Breeding for Proper Size

 


There is an old wives tale which suggests: "To reduce size... inbreed".

Although inbreeding can slightly reduce height... studies have shown that the levels on inbreeding necessary to noticeably reduce height usually have a devastating effect on other genetic traits (such as fertility, litter size and health).

Any experienced breeder can relate the tale of breeding two IG's together and producing a Heinz57 variety in litter height. Sometimes to our amazement breeding two large IGs will produce small IGs! We are not alone in this. Anyone who breeds any type of canine in which size is an issue will report similar findings.

It would be natural to assume that breeding two IG's of proper size would result in a litter of proper size puppies. We become frustrated when less than desirable outcomes occur. So as breeders how can we stop the trend of height variance? If every breeder decided to select ONLY dogs of correct height for breeding purposes would we begin to see a better result...that is... a reduction in height.

 

HOW IS SIZE INHERITED?

Size is a polygenetic trait (controlled by the total effect of many different genes). Polygenetic traits are also known as "continuous characters" meaning that they vary continuously from one extreme to the other. In contrast ...Mendelian traits, like coat color, could be considered "categorical traits." This means that colors such as fawn, blue (gray), etc. can be classified into a certain category.

Polygenetic traits can be additive or non-additive.

Additive polygenes have a cumulative effect. This refers to genes simply adding up to increase the expression of a trait .... such as making a dog taller. Additive traits can have a low hertibility, such as litter size or birth weights. Or, additive traits can have a high hertibility, such as height or growth rate. It is important to remember that many traits with low heritability are still under considerable genetic control. That is, Litter size IS highly genetic but it is NOT highly heritable.

For non-additive polygenes it is the combination of genes that makes the difference. This means that different sets of genes can combine in different ways to produce dogs who appear identical with regard to a given trait. Furthermore, a dog who is superior in any one respect may owe that superiority to his particular COMBINATION of genes more than to the specific genes themselves. Unfortunately, such outstanding animals are unable to pass on their own particular gene combinations; they simply pass on a sample of the individual genes that make up those combinations. Those genes may combine quite differently in their offspring producing very different results.

The more additive (or heritable) a trait is, the more predictably it will inherit. For the remainder of this article only additive polygenes will be discussed.

Heritability or Additive Polygenes

Heritability can be defined as the proportion of parental superiority compared to the population average for a given trait, that can be transmitted to offspring.

The heritability of a polygenetic trait can range from 0 percent to 100 percent. A purely additive polygenetic trait would have 100 percent heritability. A completely non-additive would have 0 percent heritability. The heritability of most polygenetic traits falls somewhere between these two extremes.

In genetic terms, low heritability would be defined as a value of 20 percent or less; moderate heritability would be in the range of 30 to 50 percent; and any value above 50 percent would be considered high. Only traits with moderate to high heritability will successfully respond to direct selection.

 

Inheritance of Height

Size is of moderate to high heritability 40 to 65 percent of the time. Direct selection for size should be relatively successful.

For discussion purposes let us suppose the average height in IG's to be 16". The decision has been made to breed only from a group of parents that have an average height of 14" (which is the more desirable size). Although our group of parents average two inches smaller than the breed average, the heritability value of 60% means that only 60% of this superiority can be transmitted to the next generation. This means that 60% of the 2 inches (1.20 inches) is transmitted to the next generation. Thus, the offspring of these selected parents would have an average height of approximately 15 ╝". For breeders who really care about improving height this can be frustrating ... as the puppies' heights will not be as desirable as the parents. It's important to note that if every breeder used the same selection criteria (breeding dogs only of correct height), the breed average for the next generation would start at 15 ╝" instead of 16". But unfortunately, as we know, agreement on such a large scale rarely occurs.... hence progress to overall improvement in height can be slow.

 

Predicting Height

How as breeders can we accurately predict height in our youngsters? If height were purely a matter of additive polygenes then we would be able to select two small IG's and produce immediate and desirable results. Size is a type of hereditary trait that geneticists refer to as a "continuous character" meaning that it varies smoothly or continuously from one extreme to another. As mentioned previously, height has a heritability trait of approximately 60%. The non-additive portion of size variation depends largely on gene combinations. Since a dog can't pass on specific gene combinations but only a sample of the genes it is important to make an educated guess about the kind of size these genes may actually possess. The size of littermates, parents, grandparents and other close relatives can provide useful clues. For this reason having knowledge of the bloodline in regard to height is definitely helpful.

Selecting for Height

Polygenetic traits are distributed among a population in a normal distribution sometimes called a "bell curve" -- with most members of the population falling somewhere in the middle of the distribution, on or around the "mean" or "average." As movement occurs from the mean to either extreme there are fewer and fewer members of the population to be found. This means that most individuals will be found around the average height for the breed (whatever that may be) with fewer at either extreme (i.e. very, very tall or very, very small). For Italian Greyhounds the average height has been found to be 15 to 16 inches.

The range of heights from which breeders decide to breed determine the distribution of heights that will be the starting point of the next generation. Polygenetic traits are not as easy to select as Mendelian traits. However, careful selection by a majority of breeders can move the average height lower or higher with each succeeding generation.

At the very minimum, direct selection for size would mean breeding only from in-size dogs with no rationalizations for doing otherwise. Even stricter selection for size might mean eliminating from breeding programs dogs and bitches with a preponderance of over-size relatives.

Which dog seems the better of the two sires to use - The dog who is 14" who was the ONLY one in his litter to NOT go oversize or the 14" who was the biggest in his litter?

The old arguments of needing large IG's to keep beautiful heads, bodies, structure, etc. are now passÚ. Ten years ago this may have been a partial solution to the problem. But with yesterday's breeders selecting better heads, good bone, etc. the breed has improved to the point where beautiful heads, bodies etc. CAN be handled within the breed standard height!

The key is for today's breeders to refine and redirect the wonderful work of the past into making the IG closer to standard. Just as size is controlled by polygenetic traits, other traits, such as head form, chest, muzzle length, etc. are controlled by these polygenetic traits. The heritability of such traits are anywhere from 35 to 50%. Keeping proper heads, chest, muzzle length, etc. will still continue if actively selected along with proper height.

Breeding Decisions

A small male can be more useful in the effort to produce "proper size" than a small bitch. A male can produce more offspring than a female thus getting more of those "small" polygenes into the gene pool. Perhaps more importantly, a small male is more likely to have *"smaller"* polygenes than a bitch of the same size. This is because size, like all polygenetic traits, is susceptible to environmental influences such as nutrition and gender. In practical terms this might mean that a 14-1/2" dog (in terms of his size genes) is roughly equivalent to a 14" bitch. And it works the other way, too... an otherwise "nice" 16-1/2" bitch is the genetic equivalent of a 17" DOG.

Does this mean that ALL oversize dogs should be thrown out of the breeding pool? If a breeder is actively looking to produce proper size then active selection away from large size will produce those results. As a breeding program evolves, selection for proper size should be considered. If one starts off with larger than desired IG's, then the next succeeding generations should be selected for proper size. This means letting those dogs who exhibit large size go on to live life in a wonderful pet home.

It is always difficult to make the tough breeding decisions and it's never easy to let a beautiful dog be passed over just on the basis of size. However, there are plenty of beautiful Italian Greyhounds of proper size now available for breeding purposes. As breeders begin to select for more proper size the result will be less either overly large or overly small IG's produced. The breed as a whole will benefit as a result of this practice.