CHROMOSOMES AND MEN DELI AN HEREDITY 



291 



factors by capital letters and their recessive allelomorphs by the corre- 

 sponding small letters. 



The C5rtological Basis of Mendelian Heredity. — The history of the 

 chromosomes through the critical stages of the life cycle, as more fully 

 described in the chapters on syngamy and meiosis, must be recalled at this 

 point (see Figs. 148 and 167). Each parent furnishes the offspring with a 

 set of individually different chromosomes, the two sets (represented in 

 Fig. 167 by A BCD and ahcd) together constituting the diploid comple- 

 ment present in all of the nuclei of the new individual. When gametes 

 (or spores followed later by gametes in the case of higher plants) are to be 

 formed by this individual, descendants of the homologous chromosomes of 



FEETILIZATIOH 

 Union of simplex groups 



CLEAVAGE 

 Duplex groups 

 ABCO abed 



SOMATIC DIVISIONS 

 Duplex groups 



Aa Bb Co Dd 

 SYHAPSI S 



GERM CELLS 

 Simplex groups 



Fig. 167. — Diagram of chromosome cycle. (After E. B. Wilson, 1913.) 



the two gametic sets conjugate in pairs (synapsis). In one of the meiotic 

 divisions the two members of each pair disjoin and come to lie in different 

 nuclei. In the other meiotic division the chromosomes divide equation- 

 ally. The result of the two meiotic divisions, therefore, is a group of four 

 gametes (or spores), two of which differ from the other two with respect to 

 any given chromosome pair : two of them have derivatives of A while the 

 others have derivatives of a, and so on for all the other pairs. The 

 chromosomes of the diploid complement are in this way assorted into 

 monoploid sets, each gamete (or spore) having a set made up of one 

 member of each of the pairs. This set represents the contribution made 

 to the following' generation. - , 



It is of importance to recall also that the various pairs of chromosomes 

 are independent of each other as regards their orientation in' the mitotic 

 figures and hence in their distribution to the daughter nuclei (p. 254). 



