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TEXTBOOK OF BOTANY 



the field is very large. In predicting the Fs generation no new problems 



arise. ^ 



Two characters considered simultaneously. If we wish to understand 

 the inheritance of factors when two characters are considered simul- 

 taneously, we have only to apply the same principles previously used 

 for a single character and the facts we know about the behavior of 

 chromosomes during reduction division. The ratios that result from a 

 cross when two independent characters are considered are called dihy- 

 brid ratios. 



Suppose we cross a plant whose genotype is TTRR with a plant whose 

 genotype is ddww. All individuals of the Fi hybrids will have the geno- 

 type TdRw, for they will have obtained one height factor and one flower- 

 color factor from each of the two pure-line parents. Since the factors T 

 and d are in separate homologous chromosomes (one from the pollen 

 parent, the other from the ovule parent) and the factors R and w are 

 likewise in another pair of homologous chromosomes, the hybrid with 

 the genotype TdRw may produce four kinds of sperms and four kinds of 

 eggs with respect to height and flower color (Fig. 210). We may repre- 

 sent the factors in these sperms and eggs as follows: TR, Tw, dR, and 

 dw.'^ If the homologous chromosomes that pair during reduction division 

 always separate and migrate to opposite poles of the spindle, then the 

 factors T and d would not occur in the same sperm or in the same egg; 



2 Mendel, of course, seldom obtained ideal ratios of hybrid progeny in the F^ and F, 

 generations! Some phenotypic ratios actually obtained by him from the F, hybrids are 

 given below. 



Number of Parents. All were Fi Hybrids. 

 The genotype is indicated in parentheses. 



1064 tall plants (Td) 

 929 red-flowered plants (Rw) 

 258 plants from yellow seeds (Yg) 

 253 plants from smooth seeds (Ss) 

 1181 plants with inflated pods (Ic) 



580 plants with green pods (Gy) 



858 plants with axillary flowers (At) 



Number of Progeny of the Fi Hybrids. 

 The F2 generation. 



787 tall and 277 dwarf plants. 2.84:1. 

 705 red- and 222 white-flowered plants. 3.15:1. 

 6002 yellow and 2001 green seeds. 3.01:1. 

 5474 smooth and 1850 rough seeds. 2.96:1. 

 882 plants with inflated pods and 299 plants 



with constricted pods. 2.95:1. 

 428 plants with green pods and 152 plants with 



yellow pods. 2.82:1. 

 657 plants with axillary flowers and 201 plants 



with terminal flowers. 3.14:1. 



Hereditary factors that conditioned red flowers were "linked" with those that conditioned 

 grayish-brown seed coats, and those that conditioned white flowers and white seed coats 

 were "linked." 



^ See footnote, p. 456. 



