CHROMOSOMES AND MENDELIAN HEREDITY 



303 



bined has proved difficult to obtain. In 1931 evidence of great cogency 

 was yielded by both Zea and Droso-phila. In each of these cases the 

 chromosome pair involved was heteromorphic in two regions and had two 

 pairs of genes between these regions which could be followed genetically 

 as the history of the chromosomes was being traced. 



The situation in Zea Mays is as follows (Creighton and McClintock, 

 1931) (Fig. 174). The plant selected for the test had a heteromorphic 



Fig. 174. — Diagram of evidence for the view that genetic recombination is due to an 

 exchange of portions of homologous chromosomes. In Drosophila, only the males among 

 the progeny are represented; the females also showed some crossover types. The L-shaped 

 chromosome is the F-chromosome. Further explanation in text. (Based on diagrams of 

 Creighton and McClintock for Zea, and of Stern for Drosophila.) 



chromosome pair. One member of the pair had a large knob at one end 

 and a portion of a non-homologous chromosome attached to the other 

 end ; it also carried the gene C for colored endosperm and the gene wx for 

 waxy endosperm. The other member of the pair had no knob and no 

 translocated piece, and it carried the gene c for colorless endosperm and 

 the gene Wx for starchy endosperm. The approximate positions of 

 these genes in the chromosomes were known. This plant was crossed 

 with one having two knobless, normal chromosomes carrying c wx.^'^ 

 ^^ c wx, c Wx in the original experiment; c ivx, c wx in later crosses. 



