290 



FOUNDATIONS OF BIOLOGY 



fully accounted for, provided the gene for yellow and the gene 

 for round are not borne by the same chromosome, since in 

 maturation the gametes secure one of each pair of homologous 



FIG. 147. Diagram to show the union of simplex groups of either the chromosomes 

 or of the genes of the gametes to form the duplex condition of the zygote and animal 

 body; and then 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 

 (After Wilson.) 



chromosomes (a simplex group), but not necessarily all of 

 maternal or paternal origin. (Fig. 147.) 



In short, when two gametes unite they each contribute 

 to the zygote two corresponding, simplex groups of genes with 

 the result that the offspring is of a double, or duplex gene, 

 constitution. Similarly, the gametes contribute two simplex 

 chromosome groups so that the zygote is of a duplex chromo- 

 some constitution. Thus both the chromosomes and the 



