288 FOUNDATIONS OF BIOLOGY 



homologous chromosomes of paternal and maternal origin 

 unite in pairs the process of fertilization which gave rise 

 to the individual being consummated in the ripening of its 

 own germ cells. But this union is only temporary; a suc- 

 ceeding mitosis, instead of dividing each chromosome as 

 usual, separates the maternal and paternal chromosomes of 

 each synaptic pair and delivers one of each (though rarely all 

 of the same maternal or paternal set) to the two arising cells. 

 Thus each mature germ cell contains one member of every 

 chromosome pair and the number of chromosomes is reduced 

 one half. 1 (Fig. 146.) 



Mendel postulated that the genes for alternative charac- 

 ters segregate in the formation of the germ cells of hybrids 

 so that a single gamete bears one and not both genes of a 

 pair of allelomorphs. That is the genes, which come together 

 in the zygote which forms the hybrid, separate again in the 

 formation of its own germ cells. This is just what cytological 

 studies show. Chromosome behavior exactly parallels the 

 typical behavior of the Mendelian gene, because in the matu- 

 ration of the germ cells each chromosome of paternal origin 

 separates from the corresponding chromosome of maternal 

 origin. The genes similarly situated on homologous paternal 

 and maternal chromosomes are allelomorphs and are segre- 

 gated during maturation. And further, in considering Men- 

 delian dihybrids we found, for instance, that genes for yellow 

 and round, and green and wrinkled seeds were inherited in a 

 fashion which indicated that yellow and round, let us say, are 

 segregated independently of each other, because all possible 

 combinations with green and wrinkled occur. This clearly is 



1 It will be recalled that in plants exhibiting an alternation of generations, the chromo- 

 some reduction occurs at the formation of the spores. (Fig. 124.) A little thought 

 will convince the reader that this difference is of no importance from the standpoint 

 of the present discussion, because we are interested in inheritance from sporophyte to 

 sporophyte and can neglect the gametophyte which intervenes. 



