Flaxen Color Genetic Research in Progress

Understanding the genetics of horse color could be a gateway to a deeper understanding of the molecular biology of the horse. Worldwide collaborative efforts, such as the Horse Genome Project, seek to identify and understand the entire DNA sequence of the domestic horse. Discovering the mode of inheritance of observable traits such as color can offer useful clues that may help advance molecular research of equine genetics.

Genetic tests for the flaxen trait could be developed if the genetic basis for inheritance of this color pattern were better understood.
Having blond hairs in the mane or tail of chestnut horses creates a color pattern which is called the "flaxen" trait. The genetic basis for inheritance of this trait has not been well studied, and thus, remains elusive and perplexing. Looking back into the American Morgan Horse Association's archives, one can see that some of the early Morgans showed the flaxen trait, although the registry did not consistently record the trait. Recently the Morgan breed has experienced a surge in the popularity of a variety of colors, including flaxen chestnuts.

To make progress in understanding the inheritance of the flaxen trait in Morgan horses, we began collecting and studying photographs and pedigrees of flaxen Morgans in 2003. This research is facilitated by computer software and Internet-based communications with Morgan breeders, Morgan enthusiasts, and veterinary scientists. We aim to explain and predict the likelihood of producing the flaxen trait within Morgan breeding programs using pedigree information.

Ultimately, genetic tests for the flaxen trait could be developed if the genetic basis for inheritance of this color pattern were better understood.

Thus far, our data suggest that the flaxen trait is a heritable characteristic within Morgan horses. Among the chestnut Morgan horses currently included in our study we have found some predictability in inheritance of the flaxen trait: flaxen chestnut horses mated with flaxen chestnut horses consistently produce only flaxen chestnut progeny, and non-flaxen chestnut horses mated with non-flaxen chestnut horses consistently produce only non-flaxen chestnut progeny. Mixed parents (one flaxen chestnut and one non-flaxen chestnut) produce variable progeny, some flaxen and some non-flaxen.

Flaxen families

The expression of the trait among parents and offspring is highly variable, and the inheritance mode appears to be more complicated than a simple recessive when a flaxen mates with a non-flaxen or when non-chestnut horses are involved.

The mode of inheritance of the flaxen trait appears to be complex, and more research is needed.

As the example photographs show, two barely flaxen parents can sometimes produce very intensely flaxen offspring. In some horses, the manes and forelocks can have a lighter color than the tails of the same horses. Among the flaxen Morgans in our study, there is tremendous variability in the mane and tail color of flaxen chestnut horses, highlighting the importance of establishing a consistent operational definition of the flaxen trait. The flaxen trait can be expressed as a continuous variable ranging from barely flaxen in the mane and non-flaxen in the tail to very boldly flaxen in both the mane and the tail.

Flaxen Inheritance Patterns Unraveled

The genetics of the flaxen trait in chestnuts remains mostly unknown, even though the inheritance of the chestnut body color itself is well understood. Chestnut horses have two copies of the "Extension" Ee allele, and carriers of this gene can be established by a DNA test. The flaxen trait is apparently caused by a genetic mechanism which affects the red pigment produced by chestnut horses, but not the black pigment produced by black, bay, and brown horses.

Heritability of flaxen coloring

If flaxen were a simple recessive trait, breeding two non-flaxen chestnut carriers (top row) would be expected to produce one flaxen chestnut foal for every three non-flaxen foals (bottom row) in a large sample of such breeding pairs. But this pattern does not fit with studies tracking the flaxen trait. Researchers believe that the flaxen trait is not likely controlled by a single recessive gene within chestnut-bodied horses.

Thus, if one or both parents are non-chestnuts that carry but don't show the flaxen trait, the flaxen trait may be passed from parent to offspring but will not be visible unless the offspring has inherited the chestnut body color genes as well.

The flaxen trait in chestnut horses does not appear to be controlled by a single gene, even though some reports have been published stating that flaxen manes and tails are the result of a single recessive allele (reported as Ff for flaxen). The many shades of flaxen suggest that it is not simply a single gene trait which is present or absent, but more likely a polygenic trait (controlled by multiple genes simultaneously). A study in Italy by Vacchioto and colleagues in 1991 concerning the flaxen trait in 1,714 horses of the Haflinger breed concluded that "the flaxen character in the Haflinger does not appear to be due to a recessive gene, and the mode of inheritance appears complex." Similarly, our data suggest that the flaxen trait does not follow patterns of inheritance characteristic of a singlerecessive gene.

Alternatives to the recessive hypothesis are needed. Some possible explanations may include that the trait is polygenic, that flaxen is the result of a gene which acts to modify the body color, or that there are heritable non-genetic (epigenetic) controls of expression of the flaxen trait.

Ongoing Research

For our current research purposes, we define the flaxen trait as the presence of a lighter appearance in the mane and/or tail than the chestnut body color of an individual horse. For this first phase of our study, we have classified all horses as flaxen or non-flaxen based upon registry data, owner or breeder testimonials, and observations of photographs.

At this time, we have collected photographs from 459 horses, and classified these horses as flaxen chestnut, non-flaxen chestnut, or a body color that hides the flaxen trait (such as bay, brown, or black). A major source of data thus far has come from a single breeder, Susan Hanley of Lambert Morgans, and her Web master Steven Giberti.

However, numerous other valuable contributions have been made by other Morgan owners through email and a photosharing Web site. Counting the 459 horses, plus their progeny and ancestors, there are 1,322 unique horses in the dataset. Among these, we have 297 chestnut nuclear families consisting of a chestnut horse and both chestnut parents where all three have been classified as being either flaxen or nonflaxen.

We continue to work on increasing this sample size and verifying with photographs. Lineage and inheritance patterns are visualized in pedigrees enhanced to include siblings, matings and offspring, with color-coding to represent the flaxen trait.

Our own custom software produces these pedigrees and analyzes the data. We use online pedigree data from the American Morgan Horse Association registry and from the Web site allbreedpedigree.com.  

How You Can Participate

For the Morgan enthusiast, owner, or breeder, the observations from our research so far suggests that particular sire/dam pairs may have predictable probabilities of producing the flaxen trait in their offspring. However, more horses are needed to let us understand the underlying genetic mechanism well enough to make such predictions and to study the variability of the trait's intensity.

Currently, only horses registered with the American Morgan Horse Registry are eligible for inclusion in this study. Through collaboration among horse owners, breeders, and scientists, we are striving to soon unravel the mystery of the flaxen trait in Morgan horses.--Sonya Sokolow, PhD; Stanley M. Sokolow, DDS; and Susanne Sokolow, DVM, PhD

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