22 



JOURNAL OF THE ROYAL HORTlCliLTURAL SOCMI-y^^ . 



MENDEL:S law of HEEEDITY and its AP.PIJ0AT10N TO 

 HOKTICULTUKE. 



By C. 0. Hurst, F.L.S. 



[Read Moy 4, 1909.] 



Mendel's law was founded on simple characters in garden peas. In 

 his experiments Mendel found seven pairs of characters that followed 

 his law, namely: — 



Rounded and ivrinkled seeds. 



Yellow and green seeds (cotyledons). 



Coloured and white seed-coats. 



Inflated and coyistricted pods. 



Green and yellow pods. 



Distributed and bunched flowers. 



Tall and dwarf stems. 



It will be observed that Mendelian characters consist of pairs of 

 contrasts. Mendel found that in each case the two contrasting 

 characters behaved in breeding as " dominants " and " recessives," the 

 first-named of each pair being dominant " and the other " recessive." 



For instance, when a pure-bred tall pea was crossed with a pure-bred 

 dwarf pea, the offspring were all tall, no matter which way the cross 

 was made. In Mendehan terms tallness is " dominant " and dwarfness 

 " recessive. " 



When the tall crossbreds produced seeds by self-fertilization, 

 Mendel found that the offspring consisted of both tails and dwarfs, and 

 on the average there were three tails to one dwarf. 



It will be noted that the " recessive " dwarf character, after 

 " skipping a generation," reappeared in a quarter of the offspring. 

 Mendel found that these extracted dwarfs bred permanently true to 

 the dwarf character, notwithstandmg the tallness of their parents and 

 ancestors. 



The tall individuals of the same generation, however, proved to be 

 of two kinds ; on the average one-third of them were pure tails, breeding 

 true to the tall character, notwithstanding their dwarf ancestors, while 

 two-thirds of them were impure tails throwing dwarfs as well as tails, 

 as their parents did before them. 



The diagram (fig. 12) will serve to illustrate the results obtained by 

 Mendel in crossing pure tall peas with pure dwarf peas and their 

 behaviour in subsequent generations. 



Mendel's Law of Segregation. 

 Mendel's law of heredity was based on the experimental facts noted 

 above. In order to give a reasonable explanation of such facts, Mendel 

 conceived the idea of the separation or segregation of characters in the 

 germ-cells. Mendel supposed, as Darwin and his successors have 



