Leopard complex (LP) appaloosa color pattern Base color any any Modified with LPLP, LPlp lplp Result appaloosa no change The appaloosa color pattern is a combination of spotting and/or diffuse roaning, which usually is fairly symmetrical on the horse’s body.
Appaloosa color patterns are found worldwide in various breeds of horses and ponies.
Until recently, the inheritance of the pattern was unknown, but gene mapping of the horse genome has greatly increased the understanding of this color pattern.
The gene responsible for the appaloosa pattern is termed the leopard complex (LP).
The “complex” portion of the term is to include all types of appaloosa patterns that are not leopard.
It is believed that LP functions as a dominant gene to cause the appaloosa coat pattern, along with other appaloosa characteristics such as mottled skin, white sclera, and vertically striped hooves.
To have the color pattern, the horse must have an LP gene.
However, modifying genes may influence the expression of the LP gene; therefore, a horse that has minimal white modifying genes may not exhibit the LP gene.
To further confuse the issue, many researchers have hypothesized that the LP gene may act as an incomplete dominant gene.
That is, a horse that inherits an LP from both parents (LPLP) will exhibit more white than the heterozygous horse (LPlp), which only receives the dominant form of the gene from one parent.
Horses that are homozygous dominant are few-spot leopards, while heterozygous horses are leopard, blanket, varnish roan, snowflake, and frosted patterns.
In this publication, the genes that control color are discussed individually, and prominent breeds exhibiting a particular color pattern are given.
However, it is important to realize that all horses have a pair of genes for every color gene discussed and many that are not discussed.
For example, a chestnut horse would have the color genotype of ee dd CC gg ww zz toto oo lplp rnrn.
Genetic tests for some color genes exist.
For example, a test can be done to determine whether an animal is homozygous or heterozygous for the black gene. effect of each gene is small.
At the same time, the environmental effects are high for quantitative traits.
This combination tends to blur the distinction between phenotypic classes.
One horse may look better than its genetic makeup would indicate because it has been in a good environment, while another may look worse than its genetic makeup would indicate because it has been in a bad environment.
As a result, when selecting breeding animals, we may choose a horse with a good phenotype due to a superior environment which, in reality, may or may not have the genotype we desire.
Similarly, we may pass up a horse because its phenotype is undesirable due to a poor environment, while its genotype actually may be highly desirable for a specific breeding goal.
In other words, the best performers may not always have the most desirable genes, and environmental factors can enhance or mask genetic effects.
Unfortunately, many of our economically important traits for horses are quantitative traits.
Performance traits, such as racing speed, jumping ability, and cutting ability, are all quantitative traits, and evaluation of performance usually has a combination of objective (such as speed and jump height) and subjective (such as conformation and disposition) measurements.
Animal breeders have a variety of formulas for predicting genetic change through breeding.
These formulas take into account a variety of factors that influence genetic change.
A simple formula is as follows: yearly genetic change = heritability × selection differential generation interval The Quandary with Quantitative Traits Quantitative traits often result from additive gene action.
That is, many genes affect the trait, and the Some horse traits have been investigated to the extent that scientists have estimated the heritability of that trait.
A highly heritable trait generally means that horse breeders can make more accurate selection decisions about that trait and can more rapidly influence the amount of genetic change with their decisions.
Heritability estimates for various horse performance traits range from 0.04 (cow sense) to 0.63 (wither height).
Different populations of horses may show different heritability estimates for the same trait because their genetic and environmental backgrounds are different.
For example, if the population studied is an elite group of horses performing at the top level of competition, these horses probably would be more similar in environment and genetics than a group of horses performing at the local or regional level of the same sport.
To use an example from color genetics, if all horses in a population are chestnut, there is little chance of producing a black.
Whenever possible, addi- 6 Alabama Cooperative Extension System tional information from relatives, weighted properly according to each relative’s relationship to the animal being selected, will increase the selection accuracy regardless of the heritability for that trait.
The selection differential is the difference between the selected individual and the population average.
Breeders who cannot predict (many economically important traits may not be measurable until the animal matures) or recognize superior animals probably will not make a lot of genetic change or may end up with the wrong type of change for their goals.
Also, breeders who have only a limited number of horses to utilize in their breeding program are at a disadvantage for this factor.
More intense selection, through retaining only the highest quality animals, allows breeders to increase the mean genetic level of their herd.
Breeders with a limited number of breeding animals often do not want to intensely cull animals because it may reduce their herd size to a number that is not economically viable.
However, producing animals of limited usefulness to the horse industry probably is not economically viable either.
The long generation interval of horses also reduces the rate of genetic change in horse breeding.
The generation interval is the average time from the birth of one generation to the birth of the next, and in horses the generally accepted generation interval is about 10 years.
Genetic change can occur more rapidly with shorter generation intervals.
The long generation interval also means that the average horse breeder has very few opportunities to influence genetic change in horses in his or her lifetime.
Poor selection decisions or constantly changing breeding goals will negatively influence the amount of genetic change a breeder can realize.
Also, when the long generation interval is combined with poor reproductive efficiency (only about 50 percent of mares bred produce a foal), this factor can greatly retard genetic change.
Increasing the reproductive efficiency can decrease the generation interval and increase selection intensity in a herd, resulting in more rapid genetic change.
Finally, the very nature of the horse industry can influence the amount of genetic change made by individual breeders.
Historically, many desirable stallions were accessible to only a few breeders due to factors such as high cost and geographical location.
This did not retard the genetic change of the breed, but it did slow the genetic change realized by the average breeder.
With the increased use of shipped semen in the horse industry, this problem is beginning to be alleviated in most breeds.
Through the use of artificial insemination, selection intensity from the male side can be greatly improved. Evaluating Individuals Breeding stock selection is usually based on a variety of individual preferences of the breeder.
Some breeders put more weight on pedigree, some on performance of the horse or its relatives, and some on appearance of the horse.
Are some of these considerations more important than others? It depends on the trait in question.
Highly heritable traits (0.4 or higher heritability) respond well to selection procedures based on individual performance, assuming that environmental effects are minimized.
Individual performance is also a useful selection criterion for economically important traits and when genetic turnover is fast.
Traits with low heritabilities (less than 0.2) respond well to selection based on family background (pedigree, performance of relatives) because the individual’s own performance is not an accurate assessment of the animal’s genetic merit.
Pedigree also can be a useful selection tool for a young horse that has not had a chance to prove itself or for a horse that was injured prior to proving itself.
Pedigree also may be a useful tool when dealing with genetic abnormalities, traits expressed later in life, or traits expressed by only one sex.
However, breeders should realize that the pedigree decreases in value in the selection process as the individual animal gains performance and progeny records.
In addition, breeders should remember that the individual horse will not have any genes that its parents did not have.
So, when utilizing the pedigree as a selection tool, breeders should emphasize the individual’s parents and grandparents.
The genetic contribution of more distant relatives such as great grandparents is very minor.
Performance tests, in which horses are put into a common environment for a period of time before traits are measured and compared, and progeny tests, in which a stallion is bred to number of mares to evaluate his offspring relative to those of other stallions for certain traits, have been used as selection tools.
Other livestock species commonly evaluate potential breeding animals using this scheme.
The performance test commonly is used as a selection tool for young stallions of warmblood breeds.
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