We have already seen that the former is always female, the latter male. It thus becomes possible to distinguish the sex of an embryo by counting the chromosomes of its cells. This has been accomplished in several cases.
In some instances[1] the conditions may be much more complex than the ones indicated—too complex in fact to warrant detailed discussion in an elementary exposition—but the principle remains the same throughout, the very complexity when thoroughly understood, strengthening rather than weakening the evidence. In a few forms an interesting reversal of conditions has been found in that the eggs instead of the spermatozoa show the characteristic dimorphism.
Just what the exact relationship between sex-differentiation and the X-element is has never been clearly established. It is possible that this element is an actual sex-determinant, in the ordinary cases one X-element determining the male condition and two X-elements producing the female condition. On the other hand it might be argued that it is not the determining factor but the expression of other cell activities which do determine sex; that is, a sex accompaniment. Or again, it may be one of several essential factors which must cooperate to determine sex.
SEX-LINKED CHARACTERS
The discovery of the remarkable behavior of certain characters in heredity which can only be plausibly explained by supposing that they are linked with a sex-determining factor still further strengthens our belief in the existence of such a definite factor. Such characters are commonly termed sex-linked characters.
Sex-Linked Characters in Man.—Since there are a number of them in man we may choose one of these, such as color-blindness, for illustration. The common form of color-blindness known as Daltonism in which the subject can not distinguish reds from greens, a condition which seems to be due to the absence of something which is present in individuals of normal color vision, is far commoner in men than in women. Its type of inheritance, sometimes termed “crisscross” heredity, has been likened to the knight moves in a game of chess. The condition is transmitted from a color-blind man through his daughter to half of her sons. Or, to go more into detail, a color-blind father and normal mother have only normal children whether sons or daughters. The sons continue to have normal children but the daughters, although of normal vision themselves, transmit color-blindness to one-half of their own sons. If such a woman marries a color-blind man, as might easily happen in a marriage between cousins, then as a rule one-half her daughters as well as one-half her sons will be color-blind.
Fig. 14
Diagram illustrating the inheritance of a sex-limited character such as color-blindness in man on the assumption that the factor in question is located in the sex-chromosome (from Loeb after Wilson). The normal sex-chromosome is indicated by a black X, the one lacking the factor for color perception, by a light X. It is assumed that a normal female is mated with a color-blind male.