The Leopard Gene in Horses (continued)

The pattern most people first think of in Appaloosa horses is the one that gave the gene its name—leopard. This pattern gives a white horse with round or oval spots of base color. There may be shading of the genetic base color on the flanks, behind the elbows or on the head.

Most people would call this horse a chestnut leopard. In fact he combines a white rump, extreme roaning or snowflake, and clearly defined spots. Note the haloes on several of the spots.

Genetically, a leopard must have at least one Pattern-1 allele in order to have most or all of the body white. In addition, it must have one leopard allele and one wild-type allele at the TRPM1 locus. Two leopard alleles will lead to a few-spot leopard, with only a few colored spots. Other factors leading to the leopard pattern undoubtedly exist, but are still unknown.

The mane and tail may be mixed in color if some of the mane and tail hair grow from colored spots. The spots may have roan edges, called haloes, which normally develop after birth. Blacks tend to have more and larger leopard spots than do chestnuts, with bay being intermediate. Also, horses with black mixed in the coat (sooty) will sometimes have the black and red colors form separate spots.

Three of the horses in Tourist Trap have leopard markings.

Another view of the same horse. Note the white “lightning strike” markings on the forelegs.

Token is the mare ridden by Flame. She is fairly tall—around 16 hands. She is a chestnut leopard, white with copper spots. Genetically, she is homozygous for the most recessive of the extension alleles, has two copies of the Pattern-1 allele and one of the leopard allele. She is wild-type at all dilution, pinto spotting, grey and roan loci. She could have genes for minor white marking on face or feet, but they cannot be seen.

Dusty is the gelding ridden by Timi, who would just as soon not be riding. He is the calmest and laziest of the group, and the easiest for a novice rider to handle. He is also the least responsive to leg pressure. Dusty is a buckskin leopard, around 15 hands tall. He has wild-type extension genes, bay alleles at the agouti locus, and one cream and one wild-type gene at the cream locus. His pattern-1 and leopard alleles are the same as Token’s. He has quite a lot of white in his mane and tail, so they are not noticeably sparse.

Penny is the guide and her horse, Freckles, is a bay leopard gelding. Freckles is a little keener than the horses assigned to Penny’s clients, but he’s a bit younger and the cross-country trip is part of his training. Freckles’s underlying bay color is a little sooty, so he has both red and black spots. Genetically he is the same as Dusty but with sooty and without the cream allele.

The other two horses have the leopard allele but are not leopards, and I’ll talk about them next time.

This was first posted, without photographs, April 2 2011.