Gene Mapping

"Community, Identity, Stability," describe the Brave New World conceived by Aldous Huxley in 1932; today we stand on the brink of a brave new world characterized by the possibilities contained inside a double helix.

Recent technological advances in human and animal genetics have lead to sequencing the human genome, mapping the severe acute respiratory syndrome (SARS) virus and in the near future, the release of the completed bovine genetic code.

Dr. James E. Womack, a distinguished professor at the College of Veterinary Medicine, Texas A&M University has spent 22 years in cattle genetics analyzing comparative maps that allow him to use the human genome as a rough key to unlocking the cattle code.

"One of the great things of working with a species like cattle is that we have the human model to follow," Womack said. "We've had to develop a few new technologies specific to cattle, particularly in the statistical analysis and the breeding structure of families but as far as laboratory bench technologies, we've pretty much followed the lead of human genetics."

When Womack began studying the bovine genome he was actively comparing the human and mouse genomes and trying to understand the evolutionary events that might have made our genomes different. In 1982, he decided it was necessary to add a third group to the equation that didn't consist of primates or rodents, and being from Texas, it was logical to choose the bovine.

"Cattle have been difficult to map because like humans, they breed slow and do not produce large numbers of offspring, however, the technologies developed for humans were generally applicable to cattle as well," Womack said.

Genetic mapping is an attempt to find mileposts along the genome of a particular organism. The genome is the total genetic material contained in every cell of a species and the DNA that makes up the genome serves as coding for a particular animal. Determining the exact sequence of the chemicals contained in DNA involves a string of 3 billion figures.

"Genetic sequencing is the ultimate map; knowing every inch of the road," Womack said. "Mapping as we currently do it in cattle is not finding every inch of the road and defining it, but finding mile posts along the way and the functional elements that actually code for something like disease resistance."

In comparative mapping, Womack studies, for example, chromosome 1 on the human genome and by comparison, identifies what genes on chromosome 1 match up in the bovine. This may mean that chromosome 1 in the human matches chromosome 3 in cattle. By using this method, if a gene coding for human disease resistance is found on chromosome 1 then Womack can look at the comparative map and hypothesize that a similar gene may be found on chromosome 3 in cattle. This comparison may help to develop a better understanding of how viruses and bacteria affect cattle.

Instead of sequencing the host species, some researchers focus on the virus or bacteria affecting that group. Sequencing a viral genome, like SARS, is typically easier than an entire species because it is very small with just a few thousand bases as oppose to billions. However, viruses have the unique ability to rapidly evolve into resistant stages making it difficult to develop an effective antibiotic treatment. In the case of the SARS virus, Womack believes a new strain of a pre-existing virus genetically changed into a more virulent form. Knowing more about the human genome and the virus sequence may help slow the spread of this and other diseases.

"Understanding infection involves understanding the pathogen and the host which both fall under genetic influence," Womack stated.

There are an estimated 30,000 - 40,000 genes in the mammal all of which have approximately the same amount of genetic information whether it be cattle or human. Finding where those genes are on a genetic map is a big step to finding the gene and determining what changes the gene makes in a sick or healthy person or animal.

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Angela G. Clendenin
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