Evaluation of animals
Methods of ranking animals for breeding purposes have changed as statistical and genetic knowledge has increased. Along with increases in breeding knowledge, advancements in computing have enabled breeders to quickly and easily process routine breeding evaluations, as well as to develop research needed to rank large populations of animals. Evaluating and ranking candidates for selection depends on equating their performance record to a statistical model. A performance record (y) can be expressed as y = g + e + ε, where g stands for genetic effects, e indicates known (categorized) environmental effects, and ε indicates random environmental effects.
The first task in estimating g is to statistically eliminate environmental effects, a process that involves setting up a system of equations to simultaneously solve for all of the genetic effects for the sires and cows. Information from relatives is included in g and increases the accuracy of evaluation of the candidates for selection. All relatives that are available can be incorporated in this type of evaluation. This model is called the animal model.
The animal model is used extensively in evaluating beef and dairy cattle, chickens, and pigs. To apply this model for evaluating large populations requires use of high-speed computers and extensive use of advanced mathematical techniques from numerical analysis. In evaluating the dairy cattle in the United States, a system of equations with more than 25 million variables is needed.
Accuracy of selection
In some cases the accuracy of selection for a trait can be measured using a calibrated tool or a scale. Thus, measurements of such traits can be replicated with high reliability. Alternatively, some traits are difficult to measure on an objective scale, in which case a well-designed subjective scoring method can be effective. An excellent example is hip dysplasia, a degenerative disease of the hip joints that is common in many large dog breeds. Apparently, hip dysplasia is not associated with a single allele, making its incidence very difficult to control. However, an index has been developed by radiologists that allows young dogs to be assigned a score indicating their likelihood of developing the disease as they age. In 1997 American animal geneticist E.A. Leighton reported that, in fewer than five generations of selection in a breeding experiment using these scores, the incidence of canine hip dysplasia in German shepherd dogs measured at 12 to 16 months of age had decreased from the breed average of 55 percent to 24 percent among the experimental population; in Labrador retrievers the incidence dropped from 30 to 10 percent.
Because close relatives share many genes, an examination of the relatives of a candidate for breeding can improve accuracy of selection. The more complete the genealogical record, or pedigree, the more effective the selection process. A pedigree is most useful when the heritability of a trait is relatively low, especially for traits that are expressed later in life or in only one sex.
Reproductive techniques can be used to increase the rate of genetic progress. In particular, for species that are mostly bred by artificial insemination, the best dams can be chosen and induced to superovulate, or release multiple eggs from their ovaries. These eggs are fertilized in the uterus and then flushed out in a nonsurgical procedure that does not impair future conception of the donor female. Using this procedure, valuable females can produce more than one calf per year. Each embryo is implanted in a less-valuable host female to be carried through gestation. The sex of the embryos can be determined in utero at about 50 days of gestation. The normal gestation for Holstein-Friesian cattle is about 280 days, so this early determination of sex saves many days and allows the breeding program to be adjusted. In particular, the donor cow could be collected again, or another superior cow could be bred to produce males. Thus, these reproductive technologies reduce the generation interval and increase selection intensity by getting more than one male calf from superior females. Both superovulation and sex determination are now commonly used procedures. Superovulation is also used when breeders want to increase the number of female calves from a valuable cow.
Progeny testing
Progeny testing is used extensively in the beef and dairy cattle industry to aid in evaluating and selecting stock to be bred. Progeny testing is most useful when a high level of accuracy is needed for selecting a sire to be used extensively in artificial insemination. Progeny testing programs consist of choosing the best sires and dams in the population based on an animal model evaluation, as described in the preceding section. A description of progeny testing in dairy breeding provides a good example. The best 1 to 2 percent of the cows from the population are chosen as bull mothers, and the best progeny-tested bulls are chosen to produce another generation of sires. The parents are mated to complement any individual deficiencies. The accuracy of evaluation of bull mothers is typically about 40 percent, and of sires that produce young bulls the accuracy is more than 80 percent. This is not as high as the industry wants for bulls to be used in artificial insemination. To reach greater accuracy, the next generation of sires is mated to enough cows in the population for each sire to produce 60 to 80 progeny. After the daughters of the young sires have a production record, the young sires are evaluated, and about the best 10 percent are used extensively to produce commercial cows. Some of the progeny-tested sires will have thousands of daughters before a superior sire is found to replace them. About 70 percent of dairy cattle are bred by artificial insemination, so these sires control the genetic destiny of dairy cattle. Consistently applying this selection procedure has been very successful.
The genetic gain has been consistent over the years. The actual first-lactation milk production varies more than the sire breeding value because differences in environmental conditions affect first-lactation production, but these environmental effects have been adjusted out of the breeding value calculations. There is no indication that the rate of gain in the sire breeding values is about to reduce. This level of achievement can only be realized if artificial insemination organizations and producers work together.
Breeding systems
Crossbreeding
Crossbreeding involves the mating of animals from two breeds. Normally, breeds are chosen that have complementary traits that will enhance the offsprings’ economic value. An example is the crossbreeding of Yorkshire and Duroc breeds of pigs. Yorkshires have acceptable rates of gain in muscle mass and produce large litters, and Durocs are very muscular and have other acceptable traits, so these breeds are complementary. Another example is Angus and Charolais beef cattle. Angus produce high-quality beef and Charolais are especially large, so crossbreeding produces an animal with acceptable quality and size.
The other consideration in crossbreeding is heterosis, or hybrid vigour, which is displayed when the offspring performance exceeds the average performance of the parent breeds. This is a common phenomenon in which increased size, growth rate, and fertility are displayed by crossbred offspring, especially when the breeds are more genetically dissimilar. Such increases generally do not increase in successive generations of crossbred stock, so purebred lines must be retained for crossbreeding and for continual improvement in the parent breeds. In general, there is more heterosis for traits with low heritability. In particular, heterosis is thought to be associated with the collective action of many genes having small effects individually but large effects cumulatively. Because of hybrid vigour, a high proportion of commercial pork and beef come from crossbred animals.
Inbreeding
Mating animals that are related causes inbreeding. Inbreeding is often described as “narrowing the genetic base” because the mating of related animals results in offspring that have more genes in common. Inbreeding is used to concentrate desirable traits. Mild inbreeding has been used in some breeds of dogs and has been extensively used in laboratory mice and rats. For example, mice have been bred to be highly sensitive to compounds that might be detrimental or useful to humans. These mice are highly inbred so that researchers can obtain the same response with replicated treatments.
Inbreeding is generally detrimental in domestic animals. Increased inbreeding is accompanied by reduced fertility, slower growth rates, greater susceptibility to disease, and higher mortality rates. As a result, producers try to avoid mating related animals. This is not always possible, though, when long-continued selection for the same traits is practiced within a small population, because parents of future generations are the best candidates from the last generation, and some inbreeding tends to accumulate. The rate of inbreeding can be reduced, but, if inbreeding depression becomes evident, some method of introducing more diverse genes will be needed. The most common method is some form of crossbreeding.
The genetic code (DNA) and immunogenetics
DNA
Deoxyribonucleic acid (DNA) is the genetic material that contains the instructions in each cell of organisms. DNA determines the genome, and thus the genetic code, which is a blueprint for development of all body organs and structures. The structure of DNA can be visualized as a spiral staircase. The handrails are made up of sugar and phosphate molecules, and the steps are composed of four nitrogenous bases: adenine, thymine, cytosine, and guanine. These bases are paired: adenine is paired with thymine, and cytosine is paired with guanine. The order of these four base pairs is the genetic code that determines the genotype of an individual. The DNA is arranged on chromosomes inside cells, with cells having two methods of dividing and replicating. In mitosis, a cell divides into two daughter cells such that each contains an exact copy of the original cell’s chromosomes. In meiosis, a germ cell’s chromosomes are duplicated before the cell undergoes two divisions to produce four gametes, or sex cells, each with half (male or female) of the original cell’s chromosomes. During the process of fertilization, male and female gametes from different organisms pair their chromosomes to form a zygote, which eventually becomes an adult.
Genetic progress in domestic animals has been made using quantitative methods to date. It would be very desirable to know the genes that control the many traits that have economic significance in domestic animals. This should make selection more accurate. Information from sequencing human genes, as well as those of other species, is being used to find chromosomal segments with high probabilities of coding genes in livestock. Another approach is to scan a chromosome segment and look for associations with economic traits. Several quantitative trait loci have been discovered that are or promise to be useful in livestock breeding. For example, an estrogen receptor in pigs is associated with increased litter size—on average, an increase of 0.6 to 2.0 pigs per litter, depending on the genetic background in which the gene is expressed. Other genes have been found that control the secretion of casein in cow’s milk. Genes are also known for growth hormone, and many others could be enumerated. With improvements in sequencing DNA, more genes will be discovered that affect economic traits—genes that will need to be tested in different genetic backgrounds and environments before they can be commercialized.
It is now much less expensive to sequence DNA, which has led to new methods of evaluating animals using large segments of 30,000–50,000 bases. With the use of these large segments of DNA, animals are evaluated without looking for markers for individual traits. This is intuitively an appealing approach because much more of the DNA can be evaluated; perhaps in the future the entire genome can be used to evaluate animals. This method of selection, called genomic selection, is now being applied to dairy cattle, but results are not yet available.
