"A marker is a very short
stretch of DNA, probably 100 to 200 nucleotides long," Ashwell explains.
It helps if you think of DNA as the alphabet that spells out an organism's
traits and innermost workings. The nucleotides, then, are the chemical letters
making up that alphabet. "The differences you see in markers are sequence
differences in nucleotides," she says.
Using this technology, the Beltsville team in 1995 was first to stake a
claim on a region of chromosome 23 where genes may mediate a cow's response to
mastitis. Susceptible animals can acquire this costly disease through
Staphylococcus and other bacterial infections of their udder.
Dairy producers lose an estimated $2 billion annually to the disease.
Ideally, with marker-assisted selection techniques, commercial breeders could
select for resistance traits based on such indicators as the animals' somatic
(white blood) cell scores. This refers to the concentration of cells per
milliliter of cow's milk. Ashwell cautions they're still uncertain how
"statistically significant" the finding is and says it needs to be
studied more in contemporary animals.
In the meantime, the team has taken their molecular prospecting to other
chromosome regions. One of those, located on chromosome 27, is associated with
"dairy form." This trait describes a cow's physical appearance, and
may also be an indicator of ketosis, a metabolic disorder that typically
affects cows with newborn calves. For ketosis, the scientists' goal is to come
up with a fast, accurate method of identifying animals less likely to suffer
the disorder.
Determining Traits Expressed by Genes
Whether dairy form or ketosis, the challenge for Ashwell and her colleagues
is linking what's seen on chromosomes in the lab with physical evidence in the
animals. In some cases, the team's findings run parallel to those of other
researchers. At Pennsylvania State University, for example, geneticist Gary
Rogers is investigating relationships among measures for body fat deposition
and incidence of disease in dairy cows. Farther west, at ARS' U.S. Meat Animal
Research Center in Nebraska, geneticist Eduardo Casas is focusing attention on
marbling traits in beef carcasses. Marbling affects the tenderness of meat cuts
like steak and is associated with an animal's fat content. Based on this,
Rogers' data, and marker work at Beltsville, "we think the same gene or
gene cluster could be involved in marbling in beef cattle and fat content in
dairy cows," reports Ashwell.
She is quick to credit the ongoing work of human genome mappers for helping
them blaze the trail in bovine studies. Much of that's because animal
researchers can extrapolate data from human genome mapping to zero-in on the
equivalent on cow chromosomes.
For example, "using fluorescent tags, we use markers from bovine
chromosome 27 (there are 29 total in cattle, plus the two sex chromosomes, X
and Y) and see what region that corresponds to in human chromosomes," says
Ashwell. "So much is known about human chromosomeswe try to feed off
that knowledge and identify candidate genes that may explain the differences in
dairy cattle."
But selecting these candidate genes isn't always easy. For example, the
Beltsville team recently identified an association between DNA markers on
chromosome 6 and the amount of protein in milk. Many of these milk proteins are
called caseins.
"These genes would be obvious candidate genes that could have explained
the differences we see in protein content," Ashwell says. "But the
casein genes are located close to the chromosome's bottom, and we're seeing our
effect at a different location on the chromosome much further up. This suggests
that there are several different genes that affect important traits like
protein content."
Eventually, information gleaned from this genetic riddle may offer a way for
breeders to select animals whose milk contains even more protein. Casein
proteins are especially important for cheesemaking. But some caseins are better
suited than others to this. With the right genetic tools, Ashwell ventures,
there may come a day when cows are bred for milk that contains specific
proportions of these cheese-friendly proteins.
By marking off chromosome regions, scientists can begin creating a genetic
roadmap that can be used to put desirable traits into cows. "If we can get
this mapping technology perfected," says Ashwell, "we're going to
breed better and better cows."By Jan Suszkiw, Agricultural Research
Service Information Staff.
This research is part of Animal Genomes, Germplasm, Reproduction, and
Development, an ARS National Program (#101) described on the World Wide Web at
http://www.nps.ars.usda.gov/programs/appvs.htm.
Melissa S. Ashwell,
Curtis P. Van Tassell, and Tad S. Sonstegard are with the
Gene Evaluation and Mapping
Laboratory, 10300 Baltimore Ave., Bldg. 200, Room 8, Beltsville, MD 20705;
phone (301) 504-8543, fax (301) 504-8414.
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