INHERITANCE 737 



In consequence of the independent inheritance of these four pairs of 

 characteristics, there result from crosses certain curious combinations in 

 which there is lack of harmony in the parts of the individuals. Thus the 

 eyespot, S, is inherited independently of the size of the cell (D or d). 

 The eyespot is originally in the large race, D, where its length is about 

 one-sixth the length of the cell. By crosses it may be transferred to the 

 small race, d. Here it retains its large size, so that there are produced 

 small individuals with eyespots about half the length of the individual. 



The fact that from any single zygote but two types or combinations 

 appear among the offspring shows that the reduction of the chromo- 

 somes must occur at the first division of the zygote. If it occurred at the 

 second division, there would be in some cases four different types or 

 combinations from a single zygote. 



For suppose that we have a two-factor cross, such as Fp X^P- Then 

 the zygote has the combination FpfP. If reduction occurs at the first 

 division, in some zygotes the two cells produced are Fp and fP. Each 

 now divides equationally, giving two cells that are Fp, two that are fP. 

 In other zygotes the reduction division yields FP and fp; again the second 

 division yields two cells of each type. In either case but two types are 

 produced from any one zygote. 



But if reduction occurs at the second division, then after the first divi- 

 sion there are two cells present, both with the combination FpfP. Now 

 by reduction at the second division, one of these may yield Fp and fP, 

 the other FP and fp, so that four different combinations would be pro- 

 duced from a single zygote. 



Linkage and crossing over. — Besides the four independent pairs of 

 characters just described, there are in Polytoma others that are linked 

 with some of the four. Such a character is length of the flagella. In the 

 original types, the length of the flagella is proportional to the length of 

 the body; large cells (A, B, C) have long flagella, small cells (D) have 

 short flagella (Fig. 169). This proportionality usually holds among the 

 crosses; those that have large bodies have long flagella, those with small 

 bodies have short flagella. This indicates either that the factors for size 

 (D and d) and those for length of flagella are close together in the same 

 chromosome, and so linked, or that the relation is merely a physiological 

 one, cells of a given size always producing flagella of length propor- 

 tional to the size. 



