There is no way from Mendelian data by which this difference between a true case of multiple allelomorphs and one of complete linkage (as just illustrated) can be determined. There is, however, a different line of attack which, in a case like that of Drosophila, will give an answer to this question. The answer is found in the way in which the mutant factors arise. This argument has been fully developed in the book entitled "The Mechanism of Mendelian Inheritance," and will therefore not be repeated here. It must suffice to say that if two mutant
types that behave as allelomorphs of each other arise separately from the wild form, one of them must have arisen as a double mutation of two factors so close to each other as to be completely linked—a highly improbable occurrence when the infrequency of mutations is taken into consideration.[[1]] The evidence opposed to such an interpretation is now so strong that there can be little doubt that multiple allelomorphs have actually appeared.
On a priori grounds there is no reason why several mutative changes might not take place in the same locus of a chromosome. If we think of a chromosome as made up of a chain of chemical particles, there may be a number of possible recombinations or rearrangements within each particle. Any change might make a difference in the end-product of the activity of the cell, and give rise to a new mutant type. It is only when one arbitrarily supposes that the only possible change in a factor is its loss that any serious difficulty arises in the interpretation of multiple allelomorphs.
One of the most striking facts connected with the subject of multiple allelomorphs is that the same kind of change is effected in the same organ. Thus, in the quadruple system mentioned above, the color of the eye is affected. In the yellow-spot system the color of the body is involved. In mice it is the coat-color that is different in each member of the series. While this is undoubtedly a striking relation and one which seems to fit well with the idea that such effects are due to mutative changes in the same fundamental element that affects the character in question, yet on the other hand it would be dangerous to lay too much emphasis on this point, because any given organ may be affected by other factors in a similar manner, and also because a factor frequently produces more than a single effect. For instance, the factor that when present gives a white eye affects also the general yellowish pigment of the body. If red-eyed and white-eyed flies are put for several hours into alcohol, the yellowish body-color of the white-eyed flies is freely extracted, but not that of the red-eyed flies. In the living condition the difference between the body-colors of the red- and of the white-eyed flies is too slight to be visible, but after extraction in alcohol the difference is striking. There are other effects also that follow in the wake of the white factor. Now, it is quite conceivable that in some specific case one of the effects might be more striking than the one produced in that organ more markedly affected by the other factor of the allelomorphic series. In such a case the relation mentioned above might seemingly disappear. For this reason it is well not to insist too strongly on the idea that multiple allelomorphs affect the same part in the same way, even although at present that appears to be the rule for all known cases.
SEX-LINKED LETHALS AND THE SEX RATIO.
Most of the mutant types of Drosophila show characteristics that may be regarded as superficial in so far as they do not prevent the animal from living in the protected life that our cultures afford. Were they thrown into open competition with wild forms, or, better said, were they left to shift for themselves under natural conditions, many or most of the types would no doubt soon die out. So far as we can see, there is no reason to suppose that the mutations which can be described as superficial are disproportionally more likely to occur than others. Of course, superficial mutations are more likely to survive and hence to be seen; while if mutations took place in important organs some of them would be expected to affect injuriously parts essential to the life of the individual and in consequence such an individual perishes. The "lethal factors" of Drosophila may be supposed to be mutations of some such nature; but as yet we have not studied this side of the question sufficiently, and this supposed method of action of the lethals is purely speculative. Whatever the nature of the lethals' action, it can be shown that from among the offspring obtained from certain stocks expected classes are missing, and the absence of these classes can be accounted for on the assumption that there are present mutant factors that follow the Mendelian rule of segregation and which show normal linkage to other factors, but whose only recognizable difference from the normal is the death of those individuals which receive them. The numerical results can be handled in precisely the same way as are other linkage results.
There are some general relations that concern the lethals that may be mentioned here, while the details are left for the special part or are found in the special papers dealing with these lethals. A factor of this kind carried by the X chromosome would be transmitted in the female line because the female, having two X chromosomes, would have one of them with the normal allelomorph (dominant) of the lethal factor carried by the other X chromosome. Half of her sons would get one of her X's, the other half the other. Those sons that get the lethal X will die, since the male having only one X lacks the power of containing both the lethal and its normal allelomorph. The other half of the sons will survive, but will not transmit the lethal factor. In all lethal stocks there are only half as many sons as daughters. The heterozygous lethal-bearing female, fertilized by a normal male, will give rise to two kinds of daughters; one normal in both X's, the other with a normal X and a lethal-bearing X chromosome. The former are always normal in behavior, and the latter repeat in their descendants the 2:1 sex-ratio.
Whether a female bearing the same lethal twice (i.e., one homozygous for a given lethal) would die, can not be stated, for no such females are obtainable, because the lethal males, which alone could bring about
such a condition, do not exist. The presumption is that a female of this kind would also die if the lethal acts injuriously on some vital function or structure.