554 



BREEDING 



BREEDING 



Mendel's law of hybrids. 



The preceding discussion represents fairly well the 

 general understanding ef hybrids until about 1900, 

 when DeVries and Correns rediscovered what is now 

 termed "Mendel's law of hybrids." These laws or prin- 

 ciples are of great value from an economic standpoint, 

 and are, furthermore, of the greatest scientific interest. 

 They should thus be thoroughly understood by every 

 practical breeder of plants. It has been known for 

 many years that a splitting-up and redistribution of 

 parental characters occur in hybrids, and it is on this 

 fact largely that the practical application of hybridiza- 

 tion in plant-breeding depended. Until Mendel's law 

 was discovered, however, there was no understanding 

 of why or how such a recombination could be made, and 

 it was necessary to experiment extensively in order to 

 determine what could be accomplished. 



If one carefully studies a number of first-generation 

 hybrids with special reference to the characters of the 

 parents exhibited in the hybrids, it will be found that 

 certain characters possessed by the male parent are 

 plainly represented in the hybrid, while other charac- 

 ters possessed by the female parent are also represented 

 in the hybrid. Many characters of the parents are thus 

 plainly represented in the hybrid, but it is probable that 

 other characters will be blends of the similar parental 

 characters, or possibly differ from any definite characters 

 distinguishable in the parents. Attention has already 

 been called to the complexity of organisms in general 

 and the difficulty of recognizing all of the unit-charac- 

 ters. Thus far it has been possible only to follow 

 carefully certain plainly marked characters. This com- 

 mingling of the different characters of each parent 

 gives the hybrid a mosaic appearance, as if certain 

 characters had been taken from each parent and thrown 

 together to make up a hybrid individual. 



Character-pairs. To understand this commingling of char- 

 acters in the first-generation hybrids, it is necessary to know that 

 the parents used in the hybridization differed from each other in 

 certain characters. One parent may have had red fruits, hairy 

 stems, and dwarf habit, while the other may have had yellow 

 fruits, smooth stems, and tall habit. Such characters are opposed 

 to each other, and such opposed qualities or characters are termed 

 *' character-pairs." A plant may have red fruits and smooth 

 stems, but it could not have red fruits and yellow fruits at the 

 same time. As an illustration of such character-pairs, may be 

 cited, scarlet and yellow fruits of peppers, reversed or erect fruits 

 of peppers (Fig. 645), starchy and sweet kernels of corn, standard 

 and dwarf size in tomatoes, stringy and stringless pods of beans, 

 and the like. Such pairs of characters have been termed by Bateson 

 "allelomorphic pairs of characters," and this terminology is com- 

 monly used in the literature on hybrids. When parents possessing 

 opposed or contrasted characters are crossed, the hybrid egg-cell 

 receives, through the male and female germ-cells uniting in the 

 fecundation, the determiners which represent the different con- 

 trasted pairs of characters, and all cells making up the first-gener- 

 ation hybrid will contain in like manner the determiners repre- 

 senting these characters, and are thus hybrid in nature. This 



645. Pepper plants: a, with scarlet-colored reversed fruits; b, 

 with yellow-colored erect fruits. The number and form of branches 

 are also markedly different. 



being the case, it might be expected that all characters in the 

 hybrid would show as blends of the parental characters or exhibit 

 some stage of intermediacy between the characters of the parents. 

 This is indeed frequently the case, but more commonly one of the 

 characters is very strong, or "dominant," as Mendel expressed it, 

 and only this character will show in the first-generation hybrid, 

 the other character remaining recessive or masked, although 

 present. As an illustration, in the character-pairs mentioned above, 

 scarlet fruits of pepper, reversed fruits of pepper (this is true only 

 in certain varieties), starchy kernels of corn and standard size of 

 tomato plants, are dominant over their corresponding contrasted 

 characters. Illustrations of blended or intermediate characters are 

 found, for example, in first-generation hybrids of round with pear- 

 shaped tomatoes, and large with small fruits of tomatoes or peppers. 



The law of segregation and purity of the germ-cells. The second 

 important principle of Mendel's law is what is termed the law of 

 segregation and purity of the germ-cells. It seems certain from the 

 researches that have been conducted that, when the germ-cella of 

 the first-generation hybrids are formed, the determiners which 

 represent the two different characters under consideration, and 

 which were united by the hybridization, ordinarily segregate 

 again in the cell-divisions, which lead to the formation of the germ- 

 cells, so that certain germ-cells include the determiner of one only 

 of the two characters. There are thus two kinds of genn-celU 

 formed with respect to this one character-pair. Choosing as an 

 illustration a hybrid of a pepper having scarlet fruits with one 

 having yellow fruits (Fig. 645), when the germ-cells were formed 

 a segregation of the determiners representing the two opposed 

 characters would take place and there would be germ-cells of one 

 kind, both male and female, containing the scarlet fruit deter- 

 miners and of a second kind, both male and female, containing the 

 yellow fruit determiners. This segregation takes place in the for- 

 mation of both the egg-cells and the sperm-cells or pollen-grains. 

 It is thus seen that the first-generation hybrid, when two such 

 allelomorphic characters are combined, forms two kinds of egg- 

 cella and two kinds of sperm-cells, so far as this one character-pair 

 is concerned. This segregation of characters, which has been termed 

 the law of segregation, is one of the most important facts of in- 

 heritance and, in enabling us to get recombinations of characters, 

 is of the highest importance in breeding. 



The law of probability in recombination of characters. The third 

 important principle of Mendel's law is what is termed the law of 

 probability, and explains what may be expected in plants of the 

 second generation of such a hybrid. Remembering that there are 

 formed in the first-generation hybrid, as explained above, two kinds 

 of egg-cells and two kinds of sperm-cells with reference to the 

 opposed characters, what would happen if the hybrid were bred 

 with its own pollen; or, in the case of an animal, if it were bred with 

 another hybrid of the same parentage? For the purpose of illus- 

 tration, suppose that a hybrid of a scarlet-fruited pepper with a 

 yellow-fruited pepper be fertilized with its own pollen, and that 

 100 egg-cells be fertilized with 100 pollen-grains of the same 

 hybrid. There are two kinds of egg-cells produced, some carrying 

 determiners of the scarlet fruit, and others determiners of the 

 yellow fruit, and the same is true of the pollen-grains. Taking 

 the egg-cells and pollen-grains without choice, as equal numbers are 

 produced of each kind, one would expect to have of the egg-cells 

 fifty with scarlet determiners and fifty with yellow determiners. 

 In the pollen-grains, also, one would expect to have fifty with scarlet 

 determiners and fifty with yellow determiners. If, then, the 100 

 egg-cells and 100 pollen-grains are brought together in fertilization 

 by chance, as would occur in nature, according to the law of prob- 

 ability, there would be twenty-five scarlet uniting with twenty- 

 five scarlet; twenty-five scarlet uniting with twenty-five yellow; 

 twenty-five yellow uniting with twenty-five scarlet; and twenty- 

 five yellow uniting with twenty-five yellow. Representing scarlet 

 determiners by the capital letter S because scarlet is the dominant 

 character, and the yellow determiners by the small letter y, as 

 yellow is recessive, the unions may be represented as follows: 



ONE HUNDRED EGG-CELLS BY 100 SPERM-CELLS. 

 Female Male Composition 

 Cells Cells of hybrids 



oc a 0= a o- aa (These do not contain determiners 



> 1 of y and will reproduce true. 



I These are hybrids so far as this 

 character - pair is concerned, 

 exactly the same as in the first 

 generation and contain determi- 

 ners of both S and y. TheM' will 



25 y X 25 S = 25 yS I not reproduce true to type and 

 will break up like second -genera- 

 [ tion hybrids. 



{These do not contain the deter- 

 miners of S, and will reproduce 

 true. 



The above illustration explains the law of segregation, and the 

 probable ratio of recombination when hybrids are inbred with 

 their own pollen, and when only one pair of characters is considered. 

 When an egg-cell with scarlet determiners unites with a sperm- 

 cell with scarlet determiners, this gives rise to a pure germ-cejl, 

 or zygote, containing only scarlet determiners, and the progeny in 

 subsequent generations will breed true so far as this character is 

 concerned. Also, when an egg-cell with yellow determiners unites 

 with a sperm-cell with yellow determiners the result is a pure 

 germ-cell, containing only yellow determiners and the progeny 

 would reproduce true, so far as this character is concerned, in sub- 

 sequent generations. In the other two cases, when in fecundation 



