The Role of the Chromosomes 



205 



e. g., that all the germ cells of the tall pea carry the deter- 

 miner for talliiess (T) and all those of the dwarf pea the 

 determiner for dwarf ness (t). If now we cross the tall (T) 

 with the dwarf (t) we shall have in the cells of the hybrid 

 both T and t, and the resnlt will be a tall pea (since tallness* 

 dominates dwarfness) carrying latent the determiner for 

 dwarfness. 



Before the germ cells of the hybrid are ready for fertiliza- 

 tion they must undergo a process of ripening or maturation 

 in the course of which the chromosomes of each are reduced 

 to one-half the number in the l)ody cells of the species. This 

 reduction is effected by the union of the chromosomes in 

 pairs and their subse(iuent division as apparently single, 

 though in reality double elements. One of these divisions, 

 kiiown as the "reducing division" is as- 

 sum,ed to separate the paired elements 

 from each other. If now we assume that 

 the determiner for tallness be carried by 

 one chromosome and that for dwarfness 

 by another, and that these two chromo- 

 somes pair with one another in the matu- 

 ration of the germ cells of the hybrid, 

 separating from each other in the reduc- 

 ing division, then the germ cells of the 

 latter will be of two kinds, e. g., those 

 containing the T chromosome and those 

 containing t. Now when the hybrids are 

 crossed with one another there will be 

 three possible combinations resulting from 

 the union of their germ cells, in the fol- 

 lowing ratio (ITT, 2Tt, Itt), which re- 

 sults from the chance combination of T and T with t and t. 

 These chance results may be demonstrated by a simple ex- 

 periment. If four billiard balls, two black and two white, 

 be shaken together in a box and drawn out in pairs, one- fourth 

 of the drawings will be two blacks, one-fourth tAVO whites 

 and one-half a black and a white. If then the behavior of 

 the chromosomes at the time of maturation and fertilization 

 is as assumed, and if secondly the chromosomes carry "deter- 

 miners" (whatever they may be) for the characters of the 

 organism ; then the Mendelian results must follow as a 

 mathematical necessity of the chance separation and recom- 

 bination of till' chromosomes in the maturation and fertiliza- 

 tion of the germ cells. 



We have used a iuntil)er of "ifs" in the above discussion. 

 Are our conclusions based purely on assumptions? Let us 

 see. In the ease cited we have assumed in the first place 



Diagram to illus- 

 trate inheritance of 

 size in' the sweet pea. 

 A cross of a tall 

 with a dwarf pea 

 produces 3 tall and 

 1 dwarf pea in the 

 second generation. 



