The morphological and anatomical characteristics of rachis disarticulation were studied in representative species and strains of Triticum and Aegihps, having different doses of the Q, locus. From the study it is concluded that the fragility of the rachis is governed by several morphological and anatomical factors, each one acting in a + or — direction. The additive effect of these factors ultimately decides the extent of fragility. It appears that all the three genomes A, B and C, have contributed factors for fragility to the hexaploid wheats. The A genome donor T. monococcum or T. aegilopoides has W sype of breaking system. The same is true with the B genome donor Ae. speltoides. But the C genome donor Ae. squarrosa possesses sth B type breaking system. This explains why all the tetraploid wheats have only the W type breaking system, while among hexaploids the W type breaking system occurs in some and both W and B systems in others. The expression of a particular type of fragility (W or B or both) is a function of the combination of a particular set of factors. Once-such a combination is achieved the further functioning of these factors resulting in the fragility of the rachis is conditioned by the presence of the appropriate dose of q. Q, effect over fragility is neutral. When Q or Qo condition is present, the fragility does not find expression. This is not because of the presence or absenc of Q, but because of the absence of q in the required dose.
