Heritability and Selection
How can two dogs with poor front assemblies breed together and have puppies with better fronts than their parents? Why is it that we can breed two tall dogs together and get small dogs? These questions and more are answered in the article below about heritability and selection for traits. Although this article is about canine genetics..it is actually written in English rather than the lingo of professional geneticists.
We can define heritability as how strongly inherited genes influence the final way a dog appears (phenotype) or conversely how strongly nutrition and other environmental factors affect the genes to influence the final phenotype or appearance.
Malcolm B Willis in his book "Genetics for Dog Breeders" suggests the following guide for defining the heritability of certain traits and the percentage that is influenced by genetics/outside factors**:
Fertility 10 - 15% heritability
Litter size 10 - 20%
Body length 40%
Chest Depth 50%
Hock Height 50%
Wither Height 40-60%
Temperament 30 to 50%
**The above are educated guesses and should not be taken literally and are just guidelines.
Let us use the example of litter size for heritability. Up to 85% to 90% of the size of the litter will be influenced by other factors such as nutrition and environment. A bitch may come from a long line of canines that consistently produce 8 puppies per litter, but environment/nutrition (up to 90%) will play a key role in allowing this genetic tendency to express itself (i.e. the phenotype).
The lower the heritability, the more impact other factors (i.e., nutrition and other environmental factors) have on the expression of these inherited genes.
Conversely, a trait with high heritibility (such as coat color) is strongly influenced by which genes the dog possesses. It's very easy to focus just on genes but always remember that the mature phenotype results from an interaction between environmental factors and inherited traits.
Nutrition can have a major environmental affect on the outcome of a dog's phenotypical appearance. Poor nutrition from overfeeding and/or poor quality foods can harm good breeding practices. It can determine the genetic potential not just for height and weight, but also various conformational features. If a breeder wants good shoulders he must breed specifically for this trait and then give proper nutrition to allow those features to be expressed.
Exercise is just as important. Handlers will often express opinions along the lines of.. "Exercise will help that "x" problem". Developing muscle tone is important so that the bones underneath have proper support. It is equally important to realize that too much exercise, especially in puppies, can be harmful. A balanced exercise program should be started after one year of age. No formal exercise should be given to dogs under this age.
As seen in previous articles, socialization is extremely important for the proper expression of inherited temperaments. Good temperaments have been ruined by inadequate socialization.
Heritability can also be defined as the proportion of parental superiority which is transmitted to offspring.For example, in a previous article the inheritance of height was discussed. This article showed that if an average population was 16" in height (which is quite tall for an IG) and only parents who were below this average at 14" (and therefore superior) were used for breeding only a proportion of their superiority would be inherited in their offspring. By actively selecting the 14" IGs and breeding only dogs who met that height requirement..we could eventually produce a population of 14 inchers. By active selection we could increase or decrease height in succeeding generations thus changing the average for a breed.
As breeders, we do not usually select for a single trait. For instance, we would not breed for beautiful heads alone. Some features seem to be linked together, so in selecting for one, we may increase the likelihood of another. For instance, in selecting for decreased size, we may see a decrease in weight. However, there have been no formal studies on how many traits in dogs are linked together, if any at all. When using selection methods to evaluate what a breeder will or will not breed from/to, it is important to keep it basic and not select for too many characteristics. The more characteristics selected, the more difficult it is to find all of them in "one" dog. Breeders should keep selection simple and confine themselves only to those traits which are high in hertibility. For example, selecting purely on litter size would be slow progress because of the outside influences that directly affect this inherited trait.
Some breeders like to use a tandem method. The breeder may select his initial breeding on beauty alone and once locked in begin selecting for proper fronts, and then begin to breed for proper tail sets. The problem with such a method is two-fold. If the selection criteria changes in mid stream, the breeder potentially begins to loose the first selection that was made as subsequent breedings take place. Too, such types of methods can be time consuming and take years to accomplish.
The best method for selection is setting minimum standards for each trait that is considered important. There is a danger in setting too high a standard during the selection process. As a result some very good genes may be excluded from the breeding pool over something as minor as perhaps eye color. Just because a dog may have very little to criticize does not mean that such a dog is better than the dog who may excel in areas such as proper shoulder placement but is lacking in proper eye color. The obvious choice is to use the dog who is above the breed average in traits which the breed needs in order to improve itself even if that same dog has some minor failings in areas that are easy to improve. (note: although a breeder typically would not breed two such animals together who display the same areas of weakness).
In conclusion, the criteria used in the selection process must be thought out beforehand and written down. Pedigrees, performance tests, titles, progeny results, and family history are great tools to assist in the selection process.
Genetics of the Dog, Malcolm B. Willis
Practical Genetics for Dog Breeders, Malcolm B Willis
Inheritance of Color in Dogs, C. Little
Control of Canine Genetic Diseases, George A. Padgett, DVM