Lethal Alleles
by C. Williams

Some alleles can cause some type of condition that is lethal. In the case of horses, there are several known lethal alleles. Most lethal alleles are only lethal when the animal is homozygous for the gene, leaving heterozygotes to be "carriers". Examples of lethal genes/alleles include Lethal White, and Roan.

To demonstrate how lethals work in a population, this discussion will revolve around the Roan gene, or locus. There are several patterns involving mixtures of white hairs mixed in the body color, but only one variety of this patterning is known to be lethal when the animal is homozygous.

This type of roan, is often called "Classic Roan", and may also be referred to as true roan or standard roan. Typically, the central portion of the body is mixed with random white hairs (the amount of white can vary) yet the points tend to be darker, and often the solid color the horse would otherwise be. Classic Roan is found in a number of breeds including Quarter Horses and Belgians.

To demonstrate how a lethal such as Roan works, we're going to use the following nomenclature:

• R = Roan patterning
• r = Non-Roan

(it should be noted, that Sponenberg uses the designations of Rn^Rn to denote roan, and R⁺ to denote non-roan coloration.)

Rr Chestnut Roan

rr Chestnut Non-Roan

Let's cross two heterozygous roan individuals:

Roan X Roan		Sire
		Allele #1 R	Allele #2 r
Dam	Allele #1 R	RR unknown	Rr Roan
	Allele #2 r	Rr Roan	rr Chestnut

Hey! What happened?

Well, despite what such probabilities might indicate... that perhaps a homozygous roan is also a roan, actual numbers of foals born from Roan x Roan matings show the above results.

• 1/3 Chestnut
• 2/3 Roan

Whenever numbers like this are present, it often indicates the presence of a lethal allele, as genetic percentages do not come in thirds. In the case of Roan, what should be the remaining 25% is unknown... such foals are never actually born. We don't really know what those foals would look like, because apparently a double dose of the R allele causes death to the embryo or fetus early-on.

Other lethals, are not so kind. One of these is a type of Lethal White associated with the Frame Overo pinto pattern locus. Such foals are born all or nearly white, but die within a few days due to a condition known as atresia coli... an incomplete lower colon which does not allow the foal to defecate. This can be particularly disheartening to breeders.

While it is thought that these particular alleles cause the lethal conditions, as well as the phenotypic color markers... it may not be quite that clear. Another possibility is a genetic mechanism known as Linkage.

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Views expressed herein are those of the writers and compilers of the various information. Reference sources are cited where applicable. Copyrights are the property of the respective authors.