The external mechanics of the chromosomes IV—Abnormal mitosis and meiosis

Abstract

The nuclear divisions in the pollen grains are of special importance because each pollen grain is a genetical individual genetically distinct from all others. A study of pollen grain behaviour is therefore a genetical experiment of a simple kind. In the pollen grains of a hybrid we can not only distinguish the individual segregates which differ in the form and number of their chromosomes, but we can also compare their viability. We may even find differences in the mechanical behaviour of the chromosomes in the segregates, depending on their own genetic differences or conditioned by the variable reaction of the genotype of the parent plant. Such differences usually cripple or kill the organism and are therefore never available for study in somatic mitosis; we know of none that is compatible with survival. An apparent exception, Beadle’s (1932) “ sticky chromosome ” mutant in maize, probably owes its character merely to a high rate of structural change. We may, on the other hand, expect to find them in the sexually specialized cells such as pollen grains, especially in species whose methods of propagation enable them to dispense with sexual reproduction. Beadle (1931, 1933, b ) has discovered a number of such genetic abnormalities in pollen grains, abnormalities of great theoretical significance.