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By Lana Robinson
Field Editor
Speaking at the Blackland Income Growth Conference in Waco last month, Dr. Joe Paschal, Extension livestock specialist, reviewed genetics with cattle producers, explaining what has been learned in that field, and the latest developments in genetic testing technology.
"About 800 genes have been identified in cattle. We just don't know exactly where they are," said Paschal. "Some are not inherited in any set fashion. To be useful, it has to be associated with a particular marker or location."
According to Paschal, beef cattle have 30 pairs of chromosomes with some 3 billion base pairs that make up the bovine genome in its entirety. About 50,000 are functional genes that affect physiological activities, or the bovine animal's function and appearance (four legs and hooves, a head with two ears, eyes, and a mouth, a tail, and so forth). Some is genetic "trash" (i.e. failed genes that are discarded from use but remain on the chromosome).
"Genes act together resulting in qualitative and quantitative traits," he said.
Paschal further noted that the major types of gene action at a particular site or locus on a chromosome are dominance (most often associated with coat color or polledness but also responsible for most of the heterosis or hybrid vigor in crossbreds), additive (the gene action that allows us to estimate breeding values and expected progeny difference or EPD) and epistatic (a specific gene action that occurs between genes on different parts of a chromosome and is associated with hybrid vigor).
"The genes that are responsible for the `functioning' animal, probably account for 85 percent of the total, leaving 15 percent or so—no one really knows until the entire bovine genome is sequenced and mapped— open to effective selection," he said, adding: "In this mix are most of the genes with additive effects affecting performance and all the genes with dominance effects."
Paschal said all the dominant genes tend to have large visible effects that are not affected by the environment (polledness, coat color, etc) while the additive genes tend to have small individual effects that accumulate through selection to larger increases in performance. Some of these additive genes can have rather large effects, 2, 3, 5 or perhaps 10X the level of the "average additive effect". These are called "major genes" and because of their large effect (relative to other genes) they are of great importance to beef cattle breeding. Usually the importance of these genes in selection is determined by how much of the genetic variation they account for and usually one gene will affect more than one trait, but perhaps at different levels and at different times, he suggested.
"Many of the dominant genes in cattle have been located on specific chromosomes. The Polled/Horned and the Black/Non black genes are examples, and they have specific tests. However we hit a brick wall, when we try to find the genes that affect some of the growth and carcass traits, milk production, disease prevention, and fertility. Very few of these additive genes have been found. Instead, what has been found is that performance in cattle is associated with certain known `markers,'" said Paschal.
In reality, these markers are genes (or gene sequences with different types of characteristics) that are inherited in the same manner as the gene giving the desired increase (or decrease) in performance. Hence, you have the term "marker assisted selection" (MAS).
"The actual genes are as yet not known precisely but closely linked markers, which reside on the chromosome very close to the desirable gene, have a greater chance of being inherited with the desired gene," he explained.
Hundreds of markers are now known—many for traits of interest—but only a few are known to be closely linked so they will be passed on with the genes or quantitative trait locus (QTL) of interest.
Paschal likened the whole chromosome to a city and the specific gene, or marker, as a particular house in a particular block.
Paschal said the first commercially available markers for beef cattle (besides those for coat color, polledness and some genetic diseases) were offered through the efforts of Australian geneticists in the form of GeneSTAR Marbling and Tenderness (T1).
Results for T1 indicated an 8 to 10 percent increase in tenderness for 2 vs 0 Stars. Brahman cattle had less 2 Star and more 1 Star T1 animals than British breeds.
"Probably all breeds have tenderness genes, but they are more prevalent in cattle known for tenderness," Paschal observed.
A second tenderness test (T2) has been released. GeneSTAR marbling is based on a gene (marker) for thyroglobulin and GeneSTAR Tenderness is based on markers for variants of the calpastatin enzyme that inhibits the muscle tenderizing enzyme calpain.
"The tenderness marker is important, and worth about 14 days of aging," he said.
Paschal said Frontier Beef Systems had developed a TenderGene technology, as well as DoubleBlack—a test which determines if an animal is homozygous black—and Parent Match, "a DNA fingerprinting technology that helps you make sure the parents are who they say they are."
Recently, Merial announced a marker for specific forms of leptin and coding for either cytosin or thymine. Cattle that have both pairs of genes denoted a L-tt (both coding for thymine) tend to be higher quality grading while cattle with both pairs of genes denoted as L-cc (both coding for cytosin) tend to be higher yield grading.
"Cattle can be tested for markers from DNA extracted from blood, semen, or hair follicles," Paschal said. "Current testing procedures are expensive but are attracting attention."
Most importantly, he said marker information can be combined with EPD information to aid in selection by adding precision to the expected results to the calculated EPD.
"You want to be careful that marker assisted selection (MAS) does not become single trait selection," Paschal warned. "Usually, the greatest increase in performance is seen in the first generation produced after a marker (or gene) is introduced into the population. If the heritabilty of the trait is high, MAS is much less effective than traditional selection using EPD."
In addition, Paschal reiterated that genes usually act in concert, and selection for a specific marker could have negative effects on the overall genotype.
