Two Horses, One Body?

Sounds a bit like something completely made up or from a sci-fi programme but it really can happen and the results are not only unusual but also quite stunning! Not quite the 2 headed beast that I'm sure some of you are imaging :D

Hormones normally causes a mare to release a single egg which in turn is fertilised by a single sperm. Sometimes a mare releases 2 eggs and both are then fertilised by the stallions sperm and this in turn causes 'twinning'. However, rarely, these fraternal twins become fused and start forming as one embryo (as opposed to forming individually). This fusion and development of one individual from 2 embryos is called Chimerism. 

Chimerism is a rare disorder and is usually detected using a DNA test. The individual can appear completely normal from the outside but internally they have 2 separate set of DNA (hence genetically being 2 horses within one body!). This means they could have kidneys from one set of DNA and a heart from the other types. In some cases, they may also develop hermaphroditic characteristics, meaning they have both male and female sex organs.

Chimerism is not to be confused with hybridization. Hybridization is mixing the egg from one species with the sperm of another. This process is far more common then chimerism and is the reason we have animals such as Ligers, Zonkeys, Mules, Wolfins and my personal favourite Leprons.

The most recognisable external features of equine chimeras is an unusual and somewhat unique coat pattern. As the equine embryos are developing they start to form their own coat pattern so when the embryos fuse the coat pattern also combines and becomes a mixture of the two. It is simply stunning.

Chimerism has been thought to be related to the brindle pattern of horses although its not yet been scientifically proven. It is also thought to be the reason behind male calico cats. The Calico colouring is a sex linked trait which causes pretty much all calico cats to be female. The same explanation can be used to female ginger cats, again these are typically male sex linked :)

Either way, Chimera horses are lovely looking animals! I certainly wouldn't say no to owning one :)

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