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TEXTBOOK OF BOTANY 



kinds of sperms and eggs. One merely needs to visualize the chances 

 of orientation of the three pairs of chromosomes (Fig. 211) and their 

 contained factors during the process of reduction division to see that the 

 factors in the sperms and eggs of this hybrid would be distributed as 

 follow^s: TRY, TRg, TwY, dRy, Twg, dRg, dwY, and dwg. In this case 

 the checkerboard diagram is very large. 



Other problems with hybrids. It is, of course, possible to consider two 

 or more characters simultaneously even though the parents are pure 

 lines for some of them. For example, if the genotype of the parent were 

 TTRwgg the checkerboard would be very small, for there would be 

 only two different kinds of sperms and eggs, with respect to these par- 

 ticular hereditary factors. The paired chromosomes containing height 

 factors contain the same height factor, and the paired chromosomes 

 containing seed-color factors have the same seed-color factor. But the 

 paired chromosomes containing flower-color factors have unlike flower- 

 color factors, and these are the only ones that differ in the sperms and 



eggs. 



What kind of progeny would be obtained by crossing a plant of 

 genotype TTRw with a plant of genotype Tdww? Since it does not mat- 

 ter which way the cross is made, one merely decides for convenience 

 that one of these plants is the pollen parent; the other one the ovule 

 parent. If we decide that the plant of genotype TTRw is the pollen 

 parent, the sperms will have the factors TR and Tw, and the eggs from 

 the ovule parent will have the factors Tw and dw. Hence the progeny 

 have the genotypes TTRw, TTww, TdRw, and Tdww, as shown in the 

 diagram: 



TR 



Tw 



To predict the proportion of the resultant kinds of progeny after 

 either selfing or crossing, one merely applies the foregoing principles 

 to decide (1) what kinds of sperms and eggs can be produced in the 

 parents, and (2) the possible chances of fertilization. It will be excellent 

 practice for the student similarly to predict the nature of the progeny 

 that would be obtained by selfing any one, or by crossing any two, of 

 the types of plants represented in the preceding diagrams. 



