INHERITANCE 



303 



Furthermore, in considering 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 

 are segregated independently of each other — there is an inde- 

 pendent assortment because all possible combinations with green 

 and wrinkled occur. This clearly is fully accounted for, provided 

 the gene for color and the gene for form are not in the same pair of 



Gamete 



Gamete 



Fig. 189. — Diagram to show the union of haploid groups of either the 

 chromosomes or of the genes of the gametes to form the diploid condition of 

 the zygote, body cells, and primordial germ cells. Finally their pairing at 

 synapsis, and segregation in the gametes. With four pairs of chromosomes or 

 of genes (Aa, Bb, Cc, Dd) there are sixteen possible types of gametes. 



chromosomes. Moreover, following synapsis the gametes secure 

 one of each pair of homologous chromosomes (a haploid group), 

 but not necessarily — indeed very rarely — all of maternal or 

 paternal origin. (Figs. 183A, 189.) 



In short, when two gametes unite, each contributes to the zygote 

 a homologous haploid group of genes with the result that the off- 

 spring is of diploid gene constitution. Similarly, each gamete 

 contributes a homologous haploid chromosome group so that the 

 zygote is of diploid chromosome constitution. Thus both the 

 chromosomes and the genes (characters) are in the haploid condi- 



