Chapter 6 - Simple Genetic Principles

Dogs have been and are selected for a variety of features and traits. These can be classified into the broad groups shown in the table below.

Before examining these groups, the concept of heritability should be considered. The heritability of a trait is usually expressed as a percentage and measures the degree to which any superiority or inferiority in the parents can be passed on to the offspring. It is thus a predeterminant of the speed of genetic progress that is feasible. (see table)

In broad terms low heritabilites are values below 20 percent, medium are from 20-40 percent and high values are in excess of 40 percent with very few traits being above 70 percent. Heritabilities really apply to traits that are called polygenic, i.e., traits controlled by many genes. Each gene may have minimal influence but collectively genes can influence major characteristics. However, such traits are not usually 100 percent genetic - they are also influenced by environmental factors such as nutrition and exercise.

Although heritabilities can be expressed in mathematical terms, they can best be understood by a breeder in more practical terminology. If a character is 40 percent heritable then this means that 40 percent of any superiority of parents (over the average population) would be transmitted to the next generation. If, for example, wither height in a breed averaged 25 inches (64 cm) and the breeder decided to breed parents which together average 26 inches (67 cm) in wither height, then the parental superiority is 26 minus 25 inches (67 minus 64 cm), i.e., 1 inch (3 cm). If heritability was 40 percent then only 40 percent of 1 inch (3 cm) would be transmitted to the offspring which would thus measure just under 25 1/2 inches (65.2 cm).

Clearly this is a broad generalization and on small numbers things might not work out exactly as planned. But over larger numbers heritabilities are important in predicting the results of selection. Although widely known and used in farm livestock, few heritabilities have been calculated in dogs. Nevertheless the fact that dog breeders may be ignorant of such factors does not alter the genetic principles involved. Traits that have low heritabilities such as reproductive traits will be very difficult to select for and thus, however skillful the breeder, progress will be slow.

Anyone breeding only from bitches that have large litters would not necessarily see a rapid increase in litter size in the stock because the trait is of low heritability. In contrast, selecting stock that are tall in stature would lead to quite a rapid increase in wither height. A trait like fear is quite highly inherited. Dogs that are fearful are much more likely to bite than dogs with a stable character and breeders should be firm in selecting against animals of poor character. Conformational traits are those concerned with almost all aspects of conformation and all are traits controlled by many genes and are often moderately heritable. thus the alteration of certain aspects of shape can be achieved relatively quickly. This is of course equally true in selecting for poor as well as good construction.

Qualitative traits like coat color, coat type, ear carriage and so on are relatively simple in their mode of inheritance, with few genes involved in each feature. These traits are, in effect, Mendelian traits named after an Austrian monk, Gregor Mendel (1822-84), the founder of the modern science of genetics. Although Mendelian traits are often important in certain breeds, they tend to be features that are not crucial to the health and well being of the dog. In fact, they are largely esthetic and breeders or judges should not seek to place excessive influence on such easily changed features.

Anomalies or defects are traits that are not desired but are actually best avoided. Some of them, such as progressive retinal atrophy, are known to be Mendelian and controlled by a single gene. Others, including hip dysplasis and osteochondrosis dissecans (OCD - a faulty conversion of cartilage to bone that occurs in the shoulder and elbow at about four to five months of age), are polugenic and are thus controlled to a greater or lesser degree by inheritance, as well as being influenced by external features such as nutrition and exercise.

Breeders are perpetually seeking to improve fitness traits, enhance constructional features and produce good behavioral responses while at the same time hoping to reduce the incidence of inherited defects to minimal levels. Progress actually depends upon the degree to which it is selected for. If the trait is of high heritability then progress is potentially high, but it is important that breeders select the best possible stock from which to breed.

TYPE OF TRAIT	TYPICAL EXAMPLES	HERITABILITY
Fitness	Fertility, litter size viability, longevity	Low
Behavioral	Temperament, working potential, innate instinct	Low to Medium
Conformational	Most aspects of construction	Low to high
Qualitative	Coat color, coat type, eye color	Mendelian
Anomalities	Progressive retinal atrophy, hip dysplasia, hereditary cataract	Some are Mendelian, others are polygenic
CATEGORY	TRAIT	HERITABILITY (Percentage)
Reproductive traits	Fertility Gestation length Litter size	less than 15 40 less than 20
Conformational traits	Body length Body weight Rear pastern length Wither height	20-40 20-40 30-60 40-60
Behavioral traits	Fear Retrieve (young pup)	50 20
Anomalies	Elbow problems Hip dysplasia (varies with breed.scheme) Panosteiosis	40-60 25-55 20

The percentages given are only guides to possible percentages and may not apply in any specific breed or population. Nevertheless they are educated guesses based on such data as do exist and knowledge of the situation in other livestock.

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