From horses to mice, animal inbreeding has shaped human existence

The thoroughbred

The world of horse racing is one of royal fame and fortune. Throughout history, the sport has been popular with royalty worldwide, and a good horse is worth its weight in gold. Frankel is no exception. Bred by a Saudi Arabian prince, Khalid Abdullah, Frankel was an undefeated champion in his racing days, earning nearly £3m for his owner. But if you wanted to buy Frankel, you would have to pay over 30 times that amount. Frankel’s actual worth was around £100m, but why would anyone pay so much? Because his DNA^? could help horses win races for generations to come. Frankel’s racing career was stopped short after 14 unbeaten races so he could focus his efforts on fatherhood. Horse-breeders world-wide have paid over £100,000 to mate their mares (female horses) with Frankel, in the hope that they too could produce the next champion.

Frankel over 16 metres ahead of the other competitors as he won The Juddmote International Stakes at York in 2012. Image credit: Racingfotos.com

But this story didn’t start with Frankel. Frankel’s family tree is like a royal horse bloodline, one that’s been carefully recorded for hundreds of years. He’s a member of the thoroughbred breed, an extended family that can all be traced back to three stallions (male horses) and at least 12 mares (female horses) bred in England over 200 years ago. The three original stallions were imported from the Middle East between 1680 and 1740. They were Byerly Turk, a horse captured in Hungary by a British army captain who was impressed by the horse’s abilities in battle; Darley Arabian, who was purchased by a member of Queen Anne’s council; and Godolphin Arabian, an unwanted gift for the king of France who fell into the hands of the British Earl of Godolphin. The mares were of various backgrounds, predominantly English. Breeders kept a written record of all horses that were bred from these ‘royal’ predecessors. This record has been maintained to this day and is called “The General Stud Book”. Horses can only be claimed to be thoroughbred if they are bred from other horses in The General Stud Book.

Frankel’s ancestry is a historical example of selective breeding^?. This is where humans carefully choose animals or plants with particular characteristics to mate together in the hope that the offspring will display the desirable characteristics. In the case of the thoroughbred, the historical principle has been to “breed the best to the best and hope for the best”. For modern horses, the best simply means the best at winning races and today, this is how parents are chosen.

A painting of Darley Arabian (b.1700) by an unknown early eighteenth-century artist. This horse was one of the fathers of the thoroughbred race horse. Image credit: The National Heritage Centre for Horseracing & Sporting Art, copyright Trustees of G W Darley

Breeding relatives

It’s unlikely that all of Frankel’s descendant foals will be as successful as Frankel. Although you increase your chances of having a champion racer if you breed from champion racers, some of the foals might still lose the genetic ^?lottery, and they won’t inherit the characteristics behind their parent’s success.  If you’ve paid £100,000 for a champion parent, that’s a big risk to take, but there is a way to increase your odds. Thoroughbreds are all distantly related, but by breeding horses more closely related, you can increase your chances of getting the genes^? you want. Frankel’s parents are both recent descendants of another champion horse, Northern Dancer, who won 14 out of 18 races. Similarly, Frankel’s first born son, Cunco, was born to Chrysanthemum, another descendant of Northern Dancer and Frankel’s cousin. A horse is described as inbred^? when its parents are related to each other. By inbreeding the descendants of Northern Dancer, the foals are more likely to inherit Northern Dancer’s desirable racing genes, but this still doesn’t guarantee a champion. Although Cunco was bred to succeed, he only won one of the seven races he started.

Cunco’s family tree, showing how he was an inbred descendant of Northern Dancer. Image credit: Genome Research Limited

Breeders can take inbreeding to extremes. If first-degree relatives, like siblings, are bred together over several generations, eventually the offspring will be genetically identical clones^? of their parents. These clones will give birth to more clones when bred together. Breeders call this ‘pure breeding’ or ‘breeding true’. When you know that the offspring will be just like the parents, there’s no genetic lottery to lose, but this is not without its consequences. Although inbreeding makes it easier to predict what characteristics will be passed on, heavily inbred animals are more likely to inherit negative genetic characteristics as well as positive ones from their parents. Although thoroughbred horses are a result of inbreeding, they are not actually very purebred at all. Genetically, they are reasonably different and therefore do not breed true.

The principles of inbreeding have been applied to many other animals, for many different reasons. Domestic dog breeds are inbred enough to always look the same. They can be said to breed true for certain characteristics, although they are not clones. Farm animals are also inbred for predictable physical characteristics, such as meat or milk production. There is one animal, however, where breeding true is more important than any other: a mouse by the name of ‘C57Black6’, or C57BL/6 for short.

Breeding clones

Abbie Lathrop was an ex teacher who in the early 1900s decided to buy a pair of ‘waltzing’ mice, a type of mouse that has a strange behaviour of whirling around in circles, rapidly and constantly. She sold these mice as pets, and then did the same with another type of mouse called ‘fancy’ mice, which were bred for their appearance and temperament. The collection of mice on Abbie’s farm grew to about 11,000 mice and she kept careful records of every mouse. Scientific researchers then began to buy her mice to use for medical research. She even worked with scientists to study the mice and co-authored her own scientific papers in prestigious journals like the Journal of Cancer Research.

But why did scientists want to study mice? Mice are very similar to humans and can be bred rapidly in a controlled way, which isn’t as easy with other mammals. For almost every gene in a human, there is an equivalent gene in the mouse. By studying the mouse we can therefore find out more about the biology of mammals, including humans. For example, Abbie noticed that some families of mice on her farm developed the mouse equivalent of breast cancer^? more than others, which also happens in some human families. When she removed the ovaries from these mice, it reduced the likelihood of them developing breast cancer. Later studies in humans have shown that removing the ovaries of women who are at a very high risk of breast cancer can also significantly reduce their risk of developing the disease.

Clarence Little, an ex-army major and birth-control activist used mice from Abbie’s farm to study genetics and cancer. In 1921, Clarence began inbreeding these mice over many generations. If the colony of mice died he would start again and if unhealthy mice were born he would exclude them from further breeding. He persisted with the inbreeding until he got a mouse that was robust, and could withstand being inbred. Eventually, he succeeded when he bred a female mouse 57 with a male mouse 52. The resulting colony, ‘colony 6’, produced robust, pure-breeding mice with black fur. This colony continues today and has been invaluable in helping us to understand human genetics and disease. We now know it as the C57Black6 mouse.