Of these 64 zygotes one is pure yellow — smooth — tall ; 

 one pure yellow — smooth— dwarf ; one pure yellow — wrink- 

 ed— dwarf; one pure green— smooth— tall ; one pure green 

 — smooth— dwarf ; one pure green — wrinkled — tall; and one 

 pure green — wrinkled — dwarf. The remainder are impure 

 forms. 



Phenotypically however, there are 27 yellow — smooth 

 — tall, 9 yellow — smooth — dwarf, 9 yellow — wrinkled — tall, 

 9 green— smooth — tall, 3 yellow — wrinkled — dwarf, 3 green 

 — wrinkled — tall, 3 green— smooth — dwarf, and one green 

 —wrinkled— dwarf,— the ratio being 27 : 9 : 9 : 9 : 3 : 3 : 

 3:1. On examination of the zygotes it will be found that 

 there are 27 different genotypes, and 8 phenotypes. 



Explain how Biffen was able to breed a wheat that was 

 hard, heavy -yielding, and immune to rust from a variety that 

 was soft, heavy-yielding, and susceptible to rust, when hard- 

 ness and susceptibility to rust are dominant characters. 



Explain the following formulae for segregation given 

 by Morgan: 



3 1 One pair of factors. 



9:3 3:1 Two pairs of factors. 



27:9 9:3 9:3 3:1 Three pairs of factors. 



It is important to observe that the first two pairs of 

 characters in Mendel's list (page 83) pertain to the 

 cotyledons which form the first or seed leaves of the next 

 generation, and the following five pairs of characters per- 

 tain to the seed coat, pod, flower or stem, which belong to 

 the parental generation. In actual breeding operations, 

 therefore, when a pure yellow pea is crossed with a green pea 

 the yellow color of the cotyledons of the hybrid will be ob- 

 tained the same year. But when a pure tall pea is crossed 

 with a dwarf pea, the tall hybrid will not be obtained until 

 the following year after the seed is sown. 



Similarly when a tall yellow pea is crossed with a green 

 dwarf pea, the characters of the yellow tall hybrid will not 

 find expression at the same time.' 



Mendel chose the garden pea because it is easy to culti- 

 vate, has a short period of growth, and has constant differ- 

 entiating characters. With true scientific insight he paid 

 attention to the following problems : 



1. "To determ'ne the number of different forms under 

 which the offspring of hybrids appear. 



(1) (See also Vol. XI. No. 4, 'Journal of Agricultural Research" on the 

 Researches on the 35 Factors of the Genus Pisum). 



90 



