Understanding the genetic contributions of ancestors within a pedigree is crucial for effective breeding programs. The concept of “percent of blood” offers a quantifiable way to estimate the genetic influence of a common ancestor on an individual horse. This metric helps breeders make informed decisions about perpetuating desirable traits and introducing genetic diversity.
Calculating Percent of Blood
The percentage of blood an individual has from a common ancestor is determined by the number of times that ancestor appears in the pedigree and the generation in which they appear. Each appearance contributes a specific percentage, which is then summed up.
- First generation (parents): 50.00 percent contribution
- Second generation: 25.00 percent contribution
- Third generation: 12.50 percent contribution
- Fourth generation: 6.25 percent contribution
- Fifth generation: 3.125 percent contribution
- Sixth generation: 1.5625 percent contribution
- Seventh generation: 0.78125 percent contribution
- Eighth generation: 0.390625 percent contribution
For example, if a particular ancestor appears twice in the second generation of a pedigree, the individual would have 25.00% + 25.00% = 50.00% of that ancestor’s blood.
Percent of Blood as an Estimate
It is important to recognize that “percent of blood” is an estimation rather than an absolute number. Genetic recombination and mutations can lead to genes changing places or altering on the chromosome. Therefore, while an ancestor’s genetic material is passed on, it may not always be in the exact form expressed by that ancestor. Percent of blood represents the maximum potential genetic contribution from a common ancestor.
Types of Pedigree Crosses
Breeders utilize different crossing strategies based on genetic relatedness:
- Outcross: This involves breeding two unrelated individuals within the same breed.
- Line cross: A specific type of outcross where two distinct families within a breed are bred. An example would be breeding a mare from a family line-bred to “King” with a stallion from a family line-bred to “Wimpy.”
Setting Type and the Role of Inbreeding
Percent of blood serves as a tool for breeders to monitor the percentage of genes from a common ancestor within a family of horses, allowing for the perpetuation of those genes across generations. This is particularly relevant when using inbreeding and line breeding to establish specific traits or “type.”
- Homozygous gene pairs: These pairs carry two identical genes for a particular trait, ensuring that an individual with homozygous gene pairs will consistently pass that trait to its offspring. Inbreeding aims to increase the number of homozygous gene pairs.
- Inbreeding: This practice concentrates both desirable and undesirable traits from an ancestor.
- Line breeding: Breeders often progress from close inbreeding to line breeding. While all line breeding is a form of inbreeding, the distinction lies in the generational distance of the common ancestor. Generally, if the common ancestor appears in the third generation or further back, it’s considered line breeding. Ancestors appearing in the first or second generation are typically classified as inbreeding. However, definitions can vary across breed registries.
Line breeding aims to maintain a high percentage of genes from a specific individual without necessarily focusing as intensely on homozygosity as close inbreeding.
The Importance of Numbers and Culling
Historical breeding programs, such as those at the King Ranch with “Old Sorrel” and Hank Wiescamp’s program, highlight the significance of having large numbers of individuals when implementing inbreeding and line breeding. These programs were successful not only due to the breeding strategies but also because of a rigorous culling process, where undesirable individuals were removed to ensure only the best were carried forward. This ability to maintain and then eliminate horses based on performance and traits is often difficult for modern breeders who may not have the resources for hundreds of animals.
The Ultimate Goal: Hybrid Vigor
While inbreeding and line breeding are methods to refine traits within a line, the ultimate goal for true genetic improvement is often achieved through introducing hybrid vigor via an outcross. Introducing outside blood at strategic times can provide a significant boost in genetic diversity, leading to the development of superior individuals. Successful breeders like Hank Wiescamp were adept at introducing outside bloodlines at critical junctures. For instance, the introduction of “Mr San Peppy” into the King Ranch program provided the necessary genetic diversity to overcome a plateau reached with their line-bred mares. Line breeding can eventually lead to a point where outcrossing is necessary for continued improvement.
Percent of Blood: Just One Tool
It is paramount for breeders to never rely solely on percent of blood or pedigree analysis. Conformation, performance, and disposition are equally, if not more, important factors in selecting breeding stock. Percent of blood and pedigree are valuable tools, but they are just one component of a comprehensive breeding strategy.
