Trisomic Inheritance in the Poinsettia Mutant of Datura

Abstract

(1) The (2n + 1) mutant Poinsettia in Datura Stramonium is briefly described. (2) The frequency of its new occurrence in offspring of 2n parents is about once in two to three thousand individuals. (3) The extra chromosome responsible for the peculiarities of the Poinsettia mutant, (a) is not transmitted through the pollen or to only a slight extent; (b) is transmitted through only about 30 per cent. of the egg cells; (c) is transmitted to a slightly larger proportion of the offspring when the Poinsettia parent is pollinated from a normal or from another Poinsettia than when it is selfed. (4) It is, therefore, concluded: (a) that the condition in the Poinsettia is similar to that previously found for the Globe mutant in that gametes with the extra chromosome are less viable than those with the normal (n) number, and (b) that crossing the mutant with a normal or with another Poinsettia in comparison with selfing probably gives increased vigor and therefore increased survival value to the (2n + 1) zygotes which result. (5) The theoretical ratios are given for trisomic inheritance based upon non-transmission of the extra chromosome through the pollen and upon the assumption of random assortment of chromosomes within the trisome. (6) Both the phenotypic and genetic ratios for the inheritance of purple pigmentation in the offspring of Poinsettia parents agree closely with the theoretical ratios. (7) It is therefore concluded: (a) that Poinsettia has its extra chromosome in the set which carries the genes for purple and white flower color, and (b) that the three chromosomes in this set assort at random.