July 20, 1912. 



THE GARDENERS' MAGAZINE, 



551 



MENDELISM* 



Inheritance of Characters. 



(Contintied from page 538.) 



On the formation of the sexual cells, the 

 opposing characters of each pair will sepa- 

 rate. Consequently, no pollen grain or ovule 

 will be found containing the factors for 

 yellowness and greenness, nor roundness and 

 wrinkledness. But there is nothing to pre- 

 vent the association in the same pollen grain 

 or ovule of yellow with either round or 

 wrinkled; or green with round or wrinkled. 

 We thus get in approximately equal num- 

 bers four sets of pollen grains, and four sets 

 of ovules carrying the factors YR, Yw, gR, 

 and gw. ' The chances that any one pollen 

 arain will mate with one or other of each 

 Sf the four types of ovule are equal, i.e., the 

 union of YE with YE. is just as likely to 

 occur a^ its union with either Yw, gR, or 

 gw. The constitution of the sixteen plants 

 of the second generation can therefore be 

 found by combining each of the pairs in the 

 series with itself, and the others in turn. 

 The diagram shows the result, the letters at 

 the side and top representing the factors 

 present in the pollen grains and ovules re- 

 spectively ; the letters in each square repre- 

 sentino- the constitution of each of the six- 

 teen plants. 



YR 



YW 



OvuJ«i. 



OR 



OW 





YR 





YY RR 



YY RW 





YG RR 



YG RW 







I 





i 



3 





4 



m 

















o 



YW 





VY RW 



YY WW 





YG RW 



YG WW 



m 



o 





6 





6 : 







8 



















£ 



w « 



OR 





VG RR 



YG RW 





OG RR 



GG.RW 



o 





9 





10 



11 





13 





GW 





Y G RW 



YG WW 





GG.RW 



GO WW 







I*! 





T4 







Iff 



Dominant and Recessive 



Characters. 



Examination of this table will show that 

 only one plant of each of the four forms ob- 

 tained will breed true. Tlius of the nine 

 plants containing both dominant characters 

 (Nos. 1, 2, 3, 4, 5, 7, 9, 10 and 13), only 

 one YY.ER, is pure, i.e., both dominants 

 have paired. Of the remainder, four are 

 hybrid in regard to one pair of characters, 

 four hybrid in regard to both pairs. Segre- 

 gation of these characters must follow in the 

 third generation. 



Only one plant has both recessive charac- 

 ters, gg.ww, and this plant will breed true. 



NoAv examine the two types of plants where 

 the characters of the original parents have 

 been rearranged, i.e., the yellow wrinkled 



and green round. 



Taking the yellow wrinkled first, we see 

 that No. 6 has the dominant characters 

 paired and the recessive characters paired. 

 Its constitution is YY. ww. This plant will 

 therefore breed true to these characters. Of 

 the other two. No. 8 is hybrid in regard to 

 colour (Yg), but pure in regard to the out- 

 line of the seed (ww). The same is true of 

 No. 14. The yellow and green characters will 

 therefore segregate in the next generation. 

 Similarly in regard to the green round forms. 

 N'o. 11 is pure in regard to both characters, 

 but Nos. 12 and 15 are hybrid in regard to 

 outline of seed. No. 11 will breed true, 

 Nos. 12 and 15 will se^gregate. 



Now suppose that our original intentions 

 was to obtain a green round form from our 

 cross. In the first generation, the type de- 

 sired is not present. It appears, however, 

 3n the second generation, and further, one 

 out of every three will be pure. 



We h ave then to separate tTie pure forms 

 from the impure. The older hybridists ob- 

 tained this end by selection, which if con- 

 tinued for a number of years would probably 

 he successful. The Mendelian, however, can 

 tell his pure type in the following year, for 

 keeps and sows the seed of each plant 

 separately. All the plants from the seed of 



he 



two 



factors, 



blue " 

 " pink " 

 A pink 



In any cross, therefore, every possible com- 

 bination of characters will appear in the 

 second generation, provided we have a suffi- 

 cient number of individuals. A certain 

 number of these combinations will consist of 

 a rearrangement of the characters of the 

 original parents, and a definite proportion 

 of each of these new forms will be ** pure *' 

 and give rise in future generations to identi- 

 cal offspring. 



eversion. 



Eecent Mendelian work has thrown much 

 light on the puzzling phenomenon known as 

 "throwing back" or reversion. On cross- 

 ing certain varieties we get a hybrid which 

 shows neither simple " dominance *' nor 

 " blending/ but can best be described as ex- 

 hibiting the characters of the wild " ances- 

 tral " form from which the parental varie- 

 ties are supposed to have de^scended. The 

 colour of the flower of the culinary pea is an 

 instance. 



Three colours are known—white, which is 

 the commonest; pink, which is found in seve- 

 ral varieties; and so-called purple, which is 

 almost identical to the colour of the wild 

 ancestral type. On crossing white with pink, 

 purple flowered plants result. 



When self-fertilised, the purple flowered 

 plants produce the three forms, purple, pink, 

 and white in the proportion of 9 : 3 : 4. Now 

 this proportion at once suggests that we are 

 dealing with a case of dihybridism, similar 

 to the cro*^ between a yellow round and a 

 green wrinkled. But in that instance the 

 proportion was as 9:3:3:1. How do we get 

 4 in place of 3 :1 ? 



Each parent must contain 

 which in this case are supposed to b 

 (B), and absence of blue (b), 

 (P), and "absence of pink" (p). 

 flower will therefore have the constitution 

 P.b. (i.e., "pink" and "absence of blue.'') 

 A white flower has the constitution Bp (i.e., 

 "blue" and "absence of pink.") But why 

 is this flower not blue ? For the simple rea- 

 son that "blueness" only manifests itself 

 when the pink character is also present. 

 That this is so becomes clear when we 

 examine the following generation. 



The plants of the first generation are 

 purple. They contain both colour factors, 

 blue and pink, which gives purple. In the 

 second generation we have 9 purple, 3 pink, 

 4 white. These nine purple, though not 

 identical all contain the blue and pink fac- 

 tors. Hence all are purple. Tlie three 

 pinks contains the pink factor, but not the 

 blue. Hence all are pink. But of the four 

 white, three contain the blue factor but not 

 the pink, and since blue cannot be seen m 

 the absence of pink, they remain white. The 

 fourth white contains neither colour factor, 

 and is therefore a pure white. 



We now understand why reversion takes 

 place when the pink flower is crossed with 

 the white. Each parent possesses one of the 

 necessary factors for the production of the 

 purple colour. In the hybrid, the two fac- 

 tors are once more combined, and purple- 

 ness results. The facts also suggests how 

 the pink and white varieties arose. In the 

 course of evolution each of these forms must 

 have lost one of the factors. Tlius the pink 

 flower lost the blue factor, and the white 

 flower the pink. It is also probable that the 

 " loFS " of these factors was due to imperfect 

 segregation. If this suggestion be correct, 

 both these forms must have appeared sud- 

 denly, just as the "Shirley" poppy is known 

 to have done. 



Present and Absent Factors. 



A somewhat similar instance is to be found 



The 



owx the seed of 

 bit segregation. 



in the sweet pea. 



"Emily Henderson" has two forms, which 

 are outwardly indistinguishable. The cross 

 between these, results in a purple, recalling 

 the appearance of the wild species of Europe. 

 Self fertilisation of these hybrids gives 9 

 coloured to 7 white. The explanation is 

 similar to the previous case. Each form con- 

 tained certain of the necessary factors for 

 r/^loiir and the cross resulted in a re-combi- 



nation of these factors. In the following 

 generation segregation in the proportion of 

 9:3:3:1 took place. ^Plie nine plants 

 were coloured because they all contained 

 the colour factors. The remaining seven 

 are white, because they contained either 

 half the factors or none of the factors. The 

 analysis of the nine coloured forms is of 

 special interest to the horticulturist. They 

 were not identical, but consisted of six 

 forms occurring in definite proportions:-— 



1. Purple bicolor ("Purple Invincible.") 



2. Deep purple (colour deeper than No. 1). 



3. Dilute purple (" Picotee.") 



4. Ked bicolor (" Painted Lady.") 



5. Deep red (Miss Hunt). 



6. Dilute red or Tinged white. 



It will be seen at a glance that these forms 

 constitute a series. The purples were to the 

 reds as 3: 1. Purple is therefore dominant 

 to red, and each red differs from the cor- 

 responding purple by the absence of the 

 purple factor. Further, each of the series of 

 purples and reds appeared in the proportion 

 of 9: 3: 4. Two factors must therefore be 

 concerned, these factors being stated to be 

 (1) a light wing factor, making bicolor domi- 

 nant to the dark-winged form; (2) an intense 

 colour factor. 



If purple l>e rcpre^vented by B, light wing 

 by L, and intense colour by I, the purple 

 bicolor has the constitution BLI. The other 

 forma mav be derived from this by the omie- 

 sion of each factor in turn, representing the 

 absence of each factor by the corresponding 

 small letter, we get 



1. Purple bicolor BLI. 



2. Deep purple BII. 



3. Dilute purple BLi. 



4. Red bicolor bLI. 



5. Deep red bll. 



6. Dilute red bLi. 



This again suggests that the various varie- 

 ties now in cultivation arose by the loss of 

 one or other of the factors in turn. 



Professor Punett has an interesting his- 

 torical note on the subject. To quote his 

 own words, "The wild sweet pea first 

 reached this country in 1699. . . . Some- 

 what late-r we hear of two new varieties, the 

 red bicolor or Painted Lady, and the white, 

 each of which may be regarded as having 

 'sported' from the* wild purple by the omis- 

 sion of the purple factor, or of one of the 

 two colour factors. In 1793 we find a seeds- 

 man offering also what he called black and 

 scarlet varieties. It is probable that these 

 were our deep purple and Miss Hunt varie- 

 ties, and that somewhere about this time the 

 factor for the light wing (L^ was dropped 

 out in certain plants. In 1860 we have evi- 

 dence that the pale purple or Picotee, and 

 with it doubtless the Tinged white, had come 

 into existence. This time it was the factor 

 for intense colour which had dropped out. 

 And so the story goes on until the present 

 dav, and it is now possible to express by the 

 same simple method the relation of the 

 modern shades of purples and red, of blues 

 and pinks» of hooded and waved standard, to 

 one another and to the original wild form. 

 The constitution of many of these has now 

 been worked out, and to-day it would be 

 a simple, though perhaps tedious, task to de- 

 note all the different varieties by a series of 

 letters indicating the factors which they 

 contain, instead of by the present system of 

 calling them after kings and queens, and 

 famous generals, and ladies more or less well 



known." 



In these cases of "reversion" the factors 

 concerned have been dscribed as " present " 

 or "absent." This idea of "presence and 

 absence " has been formulated to explain the 

 phenomenon of dominance. 



The dominant character is believed to be 

 due to the presence of something. The reces- 

 sive character is due to its absence. Thus 

 the tall pea is tall because it contains the 

 factor for tallness. When this factor is 

 absent the pea is a dwarf. All peas therefore 

 are dwarf, but some are dwarf plus a factor 

 which converts them into tall. 



