Population genetics (2)
The characteristics we have discussed so far were those that had clearly distinguished classes; no endless values in between. A dog was brown or black, but could not have one of many shades in between those values. Other properties are quantitative; there is an endless number of possible values. A few examples: milk production, weight, age of first heat. A number of these properties are more or less hereditary. So, selection will influence the values of those characteristics.
These quantitative properties are often determined by a large number of genes. We call them polygenous. The alleles of these genes and the genes show mutually the same interactions as we have seen before in Interaction between alleles and Interaction between genes. We only can't detect the individual interactions. Another factor is the combination between genes that indirectly influence each other. Weight is for example determined by efficiency of digestion, various hormones that influence metabolism of stored energy, storage of water, etc. and the properties of muscle and fat tissue.
Besides that, environmental factors also play a part. The body weight will also depend on the amount of exercise and the amount and type of food. The quantitative properties are often not only determined by genetic material, but also by the environment.
The fact that many quantitative characteristics are influenced by the environment as well as by the genes, leads us to the heritability index. The heritability index indicates which part of the superiority of the breeding animal can be found in his offspring. The heritability index is a value between 0 and 1, 0 indicating the situation that selection has no influence at all and 1 indicating the situation that the property can be fully controlled by selection. The heritability index shows us how successful selection for this property can be. It can be expressed by the formula:
h2 : heritability index|
Vgenetics : Variation by genetic influences
Venvironment : Variation by environmental influences
Let's take hip dysplasia (CHD) as an example. Research produced an estimate of h2 =0.2 - 0.3 1).
Other research delivered values for the heritability index between 0.25 and 0.4 . Let's assume the average to be 0.3 . This is actually a discouraging figure, because only 30% of the difference between the values of the selected parents and the average of the population can be seen in the offspring. In other words: assume the average value of CHD in the population to be 50 (on a scale of 0 - 100, 100 being the best), if we select a group of parents with an average of 80, only 30% of the difference (80 - 50 = 30) remains visible in the children (30 x 0.3 = 9), which leads to an average among the offspring of 59.
One of the factors in the formula is the variation by genetic influences. We have seen before that selection decreases this genetic variation. So selection also decreases the heritability index a bit more. In the end the heritability index approaches zero and the genetic variation for this characteristic has gone.
The parents of the new generation can be selected in a couple of ways (we're not discussing the actual criteria here).
- Individual selection
- Evaluation takes places based only on the properties of the animal itself.
- Family selection
- Evaluation is based on the characteristics of close relatives of the dog. It's often necessary if the property isn't visible in the stud dog itself (gender restricted).
- Combined selection
- Combination of other selection methods
Especially suitable if
Individual selection is much more accurate than family selection, which increases the result of selection.
- The heritability index of the property is rather high (more than about 0.4) and/or
- The property is visible in the breeding-animal itself.
Only closely related family members produce an accurate image of the properties of the breeding-animal. Further examination than half brothers and sisters is not useful.
Selection via full brothers or sisters
Brothers and sisters have half their genetic material in common. This method is only useful if:
If the litter mates grow up under the same circumstances, the value of this method is decreased. The measured superiority can be due to environmental factors in that case. Dogs usually grow up in different families though.
- The heritability index is low (lower than about 0.3) and/or
- The size of the litter is considerable (larger families) and/or
- The property is gender restricted
Selection via half brothers or sisters
This is less effective than selection via full brothers or sisters; half brothers (and sisters) only have a quarter of their genes in common. This method is only effective if the heritability is very low (less than about 0.2). The advantage is that usually more half brothers and sisters are available for evaluation and that they grew up (even before birth) in very different environments.
The value of a breeding-animal can be evaluated by the results of his/her offspring. Especially for males this method is usable if:
The main disadvantage is that value for breeding can only be determined after the sire has produced pups. If the dog has produced a few litters, this method is a good one for coming litters.
- The heritability index is low and/or
- The sire does not show the property and/or
- The litter size is small and/or
- The sire can get a large number of pups
Especially if one selects on more than one property (and we all do that), a combination of the methods described before is recommended. Select a suitable method for each property.
In the light of quantitative characteristics pedigree examination is not very worthwhile. Each animal has obviously only got two parents, which makes the number of animals that can be evaluated very low. Later we will see that for qualitative characteristics such as inherited diseases, this is a valuable tool.
Inbreeding and outcrossing
We've seen before that inbreeding and selection increase the number of homozygous genes. After breeding this way for a while various characteristics could have reached a ceiling. Further selection has no effect. In these situations crossing two different lines can produce superior (at certain characteristics) animals. This can be explained by effects such as overdominance (a heterozygous dog is better than a homozygous one (for a certain property)).
A number of properties can be improved this way. Especially the properties where heterozygous animals are preferred. People always appreciate an average amount of working abilities such as power over sheep and movement. A dog that has enormous power is not very handy, neither is a dog with very little power. Through selection and inbreeding the chance of homozygous genes and because of this also the chance of extreme values for these characteristics increases. Properties like fertility and resistance against infections are usually better in the offspring of an outcross. Homogeneity of the properties for exterior has been lost to a certain degree, which makes outcrossing not very common in breeding show dogs. This proves that the breeding method is highly determined by the goal that a breeder has.
1) SMITH, G.K., D.N. Biery, New concepts of coxofermoral joint stability and the development of clinical stress-radiographic method for quantitating hip joint laxity in the dog, Journal of the American Veterinary Medical Association, 1990, vol. 196, p. 59.
Copyright © 1998-2013 Jigal van Hemert & Danielle Boshouwers
This page last modified: Wednesday, 30-Jul-2008 16:46:29 CEST