CHAPTER XXXVIII 

 CROSS-FERTILIZATION AND HYBRID SEGREGATION 



<^0<><X><J><><>C><£><><^OC><c>^^ 



From the facts presented in the preceding chapter it must be evident 

 that heredity is definitely related to several basic processes in cell divi- 

 sion. The characters we see developing in a plant as it grows are condi- 

 tioned by the hereditary factors that are transmitted from one generation 

 to the next in the sperms and eggs. Many of these hereditary factors 

 we now know to be the genes located in the different chromosomes. We 

 have seen how the chromosomes with their particular hereditary factors 

 assemble in pairs and then become separated during reduction division, 

 and are still separated in the spenns and eggs; and how they are brought 

 together again in different combinations in the fertilized egg. 



As a consequence of this definite pairing, segregation, and recombina- 

 tion of chromosomes during sexual reproduction, the progeny of a hybrid 

 plant are not all alike. When our attention is centered upon only one 

 character, such as height of plant or flower color, the different kinds of 

 hybrid progeny may appear in simple proportions such as 1:2:1, 3:1, 

 and 1:1 as described in the preceding chapter. Since Mendel was the 

 first to discover these definite ratios of the different progeny of hybrids 

 they are often referred to as Mendelian ratios. 



Owing to the fact that during reduction division the orientation of 

 the pairs of chromosomes on the spindle of the dividing cell occurs at 

 random, and the subsequent union of the different kinds of sperms and 

 eggs also occurs at random, these ideal mathematical ratios are seldom 

 olDtained in experiments. One seldom obtains an exact 1:1 ratio of heads 

 and tails when a penny is tossed several times, or an exact 1:2:1 ratio 

 when two penines are tossed simultaneously a certain number of times. 

 The ideal ratio, 5:5, is occasionally obtained when a penny is tossed 10 

 times, but sometimes deviations as great as 9:1 or 1:9 occur. As the 

 number of tosses is increased, the magnitude of deviation from the ideal 

 ratio decreases. 



In the study of heredity we must recognize the role of chance during 

 reduction division and fertilization. Many other processes in chromo- 



458 



