Animals, at least the crop types, are outbreeders and often have a slow turnover time, especially the mammals. The gestation time is long and the animal may take years to grow to its adult state when it can be selected for or against. The litter size is often small so that building up a large population from a selected stock can take many years. Consequently artificial insemination and embryo transplantation have become important aspects of animal breeding programmes. The semen from a high yielding bull can be collected and diluted with an extender to supply a large number of cows (usually about 350 but in theory up to 1400). The semen can be stored in a sperm bank either short term at 4°C for a few days or long term at -196°C. AI is used in horses (but restricted in thoroughbreds), sheep, goats, pigs, dogs and poultry. The cost of the animal determines whether AI is a cost effective method of breeding.

Embryo transplantation combined with in-vitro AI is becoming more common in specialised breeding programmes (including humans!). Large numbers of fertile eggs, induced by application of Follicle stimulating hormone (FSH), can be collected from a superior female and the eggs fertilised by sperm from a known source animal. The eggs can be cultured and even split to clone them so increasing the available embryo number. These can then be introduced into the uteri of other female animals which are in a receptive state and many more offspring are produced from the particular cross required than could be achieved by normal means.

The embryos can be frozen and implanted later which has been useful in humans. Examples are the recent birth of an in-vitro twin a few years after her sister was born, and the use of embryo banks to produce healthy embryos early in a relationship and have them developed when the necessary money and status has been achieved. However note the recent American court case over jurisdiction of the dormant embryos which raises moral issues. The use of temporary mothers in animals has been employed for transport of embryos to distant lands, cow embryos in the uteri of a rabbit is one example. It is easier and cheaper to carry the embryos in rabbit than in the cow! Note also the use of surrogate mothers in human relationships to allow a child to be born for a couple in which the female is unable to carry a foetus to full term successfully.

Most of the animals domesticated by man have been developed into breeds, which have very clear cut characteristics. Many societies have been formed to supervise the particular breeds. They record the lineages of their animals, protect the purity of the breed, encourage further development of the breed, and promote an interest in the breed. The pedigree of a breed animal will show its lines of descent and record any special accomplishments of its lineage, ie champion at Crufts etc.

Many characteristics are not easy to evaluate in an individual organism and may require testing by comparing performance of parent stock with the performance of the offspring. This applies particularly to phenotypes which are not obvious in one particular sex. We refer to such phenotypes as controlled by sex-limited genes. A good example is milk yield in cattle. It is easy to evaluate a cow, but how do you evaluate a bull? The only way, is to mate the bull with a number of cows of known milk yield and to later compare the milk yield of the parent females with the milk yield of the offspring once they have developed fully. Naturally the environmental conditions must also be comparable. The bull is thus evaluated and given a rating and if this is high enough his sperm may be used in AI programmes. The process of progeny testing is time consuming and can only be used for organisms which outlive their offspring. Trying to use this method of testing with annual plants, or flies would be doomed to failure.

You must also remember that when dealing with small populations of animals in the breeding programme, there is a strong tendency for inbreeding depression to occur and this will generally be overcome by crossing the inbred stock with a different male animal of high value for the phenotype being selected. This introduces new allele combinations and heterosis occurs.