214 HEREDITARY TRANSMISSION [CH. 



differently, the germ-cells will bear the factors MNO, MNo, 

 Mno, mNO and so on. It is evident, as SUTTON pointed out 

 in 1903, that the correspondence between the distribution 

 in the germ-cells of the factors for Mendelian characters 

 and that of the paternal and maternal chromosomes is so 

 exact as to make it hardly possible to doubt the existence 

 of a causal connection between them. 



In matings in which both parents belong to the same form 

 the homologous chromosomes A and a etc. would of course 

 be alike, and would bear identical Mendelian factors. When, 

 however, a mutation takes place, it must be supposed to 

 arise by a change in, or loss of, a factor in some part of one 

 chromosome. This may be represented by saying that factor 

 M is changed to m. If, then, chromosome a now bears factor 

 m instead of M, there will be Mendelian segregation between 

 the factor M borne by chromosome A and factor m borne 

 by a. This may perhaps be made clearer by a specific in- 

 stance that will be discussed more fully later in this chapter. 

 In the fly Drosophila there are reasons for believing that the 

 factor for the red eye, which is characteristic of the normal 

 fly, is borne by the X (sex) chromosome. A mutant form 

 of the species has white eyes, and the red and white factors 

 are allelomorphic. It is therefore supposed that the factor 

 for white eye is due to a change in, or loss of, that unit in 

 the X chromosome which in the normal fly determines the 

 red-eye character. In the heterozygous (hybrid) fly pro- 

 duced by mating red with white eyed, one X chromosome 

 bears the factor for red, the other for white eyes, and 

 since these chromosomes separate from each other at the 

 maturation of the egg, the red and white factors must 

 segregate in the Mendelian manner. 



There are, however, several difficulties in the way of ac- 

 cepting the hypothesis that the segregation of the chromo- 



