The external appearance of two cloned animals is not identical. This has been observed in many animals that have been cloned when compared to the animals that generated them. To answer this question we must first understand, how similar two cloned animals are and what factors affect their resemblance?
Are cloned animals the same as identical twins? Whatever they might have told us, the answer is just no. When we speak about genetic material we immediately think of DNA and in fact, the genetic information that allows a complete individual to develop and fulfill his functions is all contained in this amazing molecule of life called DNA. In sexual reproduction, there is a fixed number of genes for a given species (made of DNA) to which both maternal and paternal parents contribute. This DNA we are talking about is the DNA present in the nucleus of the cell. However, inside the cell, there is an organelle called mitochondria, responsible for providing energy to the cell, that contains its own “proprietary” DNA.
This mitochondrial DNA is inherited only from the mother thus an individual such as a dog has in each of its cells, mitochondria that were provided by his mother’s egg. Paternal mitochondria contained in sperm and are not part of the new being as they are eliminated shortly after the sperm penetrates the egg during fertilization. In this way, clones share nuclear DNA but not mitochondrial DNA. Therefore, cloned animals are not 100% identical in genetic terms, but yet, they share all the genes contained in the cell nucleus.
Nuclear DNA is responsible for most of the inherited traits. Identical twins. Are formed once the embryo is developing and is composed of a few cells. Each of those cells is each able to generate a new individual. During these early stages of development, this embryo can branch off another identical embryo that comes from such cells and in the end form two new individuals that are identical from the genetic point of view, which is what we call identical twins. Both identical individuals are so because they share nuclear and mitochondrial DNA.
Cloned animals are also different from each other in terms of the activity of their genes. This is what is known as epigenetics. There are normally a finite number of genes within the nucleus of each cell of an individual. Dogs have a total of 19,000 genes. Amazing right?
At any given time and in each type of cell in a dog it is just not possible that all these 19,000 genes that make up the dog genome are activated simultaneously. Depending on the cell type we are talking about, for example, a neuron, a percentage of these genes will be active and another percentage will be inactive at a given time and this is what we know as the epigenetic profile.
A neuron, for example, will have a very different epigenetic profile from a muscle cell since they perform different functions so that each of them must produce proteins associated with its specific function. In other words, the epigenetic profile of a cell will determine what proteins are produced and which are not, allowing the cell to perform its particular and very specific functions.
One of the problems that have been identified in cloned animals is that the global epigenetic profile in their bodies presents variations compared to individuals that have reproduced naturally. This could explain, at least in part, why several reports establish that <cloned individuals have health issues during development>.
In short, a cloned dog is not genetically exact as the dog that gave birth to it.
Now it is time to ask ourselves what genetic factors make two cloned animals different? Let’s take a look at the following images to try to understand the differences in appearance that can occur between cloned animals as we mentioned at the beginning of this article:
This image represents two cloned dogs in which we can see differences in their color coat. The genes that encode the proteins within the cells that generate the hair pigmentation in dogs are genes located in the DNA of the nucleus (the central organelle in cells). So, we expect two cloned animals (with the same nuclear DNA sequence) should not present these differences. Then, what are these differences in the fur of cloned animals? To answer that we must review one of the fundamental equations in genetics:
Phenotype = Genotype + Environment
The phenotype is the physical appearance of an individual or any characteristic that can be measured, e.g. weight, size, blood glucose concentration, and so on.
In our case, the cloned dogs represented in the picture share near-identical genotypes but show coat color differences. We can then say that their phenotypes are different at least in the appearance of the spots in the fur. So, if the phenotype is determined by the genes of the animal, but these animals share the same genetics contained inside their cell nuclei, which element of the equation would then be responsible? The remaining candidate element of the equation is the environment
The environment which will generate modifications to the fur of two cloned animals acts upon them inside the maternal uterus and generates these small differences in the phenotype. Coat color has to do with some specialized cells that produce pigments and these cells are called melanocytes. Melanocytes produce melanin, responsible for the darker tones of the coat. During fetal development, melanocytes migrate through the skin surface of the fetus and generate the patterns that we can observe in the fur once the animal is born. The migration of these cells called melanocytes is influenced by the internal environment of the mother (hormones, growth factors, blood flow, and other factors) and thus generate these differences in cloned animals.
In the end, the genotype is not the only factor that affects how we look (remember that two identical twins can look different if they are subjected to totally different environments). The same goes for two cloned dogs (just think of one of the cloned dogs being fed better than the other or receiving better care than the other).
One final thought on the obvious about cloned animals: a cloned dog will always be younger than the adult dog that gave origin to it. There will always be a waiting time until the dog is born and grows up. During that time, the dog that donated the cells to create the clone will continue to get old. When the produced dog clone is an adult he will be, in the best case, very similar to his “father”.
In summary, if one day we decide to clone our senior or deceased dog (the latter can be done if we have <preserved cells before the dog dies>) let’s bear in mind that the 2 dogs will never be the same individual, as fondly and perhaps influenced by science fiction movies, some people might think or feel. An important consideration is that <cloning a dog can be very expensive!>
The next time you see two cloned animals and you see little differences in the way they look you can confidently say that it was not a scam (it is the real thing!). Dog cloning is a real possibility of our times and fortunately the <cloning technique is safer for dogs than for other species>. Amazing eh!