The world's smallest horse was born in late April on a farm in New Hampshire. Weighing in at 6 pounds at birth, Einstein appears to have beaten the previous record holder by three whole pounds.

But Einstein probably won't hold his place in the Guinness Book of World Records forever, because there may be no limit to how tiny we can make our horses, said equine geneticist Samantha Brooks of Cornell University. But to get teacup horses will take many generations of breeding.

"In the last 50 years, breeders have made very good progress at making a very small horse, but they periodically hit these speed bumps," said Brooks. "It takes a while to work them out so that you end up with a horse that not only fits in the palm of your hand but is happy and healthy."

In recent years, the genetic underpinnings of height and size in mammals have generated increasing interest from scientists. In 2007, genetics researchers made the surprising finding that a single gene plays a very large role in regulating dog size, a fact that partially accounts for the tremendous variation in dog size that we see from tiny chihuahuas to enormous bull mastiffs.

Brooks is attempting to do similar genomic studies of horses, drawing on a new genetic data set she's created from the DNA of 1,300 horses ranging in size from 29-inch tall mini horses to draft horses that are more than 6.5 feet tall at the shoulder.

Until that work is done, the molecular biological systems that make Einstein so little will remain a mystery.

"In a horse, we don't really know yet what the genetics are behind the size variation," said animal geneticist Rebecca Bellone of the University of Tampa.

We also don't know why people like them so much, but they do. Since pictures and video of Einstein were posted on the internet, Cantrell, an entertainment producer who lives part-time in Barnstead, New Hampshire, has been deluged with around 1,000 e-mails a day from fans and the media. The YouTube video of the horse has been viewed 750,000 times.

"I love to ride, but when I'm around a mini, it's more of a friend thing. It warms your heart," said owner Charlie Cantrell. "A lot of people use minis like medical assist dogs or companion animals in hospitals and I understand why they do."

And even if we did understand the genetics of these tiny horses, conventional breeding techniques practically require bringing some traits that humans don't want along with the ones that we do. Two of the best examples of this problem in breeding have to do with the special pigmentation patterns in dalmatians and Appaloosa horses.

Appaloosa horses' spotting is controlled by a set of genes called the leopard complex. When they get a certain combination of these genes from the parents, they get beautiful patterning, but some genetic variations associated with desirable spots also lead to night blindness in the horses. The spots dalmatians are famous for are associated with deafness in the dogs. A full 30 percent of the dalmatians in the United States are deaf.

"This is a standard problem. Animal breeders create as many problems as we solve," said animal scientist Tom Famula of the University of California, Davis. "When we push an animal in a direction that humans want, there's usually some side effect. To the animal that has evolved in balance, we're trying to move some trait out of balance to the others. We usually mess something up."

Brooks says that this kind of problem is part and parcel of trying to breed animals with certain tightly defined characteristics.

"The trick is you have two copies of every gene," Brooks said. "Sometimes a particular mutation with only one copy may be recognized as beneficial but later down the line if you have an individual with two copies, the recessive effect might actually produce a disease."

That's because every gene in a mammal's body isn't just being used for one thing. The protein it encodes is a multipurpose tool. "Each of these genes is multitasking. It's a pretty efficient way to do things," Brooks said. "Sometimes the negative effects are in some of these other things that a gene does. When we make a change in gene X, what are the other processes that it might affect?"

The same gene that helps control appaloosas' melanin expression in their coats also plays a role in their eyes. The situation is similar in miniaturizing horses. As they get smaller, the genes that limit the growth of their skeletons can cause negative impacts. Sometimes breeders attempting to create miniature horses end up with dwarfs that have malfunctioning jaws and legs. They can be crippled. While breeders obviously try to avoid that situation, it can be difficult for them to know which mating pairs will lead to negative outcomes.

The real limits then, may not be genetic, but cultural. At some point, it may become difficult to get healthy, tiny horses. To breed ever smaller horses, we might have to deal with a lot of deformed animals.

"It brings up this question of personal responsibility and what's humane and what's inhumane. That's something the tone of which will be set by public opinion," Brooks said. "I don't think we're at the point that we're breeding monsters. I don't think we're there but maybe we will be."

The way Cantrell sees it, avoiding the downsides of mini breeding comes down to the individual breeders. "It's just like with dogs: There are puppy mills, and there are mini mills," he noted. Einstein is more a lucky break than an attempt to breed the smallest horse. Both of his parents are champion 30-inch show horses. He just happened to come out lilliputian.

Cantrell argued that despite the difficulty of breeding the minis, it's worth it because they provide humans with a special feeling that standard horses cannot.

"When you get up close and personal to a miniature horse that is 30 inches tall, there is something about being able to touch him and walk with him in a very calm and safe manner, where you don't feel like you're going to be stomped to death," Cantrell said.

Via @**sf_sj and Cute Overload

Images and Video: Charlie Cantrell

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