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INTRODUCTION TO CYTOLOGY 



placed on the silks of homozygous dominant plants {PI PI). Normally all 

 of the immediate offspring {PI pi) of such a cross are purple, because 

 of the dominant PI derived from the pistillate parent. When, however, 

 the young embryos were X-rayed (20 hours after pollination), some of 

 them developed into non-purple plants. When such non-purple plants 

 were examined cytologically, it was found that one of the satellited 

 chromosomes lacked a portion of its longer arm (Fig. 181, £"). From this 

 it could be concluded that the irradiation had somehow caused the loss 











Fig. 181. — Diagrams of inversion and deletion in Zea. A, the region between the 

 arrows in a normal chromosome (left) became inverted (right). Synapsis of homologous 

 regions of an inverted with a normal chromosome gave configuration shown diagram- 

 matically in B and semidiagrammatically in C; cf. Fig. 182, a; also Fig. 160. D, normal 

 satellited chromosome synapsed with one lacking most of its shorter arm; cf. Fig. 182, b. 

 E, normal satellited chromosome synapsed with one lacking a large portion of its longer 

 arm. Arrow indicates suspected position of gene PI. F, synapsed number VII chromo- 

 somes, one of which lacks a median portion, leaving the corresponding portion of the other 

 extending as a loop; cf. Fig. 182, c. The pair in this plant is heteromorphic for the chro- 

 matic knob, k, chromatic knob, n, nucleolus, s, spindle-attachment region. (After 

 McClintock, 19316.) 



of this portion and that the gene PI had been located in it. Loss of the 

 dominant PI left the plants with only the recessive pi derived from the 

 pollen parent ; hence the plants were non-purple. Similarly, it has been 

 shown in another such test that the gene Ig (liguleless) probably is near 

 the end of the shorter arm of chromosome II (McClintock, 19316). 



Especially instructive are certain cases in Drosophila reported by 

 Painter and Muller (1929). By the use of X-rays there were obtained 

 several flies in which one chromosome had undergone "intercalary 

 deletion," i.e., the middle region had been lost, leaving the two ends as a 

 single small chromosome (Fig. 179, 2). This small chromosome remnant 



