554 



KNOWLEDGE & SCIENTIFIC NEWS. 



[October, 1906. 



SHOWING Mendel's results in the Ckoss between Peas with Yellow 

 AND Gkeen Cotyledons respectively. 

 Cross. 

 Y? X G^ "/• r, ? X Y<f 



The same shown in general notation. 



D = Dominant. 



K = Recessive. 



D X K oc R X U 



Seeds all yellow (YG) 



Plants raised from these, 



and self-fertilised : 



Yc; X YG 



6022 Yellow seeds 

 Sample of 519 plants raised from 

 these and sell-fertilised showed 



166 (YY) plants 

 bearing only Yellow seeds 



2001 Green seeds 



353 (YG) plants bearing 



both Yellow and Green seeds 



in the ratio 



3 Y I G 



Of the 3 Y 



1 (YY) Breeds true 



2 (YG) continues to split Breeds tri 



DR (all appear iJ) 



DR X DR 



I 



3 appear D 

 are made up of 



I DO 2 DK 

 1)1) X DD UK X UK 



I KR 



KR X RK 



DD I DD 2 DR I RR RI 



Breed true ; Hybrid Breed true ; 



pure splits as pure 



Dominant above Recessive 



ceils and pollen grains all of which carry the character 

 proper to the race. When two oppo.site character.s are 

 brought tos^cther in hybridisation, they unite to form 

 the hybrid character, which in the peas so closely re- 

 sembles one of the parent characters as to be indis- 

 tinguishable in appearance from it. The hybrid, in its 

 ttirn, produces pollen grains and egg cells, which are 

 either pitrc dnmimitit or pure recessive, and on the average, 

 equal numbers of each kind are produced. Each cross- 

 l)r<d is then producing gametes in the proportion of : — 



' 50% pure Dominant. 

 50% pure Recessive. 



In 

 male 



<? 50% pure Dominant. 

 50 ",, pure Recessive. 



sell-ft-rlilisation, random mating 

 and female cells would gi\e : — 



letween these 



25 (D' xD''); 25 (D' xR^); 

 25 (R' xD'^); 25 (Rv xR^) : 

 D X R does not differ from K x D ; we have therefore 



25 (D X D) ; 50 (D x R) ; 25 (R x R). 

 And since cross-breds resemble dominants in appear- 

 ;mcc, the apparent rcstilt of the cross (DR X DR) is 

 3D: iR. 



Mendel next turned his attention to the crossing of 

 races differing from one another in respect of more 

 than one pair of characters. Suppose a race with 

 round seeds and yellow cotyledons be crossed with one 

 which has wrinkled seeds and green cotyledons. 

 Mendel found the round and yellow characters to be 

 dominant respectively over wrinkled and green. We 

 therefore expect the first hybrid seeds to be all round 

 and vellow, a result which Mendel actually obtained. 

 The reproductive cells produced by these hybrids carry 

 either the round or wrinkled, and either the yellow or 

 green character, the possible combinations being : — 



f RY J RG • \VY ■'^ WG 



•f RY ■'■ RG ■ \VY '-' WG 



Mendel's hypothesis supposed that, on the average, 

 e(|ual numbers of these different kinds of reproductive 



cells (gametes) are produced. In self-lertijisation, 



therefore, random mating would give the following 



unions and their reciprocals, all occurring in equal 

 numbers : — 



9331 



In the first column are placed all those unions to which 

 the dominant characters, round and yellow, are con- 

 tributed at least by one of the parents; the plants result- 

 ing from these unions are, to the eye, indistinguishable 

 from the pure dominants; in the second and third 

 columns are placed those unions in which the dominant 

 of one pair and the recessive of the other pair of 

 characters is present; while the fourth column con- 

 tains that union alone in which both pairs of characters 

 are represented only by the recessive member. The 

 offspring shotild, therefore, appear in the ratio of 



gRY : 3RG ; 3WY : iWG. 



a result to which Mendel's figures of 



315RY : 108RG : lotWY : 32WG. 



approximate so closely as to be well within the limits 

 of experimental error. 



The experimental results related above provide a 

 simple illustration of the principles upon which the 

 Mendelian theory of heredity is based. It would serve 

 little purpose to recount here the numerous confirmatory 

 experiments in the breeding both of animals and plants 

 which have been carried out even in the short time 

 which has elapsed since the rediscovery of Mendel's 

 memoir. 



Mendel specifically applied his hypothesis to explain 

 the behaviour of certain characters in pens, without 

 claiming to have discovered a universally applicable 

 law. \Videspread as the phenomena of dominance and 



