Before the DNA marker was finally established (Milan et
al., 2000), the differences in chemical composition between
RN phenotypes were used to identify carriers and non-carriers.
Depending on which variable was used, a more or less
pronounced bimodal distribution was obtained, and the valley
between the peaks could be used as a threshold between
phenotypes, i.e. carriers or non-carriers of the RN- allele. The
first trait used was the ‘Napole yield’, as a bimodal distribution
was found, followed by GP (Fernandez et al., 1990). In
GP lactate was included, but in our studies we obtained an
accurate division between phenotypes using the concentration
of glycogen, glucose, glucose-6-phosphate also without
lactate (Lundström et al., 1996).