The genetic map of the B system of cattle blood groups as observed in French breeds.

40 cases of irregular inheritance of phenogroups in the B system of cattle blood groups, that were presumed to have arisen from single crossing-over, have been used to establish a partial genetic map of this system. In addition, the relative positions in the map of several antigenic factors were inferred from relationships observed between particular phenogroups occurring in French breeds. The tentative map thus obtained shows an overall similarity to that established by Ruiterkamp et al. (1977) in Dutch Friesian cattle. This result, in addition to other arguments, supports the hypothesis that the genetic structure of the B system of cattle blood groups is basically the same in all taurine breeds. Evidence is given, for the first time, for the occurrence in the B system of genetic events other than single crossing-over (double crossing-over or gene conversion, and possibly deletion). The rate of recombination between the genetic determinants of the terminal factors of the system, Q and I', was calculated in the progeny of some Normande bulls heterozygous for these two determinants (447 gametes); a value of 1.34 centimorgan was obtained. However complementary data indicate that 0.7 centimorgan would be a better estimate.