animal breeding The genetic code (DNA) and immunogenetics

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.