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Candidate gene for the mealy pattern

September 3, 2020 0 Comments

A potential regulatory region near the EDN3 gene may control both harness racing performance and coat color variation in horses. Kim Jäderkvist Fegraeus, Brandon D Velie, Jeanette Axelsson, et al. May 2018. Open access.

I wanted to start with this paper because while it was published two years ago, it provides an interesting look at how genetic research is done. In particular, it shows that science is a process. Understanding comes gradually, and papers are often published that do not give final, absolute answers. Instead, sometimes they just provide the next set of clues.

The other thing that is cool about this paper is that it illustrates another important aspect of research. Sometimes what you start out looking to find, and what the results give you, are different things. The possibility of unexpected outcomes is one of the best parts of research!

What Researchers Were Hoping to Find

Geneticists have been working to identify the genes that contribute to racing success. For this study, the researchers had a clever approach. They took a more recently developed racing breed, the Coldblooded Trotter (pictured above), and compared it to the draft breed used to create it, the North-Swedish, and a breed selected for the same purpose, the Standardbred. The idea was that areas that changed in the original population, and shared by the (mostly) unrelated racing breed, might influence racing ability.

What Was Found

One region that changed was positively correlated with racing success. That area contains the gene EDN3, which is known to have a role in pigmentation. This was an interesting discovery because as the Trotters have become faster and more intentionally bred for racing, they have also lost the mealy coloring common in the North-Swedish. This raises the possibility that the EDN3 gene may play a role in the mealy pattern in addition to influencing racing ability.

To investigate this possibility, 1,915 horses from 18 different breeds were checked for the two EDN3 alleles. I found this part of the paper the most interesting. If I had been asked to give a ballpark estimate of the mealy phenotype’s frequency in the tested breeds, I would have given very similar numbers. I have noticed the near absence of mealy patterning in the racing breeds, but I assumed this was intentionally bred out of them. In the early 1900s, faded or “washy” colors were considered a fault by many equestrian writers. The mealy pattern is also notably rare in many of the gaited breeds of the American southeast. If there was a negative correlation between the mealy pattern and some aspect of harness racing performance, that would make sense given the background and influence of the Morgan and the Standardbred.

Just a Candidate

It is important to remember that this paper reflects what the earliest stages of horse color research looks like. The allele at END3 is a candidate for control of the mealy pattern. Candidate genes do not always pan out. Sometimes they are just another clue in the process of identifying the responsible gene. In the case of this particular candidate gene, there are some caveats. That will be the topic of the next post.

In the meantime, the link to the original study is at the top of this post. With future posts about published papers, I will place a similar highlighted box at the start of the discussion. I will also use these boxes for notes about unfamiliar terms or symbols that might make reading the paper more difficult. The first of those – dealing with the delta symbol – can be found below.

It is rare to see a bay dun Fjord without the fully mealy pattern. 

In the linked paper, you will see quite a few references to ΔFst. This is short for the “delta fixation index.” Delta, which is represented by the triangle, just means change. The fixation index (usually written FST) is a way of assessing how the allele frequencies differ between two populations. So this refers to the change in how the two populations differed. For this study, a high ΔFst meant that gene may have changed as the Coldblooded Trotters were selected for racing ability.

By lkathman

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