12 SEX-LINKED INHERITANCE IN DROSOPHILA. 



morph 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 allelo- 

 morphs 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 chromo- 

 some 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. 



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 there- 

 fore not be repeated here. It must suffice to say that if two mutant 



