Genetically Resistant Animals

Lyman B. Crittenden, research geneticist, Regional Poultry Research Laboratory, Agricultural Research Service, East Lansing, MI.

The production of high-quality animal products depends on healthy animals that thrive and produce. Animals need to be protected from infection by pathogens (disease-producing organisms) or from their effects. This is usually done by eradication of the pathogen, by vaccination to induce protective antibodies, or by chemotherapy.

Researchers evaluate autoradiograms of chicken blood in research aimed at identifying chickens with avian leukosis virus genes.

Since animals can inherit the ability to resist disease and to improve response to the usual methods of control, recent developments in genetics research have exciting implications for breeding more disease-resistant animals.

Genes can influence resistance at many levels in the cycle of infection, immune response, and disease development.

An animal can passively resist infection if the pathogen cannot enter the body, organ, or cell, or is inactivated by some body fluid that is present earlier. Alternatively, an animal can respond actively, often through its immune system, to develop resistance to the pathogen and eliminate it.

Therefore, genetic resistance, even to a single disease, is usually a complex trait controlled by many genes. However, single genes have been found that block infection or a single step in the origin and development of a disease. Such genes, however, often prevent only a specific disease.

Over the years, animal breeders have selected animals for breeding if the animals' relatives survived well under farm conditions.

Wings of day-old chicks show short feathers on males and long feathers on females.

Some animals, particularly in poultry, are selected for breeding based on the ability of their relatives to survive after artificial exposure to a specific pathogen. This selection is usually made only when an epidemic is causing severe economic loss and there is no alternative method of control. This is because selection for resistance to a specific disease reduces the ability of the breeder to select for other traits of economic importance, such as growth rate or egg production.

Conventional, nongenetic, methods of control have been effective for many acute infectious diseases that have troubled livestock in the past. But as such diseases come under control, genetic resistance will become more important for the control of chronic diseases. An important research goal is to find ways to select for genes that impart general resistance to a variety of diseases so that the breeder will need to select for only a few additional traits to improve resistance.

Gene Identification for Disease Resistance

The classic method of gene identification is to look for differences in a trait or characteristic, mate animals that have different characteristics, and describe the variation observed among the progeny for the next few generations. By such observations the geneticist can determine whether a few or many genes control the trait. This approach depends on the identification of genetically controlled variation in an observed trait or in a chemically defined gene product.