298 



INTRODUCTION TO CYTOLOGY 



due to the gene R showed trisomic ratios while characters in other linkage 

 groups showed ordinary ("disomic") ratios. Hence this gene and other 

 genes of the same linkage group are carried by the smallest chromosome of 

 the normal set (McClintock and Hill, 1929, 1931) (Fig. 66, d). 



In Datura stramonium, which ordinarily has 12 pairs of chromosomes, 

 similar studies on trisomic strains have led to the assignment of certain 

 genes to certain distinguishable chromosomes. In this species each of the 

 12 chromosomes produces, when present in triplicate, a characteristic 



m 



TS 



"21 



•sn 



vnr 



TL 



\ 



O 



P 



as 



br 



f 



oin 



B 



sk 

 ■tsi 



ygz 

 c 



sh 



VI 



12 



3' 

 P9' 



Fig. 170. — Diagram of chromosome set of Zea Mays, showing the relative lengths and 

 the location of the spindle-attachment regions (cross lines) in the 10 members. Inversions 

 and translocations sometimes cause variations in these features. Chromatic knobs 

 appearing in one or more of the chromosomes of certain strains are also indicated. Below 

 the chromosomes are some of the genes which they carry. Cf. Figs. 171, 66, and 152. 

 (Based on diagrams and data of McClintock, Emerson, and others.) 



appearance in the plant which may be readily recognized in the breeding 

 plot (Belling, Blakeslee). 



Such results as these serve to emphasize the fact that the chromosome 

 set is a definitely differentiated system, each member containing a group 

 of elements with specific effects upon the course of development and 

 therefore upon the characters which appear in the organism. 



Location of Genes in the Chromosome. — That the genes of a given 

 linkage group are arranged in a definite linear order in the chromosome 

 was first clearly shown by the extensive researches on linkage relations 

 in Drosophila. It was observed that some factor pairs were closely 

 linked, i.e., crossing-over occurred between them very rarely; whereas, 

 other pairs were more loosely linked, as shown by the consistently high 

 frequency of crossing-over between them. The hypothesis was advanced 



