A Revolution in Dairy Cattle Genetics

Sound the alarm. "One if by land and two if by sea" stated Henry Wadsworth Longfellow in his poem "The Midnight ride of Paul Revere". The redcoats are coming. The redcoats are coming. Today, the SNPs are coming. SNPs (pronounced snips) are actually already here. A SNP is a single nucleotide polymorphism. Translating, a SNP is a small change in DNA, the genetic material of life. The technology is now available to process large amounts of DNA and detect how individuals vary. The challenge that remains is to associate those small changes in DNA with changes in performance of economically important traits in cattle. A panel of 50,000 SNPs is being developed to assist cattle selection. Dr. Curt Van Tassell of USDA is leading the effort to identify which SNPs are part of the panel. It is my understanding that these SNPs will be widely dispersed across the genome. It is anticipated that genomic selection can be practiced on the SNPs that an individual possesses. A SNP is basically a change of a single nucleotide in DNA. The four nucleotides, also known as bases, are adenine (A), cytosine (C), guanine (G), and thymine (T). The DNA molecule consists of two paired strings of nucleotides.

One historical example will demonstrate advantages of SNPs. Earlier research at Illinois dealt with a lethal defect called "Deficiency of Uridine Monophosphate Synthase" or DUMPS. Although DUMPS was a lethal condition, it was not quickly eliminated form the Holstein population initially. A DNA test based on a change at a single nucleotide polymorphism facilitated rapid removal of the undesirable DUMPS gene from the population. The following numerical example documents the extent of benefit.

Consider that you have a herd of 100 Holstein cows, none of which have the DUMPS gene. The frequency of the DUMPS gene is zero and you would not lose any embryos to DUMPS. However, if you mate your cows to one bull that is a carrier of DUMPS, half the resulting offspring would be carriers of DUMPS. They would not have died because two DUMPS genes are needed for the lethal effect. In the offspring, from mating your 100 cows to one DUMPS carrier bull, the frequency of the DUMPS gene would be 25%. Half the offspring have one of two paired genes that are the DUMPS gene. If the offspring were mated to another bull that also was a carrier of DUMPS, 12.5% of the embryos would die from having two copies of the DUMPS gene. Obviously, using a DNA based test such as SNP and identifying male carriers of DUMPS could avoid the second mating to a DUMPS carrier.

Yes, but it would take awhile. Consider the population of offspring with the frequency of the DUMPS gene at 25%. If the population was random mated, the frequency of the DUMPS gene would reduce to 1% after 96 generations. If each generation interval is about 5 years, the reduction of frequency of DUMPS gene from 25% to 1% would take about 480 years.

To repeat, the frequency of the DUMPS gene could be increased to 25% in one generation, but waiting for the lethal gene to reduce its frequency to 1% would take almost five centuries. Selecting for a SNP is an alternative. A SNP identifies which animals are carriers of specific genes. If the genes are undesirable, the animals with those genes can be eliminated in one generation.

Using a SNP to eliminate DUMPS from the Holstein population is an easy decision to make. The benefit of using SNPs to select or eliminate other individuals is an area of future research. Most SNPs are not associated with a lethal condition. The hope is that many SNPs can be associated with favorable outcomes. To find these associations, many animals will be needed that have both SNP and phenotypic data. Selection based on SNP's is called genomic selection. It has the potential to revolutionize selection of dairy cattle.

�� Decision to keep or cull an animal could be made earlier in an animal's life.

�� Expensive progeny testing would not be required.

�� Generation interval could be reduced resulting in more rapid genetic gain.

�� Variability available for future genetic gain would be reduced.

�� The rate of genetic gain would diminish over time.

�� Incorrect associations between phenotypes and the SNPs could lead to unexpected results, some of which would be unfavorable.

�� Geneticist would have increased job security because the associations between phenotypes and SNPs would need to be monitored routinely.

�� The panel of SNPs could be useful in selecting germ plasm for genetic conservation. Only diverse gene plasm would need to be stored.