PLANT GENETICS 



27 



The diagram illustrates a change in linkage relations due to a 

 cross-over. C and W are located in the same chromosome of one 

 parent and c and w in homologous loci of a similar chromosome 

 of the other parent. If there was perfect linkage the only 

 gametes produced would be CW and cw. Owing to a cross-over, 

 however, Cw and cW are also obtained although less frequently 



FIG. 7. Diagrammatic representation of crossing-over and results. At the 

 left, the two original chromosomes. In the middle, the twisted condition of 

 the chromosomes in synapsis and their subsequent separation. At the right, 

 the four types of chromosomes which result. (After Babcock and Clausen.} 



than the combinations CW and cw. The following outline 

 expresses the result on a percentage basis: 



CW 38.7 percent.; cw 38.7 per cent. cW 11.3 per cent.; Cw 11.3 percent. 



non-cross-over gametes 



cross-over gametes 



Accepting the view that factors are located in particular places in 

 the chromosome, the value of the cross-over hypothesis in explain- 

 ing degrees of factor linkage becomes apparent. If certain com- 

 binations of factors occur with less frequency than others, this 

 means that the breeder must grow a much larger population in the 

 segregating generations in order to obtain the combination desired 

 than when the factors are independently inherited. 



Inheritance of Quantitative Characters. Many of the impor- 

 tant characters of economic plants are size or quantitative 

 characters, such as height of plants, size of seed, or relative date 

 of maturity. It was at first thought that these characters did 

 not follow Mendel's law. The discovery that color characters 

 were frequently due to the interaction of several inherited factors 

 led to the explanation of the inheritance of size characters by 

 similar means. Numerous controlled crosses have been studied. 



