Animal breeding is the selective mating of animals to increase the possibility of obtaining desired traits in their offspring. It has been performed with most domesticated animals, especially cats and dogs, but its main use has been to breed better agricultural stock. More modern techniques involve a wide variety of laboratory methods, including the modification of embryos, sex selection, and genetic engineering.
These procedures are beginning to supplant traditional breeding methods, which focus on selectively combining and isolating livestock strains. In general, the most effective strategy for isolating traits is by selective inbreeding; but different strains are sometimes crossed to take advantage of hybrid vigor and to forestall the negative results of inbreeding, which include reduced fertility, low immunity, and the development of genetic abnormalities.
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. Dog breeders, for example, 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). 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.
There are many reasons why animal breeding is of paramount importance to those who use animals for their livelihood. Cats have been bred largely for aesthetic beauty; many people are willing to pay a great deal of money for a Siamese or Persian cat, even though the affection felt for a pet has little to do with physical appearance. But the most extensive animal breeding has occurred in those areas where animals have been used to serve specific practical purposes. For instance, most dog breeds are the result of a deliberate attempt to isolate traits that would produce better hunting and herding dogs (although some, like toy poodles, were bred for traits that would make them desirable pets). Horses have also been extensively bred for certain useful qualities; some for size and strength, some for speed. But farm animals, particularly food animals, have been the subject of the most intensive breeding efforts.
The physical qualities of economic importance in farm animals vary for each species, but a generalized goal is to eliminate the effects of environment and nutrition. An ideal strain of milk cow, for instance, would produce a large amount of high-quality milk despite the type of food it is fed and the environment in which it is reared. Thus, animals are generally all bred for feed efficiency, growth rate, and resistance to disease. However, a pig might be bred for lean content in its meat, while a hen would be bred for its laying potential. Many cows have been bred to be hornless, so they cannot inadvertently or deliberately gore each other.
Although maximum food production is always a major goal, modern animal breeders are also concerned about nutritional value and the ability of animals to survive in extreme environments. Many parts of the world are sparsely vegetated or have harsh climatic conditions, and a high-efficiency producer able to endure these environments would be extremely useful to the people who live there. In addition, many people of industrialized countries are concerned not about food availability but about the quality of this food; so breeders seek to eliminate the qualities that make meat or milk or eggs or other animal products unhealthy, while enhancing those qualities that make them nutritious.
Although earlier animal breeders had to confine themselves to choosing which of their animals should mate, modern technological advances have altered the face of animal breeding, making it both more selective and more effective. Techniques like genetic engineering, embryo manipulation, artificial insemination, and cloning are becoming more refined. Some, like artificial insemination and the manipulation of embryos to produce twins, are now used habitually. Others, such as genetic engineering and cloning, are the subject of intense research and will probably have a great impact on future animal breeding programs.
Artificial insemination is the artificial introduction of semen from a male with desirable traits into females of the species to produce pregnancy. This is useful because a far larger number of offspring can be produced than would be possible if the animals were traditionally bred. Because of this, the value of the male as breeding stock can be determined much more rapidly, and the use of many different females will permit a more accurate evaluation of the heredit-ability of the desirable traits. In addition, if the traits produced in the offspring do prove to be advantageous, it is easier to disperse them within an animal population in this fashion, as there is a larger breeding stock available. One reason artificial insemination has been an extremely important tool is that it allowed new strains of superior stock to be introduced into a supply of animals in an economically feasible fashion.
The process of artificial insemination requires several steps. Semen must be obtained and effectively diluted, so that the largest number of females can be inseminated (consistent with a high probability of pregnancy). The semen must be properly stored so that it remains viable. The females must be tested before the sample is introduced to ensure they are fertile, and, following the procedure, they must be tested for pregnancy to determine its success. All these factors make artificial insemination more expensive and more difficult than traditional breeding methods, but the processes have been improved and refined so that the economic advantages far outweigh the procedural disadvantages. Artificial insemination is the most widely applied breeding technique.
To understand the techniques of embryo manipulation, it is important to understand the early stages of reproduction. When the egg and sperm unite to form a zygote, each of the parents supply half of the chromosomes necessary for a full set. The zygote, which is a single cell, then begins to reproduce itself by the cellular division process called mitosis, in which each chromosome is duplicated before separation so that each new cell has a full set of chromosomes. This is called the morula stage, and the new cells are called blastomeres. When enough cells have been produced (the number varies from species to species), cell differentiation begins to take place. The first differentiation appears to be when the blastocyst is formed, which is an almost hollow sphere with a cluster of cells inside; and the differentiation appears to be between the cells inside, which become the fetus, and the cells outside, which become the fetal membranes and placenta. However, the process is not yet entirely understood, and there is some variation between species, so it is difficult to pinpoint the onset of differentiation, which some scientists believe occurs during blastomere division.
During the first stages of cell division, it is possible to separate the blastomeres with the result that each one develops into a separate embryo. Blastomeres with this capability are called totipotent. The purpose of this ability of a single blastomere to produce an entire embryo is probably to safeguard the process of embryo development against the destruction of any of the blastomeres. In theory, it should be possible to produce an entire embryo from each blastomere (and blastomeres are generally totipotent from the four to eight cell stage), but in practice it is usually only possible to produce two embryos. That is why this procedure is generally referred to as embryo splitting rather than cloning, although both terms refer to the same thing (cloning is the production of genetically identical embryos, which is a direct result of embryo splitting). Interestingly, although the embryos produced from separated blastomeres usually have fewer cells than a normal embryo, the resulting offspring fall within the normal range of size for the species.
It is also possible to divide an embryo at other stages of development. For instance, the time at which embryo division is most successful is after the blastocyst has formed. Great care must be taken when dividing a blastocyst, since differentiation has already occurred to some extent, and it is necessary to halve the blastocyst very precisely.
Another interesting embryonic manipulation is the creation of chimeras. These are formed by uniting two different gametes, so that the embryo has two distinct cell lineages. Chimeras do not combine the genetic information of both lineages in each cell. Instead, they are a patchwork of cells containing one lineage or the other. For this reason, the offspring of chimeras are from one distinct genotype or the other, but not from both. Thus chimeras are not useful for creating new animal populations beyond the first generation. However, they are extremely useful in other contexts. For instance, while embryo division as described above is limited in the number of viable embryos that can be produced, chimeras can be used to increase the number. After the blastomeres are separated, they can be combined with blastomeres of a different genetic lineage. The additional tissue, in fact, increases the new embryos’ survival rate. For some reason only a small percentage of the resulting embryos are chimeric; this is thought to be because only one cell lineage develops into the cells inside the blastocyst, while the other lineage forms extra-embryonic tissue. Scientists believe that the more advanced cells are more likely to form the inner cells.
Chimeras could also be used to breed endangered species. Because of different uterine biochemical environments and the different regulatory mechanisms for fetal development, only very closely related species are able to bear each other’s embryos to term. For example, when a goat is implanted with a sheep embryo or the other way around, the embryo is unable to develop properly. This problem can perhaps be surmounted by creating chimeras in which the placenta stems from the cell lineage of the host species. The immune system of an animal attacks tissue it recognizes as foreign, but it is possible that the mature chimeras would be compatible with both the host and the target species, so that it could bear either embryo to term. This has already proven true in studies with mice.
A further technique being developed to manipulate embryos involves the creation of uniparental embryos and same-sex mating. In the former case, the cell from a single gamete is made to undergo mitosis, so that the resulting cell is completely homo-zygous. In the latter case, the DNA from two females (parthogenesis) or two males (androgenesis) is combined to form cells that have only female- or male-derived DNA. These zygotes cannot be developed into live animals, as genetic information from male- and female-derived DNA is necessary for embryonic development. However, these cells can be used to
Androgenesis— Reproduction from two male parents.
Artificial insemination— The artificial introduction of semen from a male with desirable traits into females of the species to produce pregnancy.
Blastocyst— An embryo at that stage of development in which the cells have differentiated to form embryonic and extra-embryonic tissue. The blastocyst resembles a sphere with the extra-embryonic tissue making up the surface of the sphere and the future embryonic tissue appearing as a cluster of cells inside the sphere.
Blastomere— The embryonic cell during the first cellular divisions, before differentiation has occurred.
Chimera— An animal (or embryo) formed from two distinct cellular lineages that do not mingle in the cells of the animal, so that some cells contain genetic information from one lineage and some from the other.
Cloning— The production of multiple genetically identical embryos or zygotes.
Crossing over— In meiosis, a process in which adjacent chromosomes exchange pieces of genetic information.
Diploid— Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.
Extra-embryonic tissue— That part of the developmental tissue that does not form the embryo.
Fetus— The unborn or unhatched animal during the latter stages of development.
Genome— Half a diploid set of chromosomes; the genetic information from one parent.
Hybrid vigor— The quality of increased health and fertility (superior to either parent) usually produced when two different genetic strains are crossed.
Parthogenesis— Reproduction from two female parents.
Placenta— The organ to which a fetus is attached by the umbilical cord in the womb. It provides nutrients for the fetus.
Totipotent— Embryonic cells able to produce an entire fetus.
Transgenic— Cells or species that have undergone genetic engineering.
Uniparental— Having only one genetic parent.
generate chimeras. In the case of parthogenetic cells, chimeras produce viable gametes. The androgenetic cells do not become incorporated in the embryo; they are used to form extra-embryonic tissue, and so no gametes are recovered.
Aside from these more ambitious embryo manipulation endeavors, multiple ovulation and embryo transfer (MOET) could soon become a useful tool. MOET is the production of multiple embryos from a female with desirable traits, which are then implanted in the wombs of other females of the same species. This circumvents the disadvantages of breeding from a female line (since a female can produce only a limited number of offspring in a specified time period). At the present time, MOET is still too expensive for commercial application, but is being applied experimentally.
Genetic engineering can produce transgenic animals—those that have had a new gene inserted directly into their DNA. The procedure involves microinjection of the desired gene into the nucleus of fertilized eggs. Although success rates vary, in many cases the new gene is reproduced in all developing cells and can be transcribed, or read, and utilized by the cell. This is a startling breakthrough in animal breeding, because it means a specific trait can be incorporated into a population in a single generation, rather than the several generations this takes with conventional breeding techniques.
There are some serious limitations to the procedure, however. The first is that many genes must work together to produce the very few traits a breeder would like to include in an animal population. Although it might some day be possible to incorporate any number of genes into an embryo’s DNA, the complex interplay of genes is not understood very well, and the process of identifying all of those related to a desired trait is costly and time-consuming.
Another problem in the production of transgenic animals is that they pass their modified DNA on to their offspring with varying success rates and unpredictable results. In some cases, the new gene is present in the offspring but is not utilized. Or it may be altered or rearranged in some way, probably during the process of gamete production.
These factors make it difficult to produce a strain of transgenic animals. However, with further research into the mechanism by which the gene is incorporated into the genome, and by mapping the target animal genome and identifying the genes responsible for various traits, genetic engineering will no doubt become a major tool for improving animal strains.
It would be extremely useful if a breeder were able to predetermine the sex of each embryo produced, because in many cases one sex is preferred. For instance, in a herd of dairy cows or a flock of laying hens, females are the only commercially useful sex. When the owner of a dairy herd has inseminated a cow at some expense, this issue becomes more crucial. In some cases, an animal is being bred specifically for use as breeding stock; in this case, it is far more useful to produce a male that can be bred with multiple females than a female, which can only produce a limited number of offspring.
Whether or not an animal is male or female is determined by its sex chromosomes, which are called X and Y chromosomes. An animal with two X chromosomes will develop into a female, while an animal with one X and one Y chromosome will become a male. In mammals, the sex of the offspring is almost always determined by the male parent, because the female can only donate an X chromosome, and it is the presence or absence of the Y chromosome that causes maleness (this is not true in, for instance, birds; in that case the female has two different sex chromosomes). Using cell sorters, researchers are now able to separate spermatozoa into X and Y components. Although such “sex-sorted” sperm produces fewer pregnancies than nonsorted sperm, more than 90% of the offspring produced are of the intended sex.
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Sarah A. de Forest