What characteristic defines the agouti gene?

The agouti gene is primarily responsible for influencing color patterns in horses, specifically by regulating the distribution of black pigment. When we talk about "color extension restriction," we refer to the gene's ability to determine where black coloration is expressed on the horse's coat. For example, the agouti gene can restrict black color to the points of the horse (mane, tail, lower legs) while allowing for a chestnut base color to show more prominently on the body. This characteristic contributes significantly to the overall appearance and color variations seen in equines.

In contrast, the other options relate to different aspects of horse characteristics. The curly coat pertains to coat texture rather than color, height moderation refers to a horse's stature, which is not influenced by the agouti gene, and body structure concerns conformation and build rather than coloration. Therefore, "color extension restriction" accurately captures the defining aspect of the agouti gene and its role in equine color genetics.