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 defect to one-half of their sons. From a glance at the diagram it is readily seen also that a color-blind female could result from the union of a color-blind man (see first row) and the daughter of a color-blind man (see third row). For half of the gametes of such a female would bear the defect as would also that half of the gametes of the male which carry X, hence the expectation would be that half of the daughters of such a union would be color-blind and half would be carriers of color-blindness; and that half of the sons would be color-blind and half normal. All the sons of a color-blind woman would be color-blind because she has only defective X-elements to pass on.
The inheritance of various other conditions in man follows more or less accurately the same course as color-blindness. Among these may be mentioned: hemophilia, a serious condition in which the blood will not clot properly, thus rendering the affected individual constantly liable to severe or fatal hemorrhage; near-sightedness (myopia) in some cases; a degenerative disease of the spinal cord known as multiple sclerosis; progressive atrophy of the optic nerve (neuritis optica); Gower’s muscular atrophy; some forms of night-blindness; in some cases ichthyosis, a peculiar scaly condition of the skin. In one of my own tabulations of a case of inheritance of “webbed” digits or syndactyly, a condition in which two or more fingers or toes are more or less united, a sex-linked inheritance is clearly indicated (Fig. 15), although from the pedigrees recorded by other investigators this condition usually appears in some of both the sons and daughters of an affected individual.
Fig. 15
Chart showing the inheritance of a case of syndactyly after the manner of a sex-linked character. The affected individuals are represented in black; squares indicate males, circles females. The condition is seen to be inherited by males through unaffected females.
The Occurrence of Sex-Linkage in Lower Forms Renders Experiments Possible.—The course followed by such characters in man can be inferred only from the pedigrees we can obtain from family histories. Fortunately, however, such sex-linkage also occurs in lower animals and we are able therefore to verify and extend our observations by direct experiments in breeding. Several sex-linked characters have been found to exist in a small fruit-fly known as Drosophila. Extensive breeding experiments with this fly by Professor T. H. Morgan and his pupils have borne out remarkably the interpretation that the characters in question are really linked with a sex-determining factor.
CHAPTER III
MENDELISM