In such cases what appears to be a mysterious procedure becomes very simple if we assume that the defective character is associated with the sex-determining factor, or to make it concrete let us say with the _X_-element. The chart shown in Fig. 14, p. 62, indicates what the germinal condition would be under the circumstances. The column to the right represents the maternal, the one to the left the paternal line. Since two _X_ means female and one _X_ male, and inasmuch as we have assumed that the physical basis of the defect to which color-blindness is due is conveyed by the _X_-element, we may represent the defective single _X_ of the male in outline only (see first row). It is obvious that after the reduction divisions (second row) the mature sex-cells of the female will each contain a single normal _X_, the corresponding sex cells of the male will contain either no _X_ or a defective _X_. Since if any member of the class of spermatozoa containing no _X_, fertilizes an egg the resulting zygote (row three) will have but one _X_ and that a normal one, the individual which develops from the zygote will be normal as regards color vision and moreover will be male because the condition one _X_ always means maleness. On the other hand, if any member of the class of spermatozoa containing the defective _X_ fertilizes an egg two _X_-elements are brought together and this of itself means femaleness. In this case one of the _X_-elements is defective but the single normal _X_ is sufficient in itself to produce normal color vision. But when it comes to the maturation of the sex-cells of this female, the pair of _X_-elements are separated in the usual way with the result that half of the mature ova contain a normal _X_ and half a defective _X_ (row four). Since in a normal male, however, the mature reproductive cells will contain either a normal _X_ or no _X_ (fourth row), any one of four different kinds of matings may result. A sex-cell carrying normal _X_ of the male may combine with an ovum containing normal X producing a normal female (row five). Or such a cell may combine with an ovum carrying the defective _X_, also producing a female but one who although of normal color vision herself, like her mother, is a carrier of the defect. On the other hand, any one of the spermatozoa without an _X_ may combine with an ovum containing the normal _X_, in which case a normal male is produced and, moreover, one who, like his mother's brothers, is incapable of transmitting the defect. However, the sperm-cell devoid of an _X_ is just as likely to fertilize an ovum carrying the defective _X_, in which event the resulting individual, a male, must be color-blind because he contains the defective _X_ alone. In other words, the chances are that one-half the sons of a woman whose father was color-blind will be color-blind, the other half perfectly normal; and that all of the daughters will be of normal color vision although one-half of them will probably transmit the