.
In summary, a cloned animal shares nearly all of its nuclear DNA with its donor and therefore exhibits a high degree of similarity in appearance and biology. Yet differences in mitochondrial genetics, embryonic development, epigenetic programming, post-transcriptional regulation, and life experience ensure that every clone remains a unique individual. Cloning reproduces a genome with remarkable fidelity, but it does not reproduce an entire biological life.
So if beloved Fido is old and soon to be put down, and you clone him, you may get a dog that resembles him, but who is very different in temperament and behavior. In other words, a different dog. So you might forgo cloning Fido, save some money, and adopt or buy a new dog.
