Cytoplasm is the material of a cell that is transmitted from parent to offspring only through the egg, or the maternal side, independent of the cell nucleus. Characters influenced by the cytoplasm respond the same as in the female parent. Cytoplasmic male sterility is, therefore, carried through the maternal side of the line. However, in the presence of the milo cytoplasm, the kafir genes produce sterility and approximately 50 percent of the F 1 are male-sterile. Fertility can be restored at any time by reversing the mating and backcrossing the sterile plants to milo.
Ratios indicate probable proportion of fertile to sterile genes.
An explanation based on the DNA-RNA concept is simple and easily understood if one assumes that the Operon and structural genes controlling sterility are not identical in the milo and kafir group. By continual backcrossing to kafir, sterility is increased but fertility is restored when the plant is backcrossed to the milo group. The DNA-RNA molecular system simply explains partial sterility because DNA may be carried in organelles in the cytoplasm. Cytoplasmic male sterility, therefore, is concerned with the incompatibility between factors in the cytoplasm of the cell and the genes of the nucleus. Genetic sterility is that form involving only the genes in the nucleus of the cell, independent of the cytoplasm.