latter the chromosomes with the factor dwarf (D). The 

 cells of the hybrid Fi, will carry a pair of chromosomes, T 

 and D. In the, maturation of the gametes of Fi the chro- 

 mosomes are reduced to one-half, and at reduction the fac- 

 tors will get into separate cells, thus giving two kinds of 

 gametes. As each sex has two kinds of gametes, the possi- 

 ble number of combinations of zygotes will be four, viz., 

 TT, TD, DT, DD, which agrees with Mendelian segrega- 

 tion. 



Thus it will be seen that both the chromosomes and the 

 factors undergo segregation. 



A later extension of the theory assumes that all the 

 factors carried by the same chromosome remain together in 

 segregation. In other words, linkage of factors in segrega- 

 tion should be expected. (S^e page 109). 



In the crossing of yelloW — tall peas with green-dwarf 

 peas, the chromosome theory explains the results as follows: 

 The yellow-tall plants have sex-cells with the factors for 

 tall (T) and yellow (Y) in separate chromosomes, and the 

 green-dwarf plants factors for green (G) and dwarf (D) in 

 separate chromosomes, but the yellow and green chromo- 

 somes are homologous; similarly the tall and dwarf chromo- 

 soes are homologous. 



The cells of the hybrid of Fi will have the chromosome 

 constitution YGTD, the factors or determiners Y and G 

 lying in identical positions in the one pair of homologous 

 chromosomes, and the factors T and D in identical positions 

 in the other pair of homologous chromosomes. All the other 

 pairs of chromosomes carry the same set of factors. 



In the maturation of the gametes of F,, when the num- 

 ber of chromosomes is reduced by one-half, two types of 

 division occur, viz., germ-cells containing the Y T and G D 

 chromosomes, or YD and GT chromosomes. Hence there 

 will be four kinds of gametes,— YT, YD, GT, GD. 



For if the chromosomes are represented by YT and 

 GD, the paternal being YT and the maternal by GD, then 

 in synapsis only Y and G can pair together, likewise T an 

 D. The first pair, however, operates independently of the 

 second, so that at reduction either member of the first pair 

 may get into a Cell with either member of the second pair. 

 Consequently the following divisions might occur: 



.YT YD 



GD GT 



138 



