Dairy cattle genomics: Tools to accelerate genetic improvement?

Traditional selection based on genetic merit calculated from phenotypic and pedigree information has been tremendously effective at improving production in dairy cattle. Hypothetically, genetic improvement could be accelerated even further for yield and other economically important traits by directly selecting upon the genetic differences underlying the phenotypes. To elucidate these genetic differences, research strategies based on genomic science have been developed to identify economic trait loci (ETL). Once resolved with respect to position in the genome, DNA markerbased tests that identify ETL can be practically applied to enhance selection in a commercial setting. To date, most dairy-related ETL have been detected in Holstein grandsire families using the granddaughter design. Because the marker intervals identifying these ETL are not resolved well enough for accurate selection in current populations, ETL analyses have been or are being extended to include ancestral animals that connect family pedigrees and current generations of nonprogenytested animals from within the founder animal pedigree. Increasing genotypic and phenotypic information in this manner alleviates two statistical limitations often associated with ETL interval refinement in experimental populations. First, population size is no longer limited or biased by previous selection. In addition, the inheritance of ETL can be traced from historic families of interest to current generations relevant to the industry. After allele frequency and contribution to phenotype are determined in current populations, those ETL most beneficial for the industry can be accurately used for selection. As an aid to ETL mapping in dairy cattle, efforts have been initiated to catalog as many bovine genes as possible by generating expressed sequence tags (EST) from cDNA clones. These clones represent gene expression at the mRNA level in various tissue types present in cattle. Mapping EST with sequence identity to human genes onto the bovine genetic map is improving the comparative map between species and will aid future investigations in determining genes that underlie ETL. Furthermore, cDNA microarrays constructed with the aid of EST data can be used for hybridization analysis to characterize gene-expression patterns and identify genetic pathways important for animal production and udder health.