8 Mendel's Method [ch. 



that the progeny from distinct individuals must be separately 

 recorded. All these ideas were entirely new in his day. 

 When such precautions had been observed he anticipated 

 that a regular result would be attainable if the experiments 

 were carried out on a sufficient scale. 



After several preliminary trials he chose the edible Pea 

 i^Pisum sativum) for his subject. Varieties in cultivation 

 are distinguished by striking characters recognizable with- 

 out trouble. The plants are habitually self-fertilised, a 

 feature which obviates numerous difficulties. 



Following his idea that the heredity of each character 

 must be separately investigated, he chose a number oi pairs 

 of characters, and made crosses between varieties differing 

 markedly in respect of one pair of characters. The case 

 which illustrates Mendelian methods in the simplest way 

 is that in which heredity in respect of height was studied. 

 Mendel took a pair of varieties of which one was tall, being 

 6 — 7 feet high, and the other was dwarf, % X^o \\ feet. 

 These two were then crossed together. In peas this is 

 an easy operation. The unbroken anthers can be picked 

 out of a bud with a pair of fine forceps and the pollen of 

 the plant chosen for the father may be at once applied to 

 the stigma of the emasculated flower. The cross-bred seeds 

 thus produced grew into plants which were always tall, 

 having a height not sensibly different from that of the pure 

 tall variety. In our modern terminology such a cross-bred, 

 the first filial generation, is called F^. From the fact that 

 the character, tallness, appears in the cross-bred to the 

 exclusion of the opposite character, Mendel called it a 

 dominant character ; dwarfness, which disappears in the F^ 

 plant, he called recessive. 



The tall cross-bred, so produced, in its turn bore seeds 

 by self-fertilisation. These are the next generation, 7%. 

 When grown up they prove to be mixed, many being 

 tall, some being short, like the tall and the short grand- 

 parents respectively. Fig. i shows such an F^ family in 

 the Sweet Pea. Upon counting the members of this i% 

 generation it was discovered that the proportion of tails to 

 shorts exhibited a certain constancy, averaging about three 

 tails to one short, or in other words, 75 per cent, dominants 

 to 25 per cent, recessives. 



