Chestnut Morgan horses with lighter manes and tails, called "flaxen," are the subject of a research project seeking to unlock some of the mysteries of coat color inheritance.

The preliminary results from the Morgan study have produced some statistics, as well as tools for measuring flaxen coloring and creating pedigrees that include visual indicators of the trait.

Flaxen chestnut color inheritance

The two flaxen chestnut Morgan offspring above are among the progeny of the bay sire and black dam.

The genetic mechanism producing the flaxen trait is not yet well understood.

In the illustration at left, neither parent show the flaxen trait, yet they produced three very flaxen chestnut offspring, which are this mare's only offspring. The stallion produced nine other chestnuts with other mares, but none were flaxen. It is known that the flaxen trait is expressed only in horses with red (chestnut) body hair pigment, not black or bay body colors, but the trait may be hidden within the sire's and dam's genetic code and can be passed on to offspring.

One study tracked 17 flaxen sires and 38 flaxen dams. All of their 121 offspring were flaxen, except three foals sired by a stallion whose flaxen expression was anomalous, decreasing as he aged. This suggests that the flaxen trait in horses is the result of a homozygous recessive genotype, under the preliminary assumption of simple Mendelian inheritance.

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