A broad strategy was implemented to bring into perspective over forty years of research on associations between blood group polymorphisms and milk and type traits in dairy cattle, and to analyse the largest data set ever available for this type of study (2.5 million records since the 1940's), in an attempt to make a final assessment regarding the usefulness of blood groups as selection aids in current Holstein breeding schemes and in order to try to formulate general conclusions relative to the approach of resolving quantitative variation into individidual locus effects. Inconsistency of results across different studies and across different analytical designs now pursued emphasize the importance of a problem which has been conveniently ignored in most of the theoretical studies published recently on QTL detection and marker- assisted selection: that posed by the intrinsic non-additivity of genetic and biological phenomena. To ignore this fact leads to inflated expectations and too easy promises regarding the usefulness of genetic markers for selection purposes. The L blood group has been found linked to gene(s) that impact milk component traits (fat and protein yields and percentages). Linear models establishing within-family contrasts, comparison of allele frequencies between extreme and opposite tails of individual families, chi-square tests assessing the occurrence of selection on a within-family basis, changes in allele frequencies across generations, and previous research results, all demonstrate beyond any reasonable doubt the biological validity of this finding.

J. L Rocha, J. F Taylor, J. O Sanders, D. M Cherbonnier

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 19. Selection and quantitative genetics; growth; reproduction; lactation; fish; fiber; meat., , 299–302, 1994
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