IRISH GARDENING 



VOLUME IX 

 No. 97 



Edited by C- F. Ball. 



A MONTHLY JOURNAL DEVOTED TO THE 



ADVANCEMENT OF HORTICULTURE AND 



ARBORICULTURE IN IRELAND 



MARCH 

 1914 



Mendelism* 



second paper. 

 By Trofessor James Wilson, M.A. 



LfRRAR' 



^EW Y(>K 



80TANICA 



QA.'<T»EM. 



In last month's number of Irish Gardening a 

 short account of Mendel's original experiments 

 was given, in which it was shown that : — 



i. When the original parents differed in one 

 pair of characters their hybrids produced two 

 types of progeny w^hich were numerically in the 

 ratio 3:1. 



ii. When the original parents differed in two 

 pairs of characters their hybrids produced four 

 types of progeny, which were numerically in the 

 ratio 9 : 3 : 3 : L 



iii. When the original parents differed in three 

 pairs of characters their hybrids jiroduced eight 

 types of progeny, which were numerically in the 

 ratio 27 :9 :9r9 :3 :3 :3 : L 



It was also shown that the different jmirs of 

 characters bred independently of each other. 



There was one other very important point 

 brought out in Mendel's experiments whicli we 

 shall deal with now. It was that among the 

 progeny of all the hybrids bred from, whether 

 there were two types, four types, or eight ty2ies, 

 there were always equal numbers of individuals 

 which bred true in each type. 



Consider what was found in the case when the 

 original parents differed in the shape of the seed 

 only. The hybrids had all round seed : none 

 had wrinklecl ; but in the next generation 

 wrinkled seed appeared again in such numbers 

 that they were a third as many as the round 

 seed. The actual numbers were — round 5,474 and 

 wrinkled 1,850. That is to say, there were 

 5,474 seeds like one grandparent and 1,85U like 

 the other. 



Mendel then put the question : How woukl 

 these seeds breed ( To find an answer he sowed 

 a number of each kind, and so produced another 

 generation. From this generation he found that 

 all the recessives bred true, but that only a third 

 of the dominants did so. He also put the same 

 question to all the other six sets of experiments 

 and always got the same answer. A third of 

 he second generation dominants always bred 

 rue and the whole of the recessives. 



As this point is very important and needs to 

 be impressed upon us, we shall give Mendel's 



actual figures. He does not say how many 

 recessives he grew ; but, as all bred tiue. there 

 was no need for him to say. The following table 

 gives the numbers of second generation domi- 

 nants so\\ n and the numbers which bred true : — 



It will be noticed that in every case practically 

 a third of the individuals sown bred true. The 

 remainder bred like their hybrid parents. They 

 were hybrids also.^^ 



But this leads to a most important considera- 

 tion. The total number of dominants in the 

 second hybrid generation is three times as many 

 as the number which breed true. The same 

 total number is also three times as many as the 

 total number of recessives in the same generation. 

 But all the recessives breed true. Thus the 

 numbers of dominants and recessives which bi eed 

 tiue are the same. Otherwise: the numbers of 

 individuals in each of the two second generation 

 grou])s Mhich breed true are the same. 



For example, if a second generation from ptu'c 

 round seeds, on the one hand, and wrinkletl seeds 

 on the other contain in all 400 seeds, 3()0 would 

 be round and 100 wrinkled. All the wrinkletl 

 seeds would breed true ; but, at the same time, 

 only a third of the round seeds — viz., 100 — 

 would breed true. This result may be made 

 clearer if represented by a diagram, thus : — 



Total Seeds 

 400 



Pouni 

 300 



Breeding 

 True 



100 



Not 



Breeding 



True 



200 



Wrinkled 

 100 



Bi-eeding 

 True 



100 



