Genetic improvement in animals is controlled by heritability, selection differential, and generation interval.
Heritability of a characteristic is the degree to which it is passed on by parents to their offspring (e.g. if one selects both dam and sire with good temperament and the offspring show superiority in temperament over others of the same sex and age, then temperament would be supposed to have a high degree of inheritability).
Selection differential is the superiority of the parents over the mean of the population they were selected from. The greater the population one is selecting from, the more marked the superiority of a few individuals over the mean of all of them. Out of five bucks, for instance, one will be slightly better than average, but out of a thousand bucks there will be two or three which are far above the average in, say, fleece weight.
Generation interval is simply the average age of parents when offspring are born. The younger they are (i.e. the shorter the generation interval), the more rapid the progress in genetic improvement. This genetic interval is halved if the does are mated as hoggets, and thus the rate of genetic progress is theoretically doubled.
Another factor that has to be taken into consideration is repeatability, which defines the consistency of a characteristic occurring. For instance, if the repeatability for weaning weight is .16, and the repeatability for milk yield is .62 (the range in both cases being O to I .0O), then does could be culled on the evidence of one lactation, but weaning weight of one kidding would not be a valid basis for culling. There is a formula for genetic improvement involving these factors:
Heritability x Selection differential
Generation interval = Genetic progress/ Year
But as far as practical farming is concerned, the main point to note is that if inheritability is low, progress is slow unless it is balanced by a high selection differential.
Also of considerable importance to goat farming, the fact is that progress in genetic improvement is in direct proportion to the number of factors or characteristics being selected for. For instance, if one were selecting solely for weight-gain, progress would be much more rapid then if one were selecting for weight-gain and fleece qualilty.
In fact it comes down to a simple mathematical formula. the rate of progress is the square root of the number of factors being selected for. the basic factor in genetic improvement is simply this, that there is a wide variation between the individuals of a population in respect of any characteristic.
Out of a thousand goats there will be some that grow fast and some that do not, it is a law of nature. To make progess through breeding, farmers select those that execl in certain areas which are significant, such as weight-gain.
But obviously little progess is going to be made if the characteristic for which an individual is selected is not inheritable, it will not be passed on to his or her progeny, or trait is in fact determined by environment, not genes.
It is therefore important in any breeding program to record performance of individuals in respect of those characteristics that are highly inheritable. It would be pointless, for instance, selecting bucks for breeding on the basis of their own growth rate if that were not an inheritable factor.
There are two types of desirable characteristics — objective ones which can be measured (e.g. milk production, live-weight, fleece colour), and subjective ones which can only be guessed at (e.g. temperament, fleece `style’).
Performance records, then, are one thing. Pedigree records are quite different; they indicate an animal’s stage. They certainly have some value — they are of course essential for angoras to meet registration requirements — but it is necessary to be able to assess that value.
As far as the goat industry is concerned, which is, after all, just at the beginning of its development, pedigree records are less meaningful than performance records simply because the environment has changed so dramatically for each generation.
Half the genes in a goat come from each parent, so that even if the pedigree and performance of a buck is known, the fact that the doe is a feral about whom nothing is known, means that pedigree is going to be less important than performance.
This is not to suggest that pedigree has to be totally ignored at anything below pure-bred matings. Much can be learned, for instance, by looking at all the progeny of a certain buck and comparing them with progeny of other bucks on the same farm.
If a thousand does are divided into ten random mobs of 100 does for single-sire mating, and all the kids by a particular buck have longer tails than the average for all kids, that is an inheritable characteristic passed on by that buck.
Genetics, in one sense, is more a matter of mathematics than of biology … it is a question of numbers, and for that reason we talk of ‘population genetics’. It is also a question of variation … the degree to which an individual animal varies from the norm or mean of the whole population to which it belongs.