Genetics and Disease

NELSON F. WATERS.

MANY diseases and pathological disorders of domestic animals result in death or make an animal unfit for economic use. Sometimes it is difficult for the livestock owner to recognize and interpret many of the causes of death as being due to heredity or influenced by heredity.

Some persons believe that heredity has a minor role in pathology. Such reasoning is not in accordance with known facts some genetic influence operates in all of the biological phenomena of life.

The science of genetics explains how resemblances and differences, which are found in all plant and animal organisms, are transmitted to the next generation. An understanding of the basic laws of heredity has resulted from a vast amount of experimentation with plants and animals contributed by many workers in nearly all the sciences.

The science of plant breeding has become so exacting that strains of corn, wheat, oats, and other plants have been so bred that they will grow and thrive efficiently only in restricted climates. These same plants are bred to resist the ravages of specific diseases. Domestic animals, notably dairy and beef cattle, swine, sheep, and chickens, are monuments to man's ability to control heredity. All these things have occurred through the application of genetic laws.

Causes of livestock mortality may be divided into three groups:

Diseases caused by infectious agents such as bacteria, viruses, and rickettsiae that may infect the animal at almost any period throughout its life;

Infections by parasites that may either destroy body tissue and cause death or weaken the animal and predispose it to many infectious agents; Abnormalities and disorders of a non-infectious nature that are inherent in the animal and are present at birth, or may develop later. Examples of such hereditary characters are achondroplasia, deformed limbs, brain hernia, cleft palate, hydrocephalus, and congenital spasm.

Many of the diseases that affect domestic animals are caused by minute parasitic forms of life, each of which has its own form of existence and is governed by the same biological laws that dictate life for the higher animals.

TO OBTAIN AN UNDERSTANDING of genetics and its relationship to pathology, we cannot confine our studies to the complex animal alone. It is necessary also to investigate the organisms responsible for diseases.

All living animals start life as one-cell structures. Within these one-cell bodies are constituents that are specific to individual species of plants or animals. Everything an animal can be from a structural and an operational standpoint is contained in this minute bit of living matter.

We are interested in the inherent capacities that are deposited in the original cell body. Some of the one-cell bodies will divide and double their number by each division until the complex animal is formed. Other cells will never advance beyond the single-cell stage.

Some of them we call bacteria. The permanent one-cell bacteria often invade and multiply in the more complex host animal. If such bacteria are disease producing or what is termed pathogenic, they may be harmful to their host and thus may cause any one of many of the infectious diseases with which we are familiar.

Besides the one-cell bacteria, there are even smaller units of presumably living protein matter capable of multiplying in numbers inside a host. They are quite specific in their host response. These smaller units are the viruses. Those with which we are familiar are pathogenic or disease producing when they invade the more complex forms of plant and animal life. Indeed, viruses are so small that they can actually invade and destroy one-cell bacteria. Viruses mostly can be seen only under an electron microscope.

Some of the lower forms of animal life, such as the coccidia, tapeworms, roundworms, flukes, and the ticks, also invade the host animal, causing disease, reducing its efficiency, and not infrequently causing death. Such internal and external parasites may act as vectors and deposit disease-producing bacteria and viruses in the animal host. All these lower forms of life likewise are regulated by the same genetic laws that regulate the higher forms.

The host animal, then, be it cattle, swine, sheep, or chicken, responds according to its genetic capacity. The lesser forms of life bacteria, viruses, or more complex parasites will invade and affect the host, according to their genetic capabilities. Thus we must recognize two independent variables responsible for disease the host and the pathogen (or disease-producing agent).

A third important variable exists environment, which may influence materially the interaction of the host and pathogen.

Any discussion of animal diseases must therefore consider not only the interrelationship of host and pathogen but also the environment that surrounds and influences all living matter.

Questions as to the relative importance of heredity and environment are ages old. Long before the Austrian Gregor Mendel provided an insight into the mechanism of inheritance, there were those who realized some of the fundamental genetic problems of breeding plants and animals. During this early period some felt that the environment was responsible for most of the variations seen in plants and animals. This difference of opinion is now history. Today's students, trained in the biological sciences, recognize heredity and environment as separate components, but often so closely interrelated that they often are difficult or impossible to separate.

When the maternal animal cell is fertilized by a male cell, it has received a quota of inheritance from its dam and a quota from its sire. Nothing genetic will be added to this animal as long as it may live. As soon as this animal is conceived, however, it becomes subject to (and influenced by) the surrounding environment established in the body of its dam. After birth the animal will be bombarded by the conditions around it.

Animals are conceived, then, with certain genetic-potential abilities. Environment can suppress or bring out the abilities, if present. But environment will never develop abilities that were not inherent in the animal.

Animals may have inherent resistance or a susceptibility to specific diseases. Only when these individuals are challenged by a disease organism do the genetic differences become apparent. The students of veterinary medicine have been too prone in animal pathology to consider the individual host per se without due consideration of the genetic heritage of either the host or the pathogen. The ability of the diagnostician to recognize and classify properly hereditary diseases and defects will add much to genetic knowledge and greatly assist in the ultimate eradication of heretofore incurable conditions.

It is necessary to distinguish between pathological abnormalities that are dependent alone on gene differences in the host and those caused by the genetic interaction of both host and pathogen.

OF WHAT VALUE is a knowledge of genes, viruses, and bacteria to a breeder of livestock?

The answer is that he should know and understand what the undesirable genes, viruses, and bacteria can do to his animals. From a practical viewpoint, our interest in viruses or bacteria lies in how they affect man and his animals.

First, we are interested in the pathological or disease conditions caused by infectious agents.

Second, we would like to know how these agents are transmitted from host to host.

Third, we would like to know how we may prevent the entrance of disease agents, or, after they enter, how we may prevent them from inflicting damage and possibly death.

Some of these things we know. Most of the diseases affecting domestic animals are reasonably well described as to symptoms and pathology. Some of the diseases can be controlled by antisera, antibiotics, and other prophylactic means. In only a few instances, however, do we understand the genetic nature of the pathogenic organism and its relationship to its host.

A rapid increase in studies of the viruses, especially as they pertain to heredity, has been stimulated by the action and the behavior of bacterial viruses. Here one can multiply into the millions both the bacteria and the viruses. Chemical analyses, behavior, purity, immunological reactions, virulence, the rates of multiplication, size, shape, and even photography can characterize or identify the virus. Genetic studies with both bacteria and the viruses demonstrate or exhibit all the phenomena of Mendelian heredity found in the higher organisms.

Our knowledge of genes the determiners of inheritance is based mostly on what they do, rather than on what they are. Their chemical nature, other than describing them as consisting mostly of desoxyribonucleic acid plus other proteins, is unknown.

A gene, as we know it, is a regulated part or segment of a chromosome structure contained within a nucleus surrounded by cytoplasm. The composite is encased in a cell wall. Because genes are capable of expressing different traits or characteristics, it seems plausible that the genes may in part differ chemically from one another.

An induced change in a chromosome or the genes of a chromosome by X-rays, ultraviolet rays, or chemicals may influence greatly the relationship of a chromosome or its genes to other chromosomes and their genes. That in turn could influence the expression of an organism. Induced changes or mutations in the less complex organisms could conceivably modify their genetic expression drastically.

There is some question whether mutations in the higher organisms, such as domestic animals, are of widespread economic importance. Circumstantial evidence suggests that the expression of the majority of genes has remained unchanged over long periods of time.

This constancy of the gene is overwhelming and past comprehension. Every gene seems to know where it belongs and how to get there. The very nature of the constancy of the gene permits some element of predictability and allows some control in planning coatings between animals.

Bacteria as living one-cell organisms should and do exhibit similar properties of cells found in other living organisms. Structurally a bacterium does not always appear to conform to the more or less conventional cell morphology of the higher animals. There is no doubt, however, that bacteria do have cell components fully comparable to those found in the more complex multicellular forms.