The Genetic Basis Of Animal Breeding
Breeders engage in genetic "experiments" each time they plan a mating. The type of mating selected depends on the goals. To some breeders, determining which traits will appear in the offspring of a mating is like rolling the dice—a combination of luck and chance. For others, producing certain traits involves more skill than luck—the result of careful study and planning. Breeders have to understand how to manipulate genes within their breeding stock to produce the kinds of dogs they want. They have to first understand dogs as a species, then dogs as genetic individuals.
Once the optimal environment for raising an animal to maturity has been established (i.e., the proper nutrition and care has been determined) the only way to manipulate an animal's potential is to manipulate its genetic information. In general, the genetic information of animals is both diverse and uniform: diverse, in the sense that a population will contain many different forms of the same gene (for instance, the human population has 300 different forms of the protein hemoglobin); and uniform, in the sense that there is a basic physical expression of the genetic information that makes, for instance, most goats look similar to each other.
In order to properly understand the basis of animal breeding, it is important to distinguish between genotype and phenotype. Genotype refers to the information contained in an animal's DNA, or genetic material. An animal's phenotype is the physical expression of its genotype. Although every creature is born with a fixed genotype, the phenotype is a variable influenced by many factors in the animal's environment and development. For example, two cows with identical genotypes could develop quite different phenotypes if raised in different environments and fed different foods.
The close association of environment with the expression of the genetic information makes animal breeding a challenging endeavor, because the physical traits a breeder desires to selectively breed for cannot always be attributed entirely to the animal's genes. Moreover, most traits are due not just to one or two genes, but to the complex interplay of many different genes.
DNA consists of a set of chromosomes; the number of chromosomes varies between species (humans, for example, have 46 chromosomes). Mammals (and indeed most creatures) have two copies of each chromosome in the DNA (this is called diploidy). This means there are two copies of the same gene in an animal's DNA. Sometimes each of these will be partially expressed. For example, in a person having one copy of a gene that codes for normal hemoglobin and one coding for sickle-cell hemoglobin, about half of the hemoglobin will be normal and the other half will be sickle-cell. In other cases, only one of the genes can be expressed in the animal's phenotype. The gene expressed is called dominant, and the gene that is not expressed is called recessive. For instance, a human being could have two copies of the gene coding for eye color; one of them could code for blue, one for brown. The gene coding for brown eyes would be dominant, and the individual's eyes would be brown. But the blue-eyes gene would still exist, and could be passed on to the person's children.
Most of the traits an animal breeder might wish to select will be recessive, for the obvious reason that if the gene were always expressed in the animals, there would be no need to breed for it. If a gene is completely recessive, the animal will need to have two copies of the same gene for it to be expressed (in other words, the animal is homozygous for that particular gene). For this reason, animal breeding is usually most successful when animals are selectively inbred. If a bull has two copies of a gene for a desirable recessive trait, it will pass one copy of this gene to each of its offspring. The other copy of the gene will come from the cow, and assuming it will be normal, none of the offspring will show the desirable trait in their phenotype. However, each of the offspring will have a copy of the recessive gene. If they are then bred with each other, some of their offspring will have two copies of the recessive gene. If two animals with two copies of the recessive gene are bred with each other, all of their offspring will have the desired trait.
There are disadvantages to this method, although it is extremely effective. One of these is that for animal breeding to be performed productively, a number of animals must be involved in the process. Another problem is that undesirable traits can also mistakenly be selected for. For this reason, too much inbreeding will produce sickly or unproductive stock, and at times it is useful to breed two entirely different strains with each other. The resulting offspring are usually extremely healthy; this is referred to as "hybrid vigor." Usually hybrid vigor is only expressed for a generation or two, but crossbreeding is still a very effective means to combat some of the disadvantages of inbreeding.
Another practical disadvantage to selective inbreeding is that the DNA of the parents is altered during the production of eggs and sperm. In order to make eggs and sperm, which are called gametes, a special kind of cell division occurs called meiosis, in which cells divide so that each one has half the normal number of chromosomes (in humans, each sperm and egg contains 23 chromosomes). Before this division occurs, the two pairs of chromosomes wrap around each other, and a phenomenon known as crossing over takes place in which sections of one chromosome will be exchanged with sections of the other chromosome so that new combinations are generated. The problem with crossing over is that some unexpected results can occur. For instance, the offspring of a bull homozygous for two recessive but desirable traits and a cow with "normal" genes will all have one copy of each recessive gene. But when these offspring produce gametes, one recessive gene may migrate to a different chromosome, so that the two traits no longer appear in one gamete. Since most genes work in complicity with others to produce a certain trait, this can make the process of animal breeding very slow, and it requires many generations before the desired traits are obtained—if ever.
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