108 GENETICS 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 F, plants of the genetic constitution, (Cw)(cW), are selfed 
segregation occurs in J’, 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. Ina 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 ¢ 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 
Digitized by Microsoft® 
