84 Heredity and Environment 



(6) Differences in the positions of the flowers, whether axial, 

 that is, distributed along the stem, or terminal, that is, bunched at 

 the top of the stem. 

 . (7) Differences in the length of the stem, whether tall or short. 



i. Results of Crossing Individuals with one Pair of Contrast- 

 ing Characters. Having determined that these characters were 

 constant for certain varieties Mendel then proceeded to cross 

 one variety with another, by carefully removing the unripe sta- 

 mens, with their pollen, from the flowers of one variety and dust- 

 ing upon the stigmas of such flowers the pollen of a different 

 variety. In this way he crossed varieties of peas which differed 

 from each other in some one of the characters mentioned above, 

 and then studied the offspring of several successive generations 

 with respect to this character. 



Dominant and Recessive Characters. In every case he discov- 

 ered that the plants that developed from such a cross showed only 

 one of the two contrasting characters of the parent plants, i.e. } all 

 were round-seeded, yellow-seeded, or tall, etc., although one of the 

 parents had wrinkled seeds, green seeds, or short stem, etc. "Those 

 characters which are transmitted entire or almost unchanged in 

 the hybridization are termed dominant, and those which become 

 latent in the process, recessive!' 



Ratio of Dominants to Recessives. These hybrids* when self- 

 fertilized gave rise to a second filial generation of individuals 

 some of which showed the dominant character and others the re- 

 cessive, the relative numbers of the two being approximately 

 three to one. Thus the hybrids produced by crossing yellow- 



* Bateson introduced the term "homo-zygote" for pure-bred individuals 

 resulting from the union of gametes which are hereditarily similar, and 

 "hetero-zygote" for hybrids resulting from the union of hereditarily dis- 

 similar gametes. The gametes formed from a homo-zygote are all of the 

 same hereditary type, those formed from a hetero-zygote are of two dif- 

 ferent types for every unit difference of the parents. The members of a 

 pair of contrasting characters are called "allelomorphs"; eacsh member 

 of such a pair is "allelomorphic" to the other member. 



