BREEDING 



BREEDING 



555 



gametes with scarlet determiners unite with gametes with yellow 

 determiners giving the combinations Sy and yS, which amount to 

 the same thing, there re.'^ult in reality, hybrids exactly the same aa 

 in the first generation and the progeny from these in the next 

 generation behave exactly the same as did the first-generation 

 hybrids in the second generation. 



In such a case as the one under consideration, in which the scarlet 

 is a strong dominant character, all combinations that contain the 

 determiners of this character, whether pure or of hyl>rid nature, 

 show this character only. Thus in the above 100 combination>5 the 

 twenty-five yy would come with yellow fruits while the seventy- 



or- 



^1 hr.'BRIO 



Ct»*f-C£LLS 



HYBfflO SCA/^L£T 



646. Representing Mendelian inheritance of scarlet S,and 

 yellow Y, color of fruits in pepper hybrids. 



five other combinations would have scarlet fruits, although fifty 

 of these would be of hybrid nature. To determine which of these 

 seventy-five scarlet-fruited plants are the combination Sy, that is, 

 scarlet with yellow, and which are SS, that is, scarlet with scarlet, 

 requires the growing of self-fertilized progeny from them to deter- 

 mine which are reproduced true to^Iype. as these would be the 

 pure scarlet. The progenies of any of these plants that produced 

 both scarlet- and yellow-fruited plants would show that the parent 

 of such progeny was a hybrid. 



In the hundred combinations there is thus produced a ratio of 

 one pure scarlet to two hybrid scarlet and yellow to one pure yel- 

 low. 1 Sri:2 Sy:l yy. or three scarlets to one yellow and this is the 

 famous 3:1 Mendelian formula. • 



This process of union of an allelomorphic pair of characters 

 in hybridization, the formation of four kinds of gcrm-cells. both 

 male and female, by the hybrid, and their four difTerent unions, is 

 graphically illustrated in Fig. 646. 



While in certain hybrids of parents possessing two opposed 

 parental characters, this ratio of probabilities is not produced, if 

 large numbers are used the ratio will be found in many cases with 

 little deviation. A sufficiently large number of cases have now 

 been studied with various plants and animals to place this con- 

 clusion t>eyond gue-ttion. It is not known, however, how many 

 characters follow Mendel's law, nor is it yet entirely certain whether 

 those charaf-ter-pairs that sometimes follow the law of segregation 

 always follow it. 



The individuals of the second generation which contain the 

 determiners of both characters of the pair, if self-fertilized or bred 

 with similar individuals containing the determiners of both char- 

 acters, exhibit in the third generation exactly the same nature 

 that first-generation hybrids exhibit in the second generation. The 

 two determiners are commingled in their cells, and to all intents 

 and purposes they are exactly the same as first -general ion hybrids. 

 When such self-fertilized hybrids are grown they give again, in 

 the third generation, the regular Mendelian proportion of 1 SS:2 8y: 

 1 yy. Here the individuals containing only determiners of one 

 character, that is. SS and yy, would come true to these characters 

 in succeeding generations, while tliose individuals containing the 

 determiners of both characters, .S and y. would be expected to 

 segregate again in the fourth generation in similar proportions. 



When dealing with more than one eharacter-pair, ratios of seg- 

 regation become complicated but are easily understood. If the 

 character of reversed fruits (R) and erect fruits fe). two plainly 

 marked characters of ordinary garden peppers, caused by the 

 pedicel of the fruit cur\-ing backward in one case and remaining 

 straight in the other, are combined with the above allelomorphic 



characters, it can be foretold exactly what combinations will occur 

 and the relative number of each. This is a second allelomorphic 

 pair of characters that behaves in inheritance the same way as did 

 the two colors of fruit. In this case, the reversed pedicel is the 

 dominant character, as in the Fj hybrids of reversed with erect 

 sorts the peaicels are always or very generally recurved. These 

 characters would thus be represented by It tor the recurved or 

 dominant character and e for the erect or recessive character. In 

 this character-pair one would expect a splitting and segregation to 

 have occurred in the formation of the germ-cells of the first gen- 

 eration so that the hybrid plants of the second generation would 

 exhibit these characters in Mendelian proportions as in the char- 

 acter-pair first described. The progeny in the .second generation 

 would thus exhibit these characters in the following combinations 

 and proportions: 1 RR:2 Ke:l ee. This theoretical proportion 

 should hold rather constantly, either in small or large numbers of 

 hybrids, though in large numbers it would be more nearly realized. 

 The determiners of the four characters, or two character-pairs, are 

 commingled in the cells of the first-generation hybrid. When the 

 egg-cells and pollen-grains are formed, however, a segregation of 

 the determiners of the two character-pairs occurs, but independent 

 of each other. Each egg-cell or pollen-grain will receive only the 

 determiner of one character of a certain character-pair but will, 

 at the same time, receive determiners of other characters belonging 

 to other character-pairs. Considering the two character-pairs 

 described in peppers, an egg-cell receiving the determiner of the 

 scarlet color of fruit S, might also receive the determiners for either 

 R or e representing the characters of recurved or erect fruits. These 

 two character-pairs would thus give egg-cells of four combina- 

 tions, SR, Se, yR, and ye. 



In the formation of the pollen-grains, the same combination 

 occurs, so that with reference to the two character-pairs described, 

 the pollen -grains that would be formed have the same combi- 

 nations of determiners as the egg-cells, namely, SR, Se. yR, and ye. 

 There would thus be four kinds of egg-cells and four kinds of pollen 

 grains so far as these two character-pairs are concerned. If these 

 are brought together, sixteen combinations are possible as follows; 



SRSR 

 SRSe 

 SRyR 

 SRy.- 



SeSR 

 SeSe 

 SeyR 

 Seye 



yRSR yeSR 



yRSe yeSe 



yRyR yeyR 



yRye yeye 



Examining these combinations carefully, and placing together 

 those combinations that contain the same character-determiners 

 as indicated by the letters, and this can properly be done as it 

 does not matter in the fecundated egg whether a certain determiner 

 is furnishe(i lay the egg-cell or the pollen-grain, there result the 

 following nine combinations, all of which are dirferent in germinal 

 constitution with reference to these two character-pairs: 



Table Showing Xumber of Germinal CoMBiNATiONa and Char- 

 acter OF F^ Pepper Hybrids with Two jVllelomorphs. 



An examination of the preceding table, in which arc grouped 



the sixteen possiole combinations when two allelomorphic pairs 

 are concerned in the liybridization will show that among these 

 sixteen there are nine groups with different germinal constitutions. 

 The visual character of the hybrid plants of these nine different 

 groups is given in the tliird column and is easily understood by 

 examining the germinal constitution and remembering that scarlet 

 S, and reversed R. are the dominant characters in the two allelo- 

 morphs and that the presence of one determiner of either of these 

 characters will cause the appearance of that character in the 

 hybrid plant. It will be observed that hy grouping the hybrid 

 plants according to the characters they show, there will be nine 

 scarlet and reversed, three scarlet and erect, three yellow and 

 reversed, and one yellow and erect. This is the Mentlelian formula: 

 9:.3:3:1. The nature of the nine different groups of hybrid plants 

 with different germinal constitution is given in the fourth column of 

 the table. When a character is pure, it m«y he expected to reproduce 

 true in succeeding generations but in those cases in which both 

 determiners of a character-pair are present, the character is of 

 hybrid nature and will segregate in succeeding generations. 



In the illustration of the character-pair, scarlet and yellow fruits 

 and the probable ratio of number of unions in K) hybrids, it was 

 shown that out of 100 unions one should expect 25 SS:50 Sy:25 yy. 

 If now the second character-pair recurved and erect fruits is con- 



