to imply that new characters have not appeared, or that those that have appeared must be due to loss in the sense of absence of something from the germ-plasm.
In regard to the first point, several of the mutants are characterized by what seem to be additions. For example, the eye-color sepia is darker than the ordinary red. At least three new markings have been added to the thorax. A speck has appeared at the base of the wing, etc. These are recessive characters, it is true, but the character "streak," which consists of a dark band added to the thorax, is a dominant. If dominance is supposed to be a criterion as to "presence," then it should be pointed out that among the mutants of Drosophila a number of dominant types occur. But clearly we are not justified by these criteria in inferring anything whatever in regard to the nature of the change that takes place in the germ-plasm. Probably the only data which give a basis for attempting to decide the nature of the change in the germ-plasm are from cases where multiple allelomorphs are found. Several such cases are known to us, and two of these are found in the X chromosome group, namely, a quadruple system (white, eosin, cherry, red), and a triple system (yellow, spot, gray). In such cases each member acts as the allelomorph of any other member, and only two can occur in any one female, and only one in any male. If the normal allelomorph is thought of as the positive character, which one of the mutants is due to its loss or to its absence? If each is produced by a loss it must be a different loss that acts as an allelomorph to the other loss. This is obviously absurd unless a different idea from the one usually promulgated in regard to "absence" is held.
MULTIPLE ALLELOMORPHS.
It appears that Cuénot was the first to find a case (in mice) in which the results could be explained on the basis that more than two factors may stand in the relation of allelomorphs to each other. In other words, a given factor may become the partner of more than one other factor, although, in any one individual, no more than two factors stand in this relation. While it appears that his evidence as published was not demonstrative, and that, at the time he wrote, the possibility of such results being due to very close linkage could not have been appreciated as an alternative explanation, nevertheless it remains that Cuénot was right in his interpretation of his results and that the factors for yellow, gray, gray white-belly, and black in mice form a system of quadruple allelomorphs.
There are at least two such systems among the factors in the first chromosome in Drosophila. The first of these includes the factor for white eyes, that for eosin eyes, and that for cherry eyes, and of course that allelomorph of these factors present in the wild fly and which when present gives the red color. In this instance the normal
allelomorph dominates all the other three, but in mice the mutant factor for yellow dominates the wild or "normal" allelomorph.
The other system of multiple allelomorphs in the first chromosome is a triple system made up of yellow (body-color), spot (on abdomen), and their normal allelomorph—the factor in the normal fly that stands for "gray."
In general it may be said that there are two principal ways in which it is possible to show that certain factors (more than two) are the allelomorphs of each other. First, if they are allelomorphs only two can exist in the same individual; and, in the case of sex-linked characters, while two may exist in the same female, only one can exist in the male, for he contains but one X chromosome. Second, all the allelomorphs should give the same percentages of crossing-over with each other factor in the same chromosome.
It is a question of considerable theoretical importance whether these cases of multiple allelomorphs are only extreme cases of linkage or whether they form a system quite apart from linkage and in relation to normal allelomorphism. It may be worth while, therefore, to discuss this question more at length, especially because Drosophila is one of the best cases known for such a discussion.
The factors in the first chromosome are linked to each other in various degrees. When they are as closely linked as yellow body-color and white eyes crossing-over takes place only once in a hundred times. If two factors were still nearer together it is thinkable that crossing-over might be such a rare occurrence that it would require an enormous number of individuals to demonstrate its occurrence. In such a case the factors might be said to be completely linked, yet each would be supposed to have its normal allelomorph in the homologous chromosome of the wild type. Imagine, then, a situation in which one of these two mutant factors (a) enters from one parent and the other mutant factor (b) from the other parent. The normal allelomorph of a may be called A. It enters the combination with b, while the normal allelomorph B of b enters the combination with a. Since b is completely linked to A and a to B, the result will be the same as though a and b were the allelomorphs of each other, for in the germ-cells of the hybrid aBAb the assortment will be into aB and Ab, which is the same as though a and b acted as segregating allelomorphs.