INTRODUCTORY. I 3 



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.^ The evidence opposed to such an interpretation is 

 now so strong that there can be Httle 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 mul- 

 tiple 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 muta- 

 tive changes in the same fundamental element that affects the char- 

 acter 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 fac- 

 tor 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. 



'For a fuller discussion see "The Mechanism of Mendelian Heredity" by Morgan, Sturtevant, 

 MuIIer, and Bridges. Henry Holt & Co., 1915. 



