Horse Genetics

1. Horse Genetics Equine Science & Technology

2. Horse Genetics Nature ordained that genetics be applied to horse breeding long before there were geneticists. Prior to the domestication of horses, there was natural selection for speed and stamina. One of the most important defense mechanisms of the horse was to outrun its enemies. • Natural selection was probably very effective in improving speed and endurance, since the slower horses were eliminated by predators.

3. Horse Genetics • Many horse breeders have been practicing genetics as they concern themselves with the are of breeding. Their guiding concept of heredity is that “like begets like”. • Breeders in the 18th century made a tremendous contribution pointing the way toward horse improvement before Mendel’s laws became known to the world. • As knowledge of genetics developed, there evolved an understanding of the science that underlies the art of horse breeding.

4. Horse Genetics Genetics • The bodies of animals are made up of millions or even billions of microscopic cells. • Each cell contains a nucleus in which there are a number of pairs of bundles called chromosomes. • In turn, the chromosomes carry pairs of minute particles called genes, which are the basic hereditary material. • The nucleus of each cell of a horse contains 32 pairs of chromosomes, or a total of 64.

5. Horse Genetics Genetics • There are thousands of pairs of genes on each chromosome. • Genes determine all the hereditary characteristics of animals from the body type to the color of the hair. • The mule has an uneven number of chromosomes and cannot reproduce. • The genotype of an animal refers to the specific genes it possesses on its chromosomes. • Genome describes the complete set of “instructions” for making an organism.

6. Horse Genetics Genetics • Dominance gene expression has a full allele effect. An example would be when a gray horse produces a gray horse. • Recessive character appears only when both members of a pair of alleles are alike. • The job of transmitting qualities from one generation to the next is performed by the gene cells- a sperm from the male and an ovum, or egg from the female.

7. Horse Genetics Genetics • There are alternate forms of each gene. The alternate gene forms are referred to as alleles. If the alleles are the same, the condition is referred to as homozygous. If the alleles are different, the condition is called heterozygous. • Selection and crossbreeding are the tools through which the horse breeder may obtain stallions and mares whose chromosomes and genes contain similar hereditary determiners-animals that are genetically more homozygous.

8. Horse Genetics Genetics • Gene changes are known as mutations. A mutation may be defined as a sudden variation that is later passed on through inheritance and that results from changes in a gene or genes.

9. Horse Genetics Simple Gene Inheritance (Qualitative Traits) • In the simplest type of inheritance only one pair of genes is involved. Thus, a pair of genes may be responsible for the color of body hair in horses. • The idea that certain basic colors may have a rather simple explanation of inheritance should not alter the fact that other genes may play an important role through their influence on basic schemes. • The possible gene combinations are governed by the laws of chance.

10. Horse Genetics Simple Gene Inheritance (Qualitative Traits) • Other possible examples of simple gene inheritance in horses might include eye color and the set of the ears on the head.

11. Horse Genetics Dominant and Recessive Alleles • A dominant allele may cover up a recessive allele. Therefore, a horse’s breeding performance cannot be recognized by its phenotype (how it looks). • Recessive genes can be passed on from generation to generation, appearing only when two animals, both of which carry the same recessive allele, happen to mate.

12. Horse Genetics Dominant and Recessive Alleles • Reputable breeders have an obligation, not only to themselves, but to their customers. Purebred animals must be purged of undesirable genes by eliminating those stallions and mares that are known to have transmitted the undesirable recessive gene.

13. Horse Genetics Incomplete Dominance • In some cases, dominance is neither complete nor absent, but incomplete or partial and expresses in a variety of ways. • Perhaps the best known case of this type in horses is the Palomino color. Genetic studies of the Palomino indicate that the color is probably unfixable- that it cannot be made true breeding, no matter how long or how persistent the effort.

14. Horse Genetics Multiple Gene Inheritance (Quantitative Traits) • Important characters such as speed are due to many genes; thus, they are called multiple-gene characters. • Quantitative inheritance refers to the degree to which a characteristic is inherited. • For example, all Thoroughbred horses can run and all inherit some ability to run, but it is the degree to which they inherit the ability that is important. • In quantitative inheritance, the extremes (either good or bad) tend to swing back to the average.

15. Horse Genetics Horse Color • Recent advances in genetics have allowed us to understand better how coat color is inherited, thus making it possible to plan matings that will produce foals of a certain color.

16. Horse Genetics Gene A • Genetic control of black color restricted to the points of a horse is through gene A, or the A allele. • The A allele is dominant. • A uniformly black horse has the recessive genotype aa.

17. Horse Genetics Gene E • Gene E, or the E allele, controls the expression of black hair on the body. • The recessive form ee, results in uniformly red-colored horses. • The E gene is related to (and can mask) the A gene.

18. Horse Genetics Gene G • Gray coloring is due to the dominant gene G. • Any horses with the G allele will be gray; non-gray horses will have the genotype gg. • Gray horses can be born any color.

19. Horse Genetics Gene W • Solid white coloring is controlled by gene W. • The dominant homozygous (WW) condition is lethal; thus, all white horses have the genotype. • All non-white horses have the genotype ww.

20. Horse Genetics Gene C and D • The C and D alleles are dilution alleles. Gene C affects only red pigment and causes the pigment to lighten. • The C gene does not affect black pigment. • Gene D differs from gene C in that it can dilute both black and red pigment. • With the D allele, black is diluted to a mouse or slate grey, and red is diluted to a yellow-tan. • Horses with the D allele also show primitive marks such as the dorsal stripe, shoulder stripes, and leg barring.

21. Horse Genetics Gene TO • The spotting of paint and pinto horses is controlled by the tobiano gene TO. • The tobiano color pattern is dominant; thus, overos must have the genotype toto. • Homozygous TOTO horses are rare and are sought after by horse breeders for their ability to always produce spotted foals. • Genetic tests are available to determine whether a horse is homozygous.

22. Horse Genetics Heritability of Performance Traits • Relatively little scientific work has been done on the heritability of performance of horses- on the genetics of working ability, racing ability, cutting ability, etc. • The horse is the last farm animal to which the science of genetics has been added to the art of breeding. • Differences in the performance ability (working, racing, jumping) are due to two major forces-heredity and environment.

23. Horse Genetics Heritability of Performance Traits • The important environmental factors in determining the overall performance of horses are nutrition (both prenatal and postnatal), health care, quality of training, ability of the handler, and injuries. • An important genetic principle is that traits as such are not inherited. The ability to respond to a given set of environmental conditions in order to produce a trait with a measurable effect is inherited. • Reliable estimates on the heritability of performance traits in horses are limited in comparison with those for other species.

24. Horse Genetics The following conditions would tend to indicate a hereditary defect in horses: • If the defect had previously been reported as hereditary in the same breed. • If it occurred more frequently within certain families or when there had been inbreeding. • If it occurred in more than one season and when different rations had been fed.

25. Horse Genetics The following conditions might be accepted as indications that the abnormality was due to a nutritional deficiency: • If it had previously been reliably reported to be due to a nutritional deficiency. • If it appeared to be restricted to a certain area. • If it occurred when the ration of the dam was known to be deficient. • If it disappeared when an improved ration was fed.