108 (iENKTICS IN RELATION TO AGRICULTURE 



w, and c and W, although the reverse of those in the former case, tend 

 to remain together in the same ratio. 



When Fi plants of the genetic constitution, (Cw)(cW), are selfed 

 segregation occurs in t\ as shown in the checkerboard in Fig. 50. When 

 like phenotypes are collected into classes, the following distribution is 

 obtained : 



39.72 with purple aleurone and starchy endosperm 

 18.36 with purple aleurone and waxy endosperm 

 18.36 with white aleurone and starchy endosperm 

 1.00 with white aleurone and waxy endosperm. 



This ratio is strikingly different from that obtained. for the former cross, 

 although exactly the same characters are involved. Unfortunately data 

 supporting this part of the analysis have not yet been presented in a 

 satisfactory manner, but the results so far as reported do show a positive 

 linkage between the factors. Moreover other cases which we shall discuss 

 in this chapter demonstrate beyond doubt that the relations described 

 above hold rigidly for cases of factor linkage. The different results 

 obtained when factors enter a cross in different combinations are, there- 

 fore, simply due to the fact that the original combinations tend to be 

 preserved in segregation in a definite fixed proportion of gametes. 



To give a chromosome interpretation of linkage we assume that the 

 factors linked are borne in the same chromosome. Thus the factor for 

 purple aleurone color is one of the chromomeres occupying a definite 

 locus in a particular pair of chromosomes in a purple starchy race of 

 corn and the factor W for starchy endosperm occupies a different locus 

 in these same chromosomes. In Fig. 51 the chromosome behavior in 

 linkage is shown graphically. In the hybrid one member of a pair of 

 chromosomes bears the factors C and W, the other member c and w. 

 During synapsis these chromosomes conjugate, and when the threads 

 representing the two chromosomes separate after conjugation they may 

 in consequence of their twisted condition break at certain points and, 

 reuniting, the free ends of different threads may join together. In a cer- 

 tain percentage of cases this breaking of the filaments may occur between 

 C and W, so that the chromosomes afterward reconstituted will contain 

 the factors C and w, and c and W rather than the original combinations. 

 More frequently the chromosomes will untwist without exchanging chro- 

 matin material or after having exchanged it in such a way as not to dis- 

 turb the original factor combinations. Exchange of chromatin material 

 between homologous chromosomes is called crossing-over. This term 

 is also applied to the formation of new combinations of linked factors, 

 and these new combinations are called cross-overs. In this particular 

 case the end result is that for the factors C and W and their allelomorphs 



