When selecting for improved performance of dairy cattle, breeders may apply a direct selection for the commercial important traits or an indirect selection by using other traits giving information about the performance of the animals. A third possibility is a combined direct and indirect selection.
The additional traits used in indirect selection are also called marker traits. They have always been of interest to breeders in order to identify animals of superior genetic merit and to achieve a faster genetic progress.
In the present paper only marker traits designated as biochemical, physiological, immunological, or cytological will be discussed. Further these will be subdivided with regard to inheritance into traits with simple mendelian inheritance, controlled by major genes (marker genes), and marker traits with a quantitative inheritance. The first group includes the polymorphic traits, while the second group contains most physiological traits. Cytogenetic traits form a third important group, including both markers with simple inheritance and chromosomal deviations.
To be of use in animal breeding the marker traits have to fulfil certain important criteria: 1) simple inheritance or high heritability, 2) close genetic correlation with performance traits of interest, 3) simple testing method with high repeatability, 4) early manifestation. The asso ciation between marker genes and performance traits has to be the result of pleiotropic effects or genetic linkage in order to be applicable in selection. Associations due to sampling error, spurious associations, or common environmental influence have to be sorted out (Petersen, 1979). Some of the marker traits may also be used for many purposes other than indirect selection for performance, such as parentage control, breed comparisons, estimation of heterozygosity, physiological and population genetic studies (see e.g. Baker & Manwell, 1980; Spooner, 1981). These areas will not be discussed in the present paper.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 5. Plenary sessions, , 387–398, 1982
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