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Abstract
Recent developments in molecular biology and statistics have opened the possibility of identifying and using genomic variation and major genes for the genetic improvement of livestock. Information concerning the basis of these techniques and their applications to the genetic improvement of animals is reviewed. Main marker molecular marker systems in animals (RFPL and microsatellites), genome maps, methods for detecting marker major gene linkages and use of marker assisted selection, genetic fingerprinting and mixture models based on segregation analysis are analyzed. The characteristics where the application of marker assisted selection can be more effective are those that are expressed late in the life of the animal, or controlled by a few pairs of genes. The first example correspond to the longevity and carcass characteristics in meat producing animals, the second, to the resistance to certain diseases or defects of simple inheritance. The detection of major genes using mixture models with segregation analysis can direct the work of identification of DNA marker genotypes towards populations and characteristics with greater probability of detecting a major gene using molecular markers. The present trend indicates that molecular, pedigree and phenotypic information will be integrated in the future through mixture models of segregation analysis that might contain major gene effects through the markers, polygenic inheritance and uses powerful and flexible methods of estimation such as Gibbs Sampling.
Highlights
Recent developments in molecular biology and statistics have opened the possibility of identifying and using genomic variation and major genes for the genetic improvement of livestock
Main marker molecular marker systems in animals (RFPL and microsatellites), genome maps, methods for detecting marker-quantitative trait loci (QTL) linkages and use of marker assisted selection, genetic fingerprinting and mixture models based on segregation analysis are analyzed
Important advances to some of the economically important characters in several species of livestock has been achieved based on phenotypic performance, several limitations of these methods of improvement based on population genetics alone are becoming evident with time
Summary
Use of molecular markers and major genes in the genetic improvement of livestock Hugo H. During the last five decades, the application of methods based on population genetics and statistics allowed the development of animals with a high productive efficiency These systems are based on simplified models of genic action that assume a large number of or genes with small individual effects in the expression of the phenotype (polygenes) and emphasizes the average genic effects (additive effects) over their interactions. The identification of RFLPs requires the use of gel electrophoresis to separates the DNA fragments of differing sizes followed by transfer of the fragments to a nylon membrane (Southern blot) and visualization of specific DNA sequences using radioactive or chemiluminescent probes exposed to an X-ray film (Drinkwater and Hetzel, 1991) This methodology was the standard method in the identification of RFLPs before 1986, situation that limited the identification of the whole genome variation in animals. PCR based Microsatellites techniques facilitated the construction of genome maps in most livestock species because its abundance in the genome, the specificity of the primers, its high degree of polymorphism which several alleles and their easy detection (Albert et al, 1994; Lewin, 1994; Bishop et al, 1995; Smith and Smith, 1993; Stein et al, 1996)
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