In genetics, a chimera is an individual formed from two different cell lines. Scientists believe that it happens when two nonidentical twin embyos (fertilized eggs) fuse into one embryo very early in their development.
The embryo develops into a normal, complete individual that has two different DNA types. He or she might have kidneys that developed from one DNA type and a heart or skin cells from the other type.
Extremely rare, chimerism has been documented in other species, including cats and humans.
"Dunbars Gold has one cell line that is female and one cell line that is male," explains Dr. Cecilia Penedo. "The cell lines have slightly different DNA types, but both qualify to his sire, so there's not a parentage issue involved. It's almost clear that there were two embryos produced, and they fused."
Although chimeric in his skin and hair, Dunbars Gold's reproductive organs were apparently formed by his male DNA type.
"It's an interesting biological development event. He went on to develop as a completely normal male," Dr. Penedo says.
The lab has tested several of his foals, and they were all produced by his male cell line.
When Dr. Penedo turned her attention to Sharp One, she again made note of the rare brindle coat pattern the mare had in common with Dunbars Gold.
If Dunbars Gold was chimeric in his skin and hair, could there be a link between the equine brindle coat pattern and chimerism? What if the problem the lab was having qualifying Sharp One as the foal's dam was also due to chimerism?
Like Dunbars Gold, Sharp One's DNA type had also been initially established from mane and tail hair samples. However, according to Dr. Penedo, there was no evidence of chimerism in those hair samples.
"I wondered if she could be the opposite of Dunbars Gold," Dr. Penedo says. "If she might show chimerism in her blood but not her hair."
When Sharp One's blood was tested, that's exactly what Dr. Penedo found: two different cell lines (and DNA types) in the mare's blood. But both of her cell lines were female.
The lab then tested Sharp One's body hair taken from the darker and lighter areas of her brindle pattern.
"We were able to identify the two cell lines in the different (body hair) patches," Dr. Penedo says.
When the lab compared the 2004 foal's DNA type to Sharp One's 2003 foal (her only other living foal so far), it found that each came from a different female cell line.
"Unlike Dunbars Gold, she is chimeric in her (reproductive organs)," Dr. Penedo explains. "She is producing two different types of eggs; they can be from one or the other cell line, which is a very interesting situation.
"Once we put the puzzle together, we were able to qualify the foal without any problem," Dr. Penedo says.
The Pieces Fit
"We now have evidence of chimerism from two different brindle horses," Dr. Penedo says. "We think that the brindle pattern in some horses could be explained by this very rare event, where two embryos fuse early on in their development and go on to make just one, single individual with different cell lines."
Dr. Penedo is quick to point out that there is evidence of a type of brindling pattern in horses that appears to be inherited, linked to a coat pattern gene, as it is in dogs and cattle.
"We can't say that it is always embryo fusion that leads to the brindling pattern in horses," she says.
"But for the very classic, clear brindling pattern like we see in Dunbars Gold and Sharp One, I wouldn't be surprised if that was caused by chimerism," she continues.
"If it results from embryo development, then there is no genetic control, and you can't really breed for it. Unless there is a gene controlling something that makes it more likely for the fusion to occur, such as something that increases the likelihood of a twin pregnancy."
Dr. Penedo thinks you would only see outward evidence of chimerism in the coat pattern if the two embryos that fused had genes coding for contrasting coat colors.
"For example, if two bay embryos fused, or two chestnuts, you wouldn't see anything," she explains. "I think you'd see the brindle pattern if you had a combination of a base color with one dilution gene. In this case, it was the dun gene. Both Dunbars Gold and Sharp One have dun in their backgrounds.
"It's been a great learning experience," Dr. Penedo adds. "It is shedding a little bit of light on the brindling pattern."
Denise Charpilloz named her foal Sharp Barcoder, aka "Deuce." Deuce is a solid sorrel, and there's nothing chimeric about him. But he is the occasion for an incredible genetic coincidence: If Denise hadn't been trying to breed for the rare brindle coat pattern, these even rarer chimeric individuals would never have met. And geneticists worldwide would not have had this once-in-a-lifetime chance of studying their DNA. What are the odds of that?