338 



INTRODUCTION TO CYTOLOGY 



the "recessive" t, so that the plants of this generation are all tall. When 

 the hybrid (Fi) produces germ cells the two factors for tallness and 

 dwarfness separate, half of the germ cells receiving T and the other half 

 receiving t. Each gamete therefore carries either one or the other of the 

 two factors in question, but never both: a given gamete is "pure" 

 either for T or for t. This segregation in the germ cells of factors pre- 

 viously associated in the individual without their having been altered by 

 this association is the central feature of the entire series of Mendelian 

 phenomena, and is often referred to as Mendel's first law. Since, now, 

 the gametes, both male and female, produced by the hybrid plants of the 

 Fi generation are of two kinds (half of them bearing T and half bearing ft 



MIRABIUS JALAPA 



FIG. 130. Blending inheritance ("incomplete dominance") in Mirabilis jalapa, showing 

 1:2:1 ratio of three genotypes in Ft. (Adapted from Correns.) 



four combinations are possible : a T sperm with a T egg, a T sperm with a 

 t egg, a t sperm with a T egg, and a t sperm with a t egg. These four 

 combinations result respectively in a tall plant (pure dominant, TT), two 

 tall hybrids (Tt and tT), and a dwarf plant (pure recessive, tt). It is 

 obvious that in the long run these three types will occur in the ratio of 

 1 :2:1. 



Mendel's researches on peas included also a study of six other pairs 

 of heritable characters (now known as allelomorphic pairs), the two 

 members of each pair behaving toward each other in a manner similar to 

 that described above for tallness and dwarfness. He further observed 

 that the seven pairs are entirely independent of each other in inheritance 

 (Mendel 's second law; now modified; see p. 384). All these phenomena he 

 interpreted on the basis of the hypothesis that each character is in some 

 way represented by a factor in the cells, new combinations of factors 



