May 6, 1911 



HORTICULTURE 



fiK9 



MENDEL'S LAW AS RELATED TO 

 HEREDITY AND BREEDING. 



By Dr. Herrmann Decker. TransLiteil from 



"Cosmos by Gustave Thommen. 



II. 



When Mendel began his experiments 

 he intentionally selected one factor 

 only for his first trials. He decided to 

 find out what principles determined 

 the height of plants. There are tall 

 sorts and dwarf sorts of peas, and 

 Mendel was well aware that the differ- 

 ence in size is not a result of any suffi- 

 cient or insufficient nourishment or 

 any certain condition of the soil. He 

 knew, of course, that the tallness or 

 dwarfness of each resided in itself as 

 a characteristic quality, and that these 

 dispositions are always present in the 

 seed. 



Mendel transferred the pollen from 

 a dwarf sort to the stigma of a tall 

 one, and waited. Popular belief would 

 expect to get as a result from such a 

 cross (tall and dwarf) medium sized 

 plants. But the seed from his cross 

 gave him plants of one size only. They 

 were all tall. Where now was the id- 

 fluence of the dwarf sort? Had it dis- 

 appeared completely? Was the pollen 

 parent, the father, of no consequence 

 whatever? Mendel left these plants of 

 the first generation to themselves. 

 They bloomed, self-fertilized and made 

 seed. This seed he planted again. This 

 time he planted the seed from each 

 plant upon a separate bed. When the 

 plants were full-grown the result 

 showed the utmost irregularity as to 

 size, apparently. In each bed Mendel 

 found the progeny of each plant to con- 

 sist of tall and dwarf plants. This 

 confirmed his opinion: "that the in- 

 fluence of the dwarf sort could not be 

 entirely lost, but had only been laying 

 dormant in the tall plants of the first 

 generation." 



The wise monk carefully counted 

 and tabulated the plants according to 

 height. Now, the veil of the secret 

 began to lift; a strange and wonderful 

 coincidence was revealed to him. Among 

 the progeny of each of the plants of 

 this first generation he found the same 

 proportions as to size. Three-fourths 

 of the plants were tall and one-fourth 

 was dwarf. Of course only approxi- 

 mately, more or less accurate, the 

 larger the number of plants the closer 

 the proportions. A rule of probabili- 

 ties. 



(The larger the numbers we deal with, 

 the more definite the proportions. As plants 

 multiply 100 and 100(1 fold, it is very easy 

 to find Mendel's proportion of numbers 

 amons the progeny, although very seldom 

 entirely pure. It is naturally more diffi- 

 cult to give a numerical confirmation of 

 Mendelian proportions in the case of ani- 

 mals, because the number of individuals in 

 any family is relatively very small, so that 

 results very often seem anythiu.g but cor- 

 rect. We know that children are born In 

 equal proportions as to sex. The very 

 small, actual difference is here of no eon- 

 sequence. So we might also' expect that of 

 six children in one family! three would be 

 boys and three girls. But we know that 



PENTSTEMON GENTIANOIDES. 



This illustration inadequately shows 

 the attractiveness of the improved 

 large flowering pentstemon hybrids. 

 These flowers are not so well known 



as their beauty in garden decoration 

 entitles them to be. Florists making 

 a business of bedding plants would do 

 well to give the pentstemons a promi- 

 nent position on their spring_ lists. 



this is seldom so, and that we will have 

 to use the average of a large number of 

 births to find that in relation to numbers 

 the sexes are eqi>all3' divided.) 



The progeny of the first generation 

 was represented as follows: [Sup- 

 pressed character is marked 0]. 



Parents. I (tUl) X»(dwarf) 

 First Generation : 



one-third proved true (tall) ; the re- 

 maining two-thirds were bastards. 



Par nts: I (tall) X» (dwarf) 

 First Generation: 



I I 1 I I I I 

 Second Generation: 



A0A0«SA®A©A0A® A0A0A©ASA®A0A€ 



I I I I I I I I I I I I I I 



Second Generation : 



A0A@ A0A0 A0A0 A0A0 



1 I I A I I I A I I I A I I 1 « 

 Thir Generatic n: 



I I A I 1 I A 



I 1 I A I 1 I A 



Again Mendel let his plants flower 

 and ripen the seed. Again he sowed 

 this seed carefully, the seed of each 

 plant in a separate row. Now the re- 

 sult. The seeds of the dwarf plants 

 produced nothing but dwarf plants 

 (and ever afterwards the same). They 

 remained true to type (the type was 

 fixed, as the gardener would say), even 

 though they had passed through the 

 tall plants of the first generation. The 

 seeds from the tall plants behaved 

 quite differently; some of the rows 

 brought tall peas only, and these after- 

 wards proved true to type also. Upon 

 the remaining rows planted with seed 

 from the tall plants, Mendel again 

 found the plants mixed in regard to 

 size, three-fourths tall and one-fourth 

 dwarf, exactly as in the second genera- 

 tion. Now mark this: The seed of 

 all the dzi'nrf plants gave him dwarf 

 plants, which afterwards proved true 

 to type in the seed. One part of the 

 tall plants also was true. In the others 

 of the fall plants the disposition to 

 dwarfness had been suppressed — "put 

 to sleep." so to speak — for the time 

 being. In counting up and tabulating 

 the result, Mendel found that "of the 

 f.eed from the second generation that 

 of all of the dwarf plants proved true 

 (dwarf) ; of that of the tall plants, only 



A0A0 A@A0 



I I 1 I I I I A I I I A AAAAAAAA 



And so it continued throughout the 

 generations. Mendel followed it up for 

 twelve generations. Tall pure, pro- 

 duced invariably tall pure. Tall im- 

 pure, always tall and dwarf impure 

 (bastards). Dwarf pure, again only 

 pure dwarf. 



These were the facts that nature 

 presented to Mendel. And what nature 

 demonstrated in regard to size of 

 plants, Mendel later on also proved to 

 hold true in regard to any of the other 

 characters, position and constitution 

 of leaves, color and form of seed or 

 flower, form of stem, etc., etc. 



Mendel experimented with many dif- 

 ferent factors. Hundreds of other in- 

 vestigators have since shown that 

 Mendel's law applies to any and all at- 

 tributes of living things. 



"In crossing tzvo specimens of plants or 

 animals, differing in any character (it is 

 immaterial which is the father), there oc- 

 curs no blending or amalgamation of these 

 characters in the resulting bastard, but one 

 of the characters is alzvays suppressed by, 

 the opposing one of the same attribute." 



This Is the "FIRST LAW OF MEN- 

 DEL." 



Mendel's discovery, "That the units 

 of all hereditary characters are indi- 

 visible and pass through the genera- 

 tions self dependently and unimpaired, 

 without ever becoming obliterated or 

 amalgamated with any others, has 

 been a great advance in the knowledge 

 pertaining to heredity. 



C To be continued.) 



