The connection between an organism’s genetic makeup and its immune system, as well as applications of that knowledge, form the young science of immunogenetics. In particular, producers must control diseases in their livestock if they are going to be profitable. While vaccines, hygiene, and other therapeutic methods control most diseases, vaccines are expensive and none of these methods is completely effective. However, there is evidence from experiments and field data of some degree of genetic control over the immune system in humans and animals. For example, bovine leukocyte adhesion deficiency (BLAD) is a hereditary disease that was discovered in Holstein calves in the 1980s. The presence of the BLAD gene leads to high rates of bacterial infections, pneumonia, diarrhea, and typically death by age four months in cattle, and those that survive their youth have stunted growth and continued susceptibility to infections. It was soon found that these calves carried two copies of a recessive gene that was present in nearly 25 percent of Holstein bulls. Cattle with only one copy of the gene, or carriers, had normal growth patterns and immune systems. Holstein bulls are now routinely tested for the BLAD gene before being used for artificial insemination. With a high percentage of Holsteins being bred artificially, a potentially major problem has been avoided.

Genetic control of the immune system is based on the DNA of the individuals. Histocompatibility genes that serve several functions are on one area of a chromosome, called the major histocompatibility complex (MHC), which exists in all higher vertebrates. There are large numbers of genes involved in the MHCs of different species. There are more than 60 different alleles at one locus and other loci are multi-allelic. There are also differences among species in the number of genes known. In addition, selection experiments have demonstrated genetic variation between lines selected for high and low response to different antigens. Some vaccinations are more efficacious when the animals have been selected for resistance to the antigen for which they are vaccinated.

Substantial progress has been made in the field of immunogenetics, but limited use has been made of this knowledge. One reason for this is that immune systems have evolved to be generally robust. Changing the frequency of some genes that control immune function may inadvertently change the function of other genes and result in adverse effects. Experiments are now under way to determine whether sires’ immune responses can be used to predict the health of their daughters under field conditions. The results indicate that there are differences among sires’ daughter groups, but the differences are not large enough to control a high proportion of the variability. The tests used were based primarily on leukocytes, which are the first line of defense when an antigen invades an animal. Application of knowledge in the area of immunogenetics must be used with caution.

It might seem that integrating molecular markers and quantitative methods would be a trivial task. However, the effect of some genes depends on the presence of others, and these interactions need to be considered along with the particular breeding scheme. Furthermore, there are nongenetic influences that may turn genes on and off. Thus, some genes act individually, some genes interact, and the environment has a further impact. Finding how these all affect the phenotypic expression of an organism is complicated. However, this challenge presents an opportunity for future research and for producers.

Many advances in reproductive technologies have been made, though many are too expensive for everyday use. Most of the advanced techniques use artificial insemination, which was developed decades ago, though refinements continue.

Cloning

Cloning, an asexual method of reproduction, produces an individual with the same genetic material (DNA) as another individual. Probably the best-known examples of clones are identical twins, which result when cells in the early development stage separate and develop into different individuals. Though the DNA in cloned individuals is the same, environmental influences may make them differ in phenotype. Thus far, the commercial use of clones has been limited. Cloning can be used to produce clones from a highly productive individual, but the cost would have to be low enough to recover the expense quickly. Animals have been cloned by three processes: embryo splitting, blastomere dispersal, and nuclear transfer. Nuclear transfer is most common and involves enucleating an ovum, or egg, with all the genetic material removed. This material is replaced with a full set of chromosomes from a suitable donor cell, which is microinjected into the enucleated cell. Then the enucleated cell, with the transplanted chromosome, is placed into a recipient female to be carried through gestation.

Determining sex from sperm

There is a commercial demand for the ability to predetermine the sex of livestock. For example, a producer may want female calves from the best cows for replacements and male calves for beef production. Dairy producers may want more females for replacing cows or for expansion of their herds. The sex of mammals is determined by the sex chromosomes, or X and Y chromosomes. Animals with two X chromosomes develop into females; animals with an X and a Y chromosome develop into males. Thus, the detection of X and Y chromosomes on sperm has been the focus of research to predetermine the sex of domestic animals.

In one process, sperm is pretreated with a dye that fluoresces when exposed to short wavelength light. The fluorescence is brighter from a sperm bearing X chromosomes, which contain about 4 percent more DNA than the Y chromosome. A stream of dyed sperm is passed through a flow cytometer, a computer determines the degree of fluorescence, and the sperm is separated into different containers. The success rate can be as high as 40 percent. When “sexed sperm” has been used on a commercial basis, though, it has had limited success. The conception rate using sexed sperm is lower in cows, though it is higher in primiparous cows. In addition, sperm are killed in the typing process, and the rate of sexing the sperm is slower than desired. While economical processing of sperm is just getting started, it is expected to become another useful tool in animal agriculture.

Albert E. Freeman
Key People:
Ilya Ivanovich Ivanov
Related Topics:
breeding

artificial insemination, the introduction of semen into the vagina or cervix of a female by any method other than sexual intercourse. The procedure is widely used in animal breeding and is used in humans when a male is sterile or impotent or when a couple suffers from unexplained infertility (when the cause of infertility cannot be identified). Impregnation of a woman through artificial insemination may also be used by women or men in same-sex partnerships who wish to produce children of their own.

Artificial insemination in animals

The first successful experiment with artificial insemination in animals was performed by Italian physiologist Lazzaro Spallanzani, who in 1780, while investigating animal reproduction, developed a technique for artificial insemination in dogs. This approach was refined in the 1930s in Russia, and the subsequent development of methods for the cryopreservation (preservation through freezing) of semen led to the widespread use of artificial insemination in animals.

The chief advantage of artificial insemination is that the desirable characteristics of a bull or other male livestock animal can be passed on more quickly and to more progeny than if that animal is mated with females in a natural fashion. Ten thousand or more calves have been produced annually from a single bull through the use of artificial insemination. In the actual procedure used, semen is obtained from a male animal and, after being diluted, is deep-frozen, after which it can be stored for long periods of time without losing its fertility. For use, the semen is thawed and then introduced into the genital tract of a female animal.

Farm in Saskatchewan
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origins of agriculture: Artificial breeding

Artificial insemination has been used to facilitate the reproductive success and conservation of threatened or endangered animals. Examples of wild animals that have been successfully impregnated through artificial insemination include big cats (e.g., the tiger, the puma, the cheetah, and the clouded leopard), the white rhinoceros (Ceratotherium simum), and the onager (Equus onager).

Artificial insemination in humans

The first recorded experiment with artificial insemination in humans occurred in the late 1700s, when Scottish-born surgeon John Hunter impregnated a women with her husband’s sperm, resulting in a successful pregnancy. In 1884 American physician William Pancoast performed a modified artificial insemination procedure when he injected sperm from a donor into a woman who was under anesthesia. The woman, who was married, gave birth to a baby nine months later and did not know that she had been impregnated with donor sperm. Her husband, whom Pancoast determined was infertile, later found out about the procedure from Pancoast.

Today artificial insemination in humans is considered a form of assisted reproductive technology. Women impregnated in this way are physically capable of conceiving and bearing children, though they are unable to conceive through sexual intercourse, usually because their husband is sterile or impotent. Fresh semen is obtained from the husband (if he is impotent) or from some other male donor (if the husband is sterile) and is introduced by a syringe into the woman’s vagina or cervix during the middle of her menstrual cycle. The semen can also have been previously frozen and stored in a sperm bank. The technique is reasonably successful in achieving conception and pregnancy.

This article was most recently revised and updated by Robert Lewis.