Genetics and Environment — Why the First Cloned Cat Looked Nothing Like Her Mom and Other Stories
In 2001, Texas A&M University made history with the birth of a kitten named CC.
CC was the first ever cloned cat. Her gene mother was a calico cat named Rainbow.
CC wasn’t calico. At first they thought they had an oops…how did they clone a calico cat and get a tabby and white. Genetic testing, though, confirmed that CC (short for Copy Cat) was, indeed, identical to the donor.
So, what was going on here?
X-Chromosome Inactivation, Otherwise Known as Why There Are So Few Male Tortoiseshells
Cats have a fairly compact genome. And at some point in their evolution, the color genes ended up…on the sex chromosomes.
Or rather, on the X chromosome. Yup, a cat’s coat color is determined by genes on the X chromosome.
In this specific case, we’re looking at the gene which determines whether a cat’s coat is red-based or black-based. This gene lives on the X chromosome, so female cats have two copies, but genetically normal male cats only have one (all male tortoiseshells are, in fact, intersex).
So, there’s an issue with carrying double genes when you only need one set. All female mammals have a mechanism called XCI (X-chromosome inactivation). This mechanism turns off one X chromosome in each cell.
You can see where this is going. Calico and tortoiseshell cats carry both genes, for red and black. Depending on which X chromosome gets randomly turned off in which skin pigmentation cell, the cell produces red and black. For some reason, likely related to how mammals make sure that internal organs get the same activated X, they clump together making those oh so pretty red and black patterns.
For some reason, every single one of CC’s skin cells got the black gene active. This may happen more than we thing as we don’t generally gene test our tabby cats.
CC also had different white spotting pattern. Let’s look at that next.
Scamper and Clayton
Scamper, officially known as Gills Bay Boy, was one of the most successful barrel racing horses in the world.
Unfortunately, he was gelded before his talent was proven (or perhaps he would not have run so well as a stallion). As you can’t ungeld a horse, Charmayne James, his owner, who apparently had quite a lot of money, opted to clone him to create a colt she could then stand at stud. That colt was Clayton.
Scamper had no white on him.
Clayton had a star, snip, and two hind socks!
Oops? Well, if we go look at CC and Rainbow again, you’ll see the white markings are different there too.
White markings on animals are determined by epigenetics. The animal has the DNA to have a white marking, say, in the middle of his forehead in horses (a common place for white), but uterine environment determines how large the star grows. It has to do with melanocyte migration. Pigment cells move to these places during early development, and some parts of the body might never get them. In horses, these generally include the face, especially the forehead and nose, and the ends of the limbs.
Even if you use the original animal’s mother as the surrogate for the clone, the uterine environment will never be the same and the white markings will be different.
This may have implications if we ever clone humans. In fact, a man and his clone may be less identical than twins, who share the same uterine environment.
What About Ability?
So, now onto the Brave New World question — do clones share the ability of their genemother or genefather?
Horse cloning might provide some answers here. Clayton was not given the chance to prove he was as good as Scamper; his owners didn’t want him to be injured while running.
In fact, the majority of horse clones are not destined to perform; they’re destined to replace their gene donors in the breeding shed. The majority of clones are of high performing geldings. In one famous incident, a polo player had six clones made…so he could play an entire match on the “same” horse. Polo has, in fact, embraced cloning more than any other equine sport.
This might make a reasonable argument that clones have the same ability as their donors.
But, and of course there’s a but: Cloned horses only have the genetics of their original.
They are generally birthed by different surrogates (by the time a horse is determined valuable enough to clone, his or her dam is usually pretty elderly, and surrogacy for clones and embryo transfer is generally a profession for mediocre, but reproductively healthy mares), raised differently and with the best will in the world, not trained the same. Those polo clones were raised and trained to be a team, which is a bit different.
No clone has yet won an Olympic medal, or even a major FEI show jumping event. This might be because few of them are old enough yet.
What horse clones teach us is that a clone has the same genetic potential as the original. Breeding from a clone is functionally identical to breeding from the original.
However, the clone still has to do well in training. There might well be an advantage, though, to a clone raised and trained by the same person who raised and trained the original; they tend to have similar dispositions and thus respond to the same training methods. But that’s not so different from training a horse after training its dam…
The implications for human cloning are that a clone will not be identical in ability to their donor. They might be inferior. Or they might be superior because they are given special treatment.
(C.J. Cherryh’s brilliant book Cyteen addresses these questions in a very thoughtful and intelligent way).
Of course, humans are proving, like other primates, to be very difficult to clone successfully…
…so it might be a moot point after all.