GENES MODIFYING NOTCH. 301 



of males, 1,095, gives 3.1 per cent of cros.sinR-ovor. (If the four (jiics- 

 tionable individuals are utilized the result is 3.4 per cent.j 



There were three classes in the males where crossing-over occurred 

 between Notch and forked, containing 413 + 14-f 1 =428 cross-overs 

 (omitting the questionable classes). Dividing the.se by the total 

 number of males, 1,095, gives 39.1 per cent of crossing-over. 



In the females there were two cla.sses where crossing-over occurred 

 between eosin and Notch, 19 + 7 = 26 (omitting the rejected clii.s.s(-s). 

 Dividing 26 by the total number of fenuiles, viz, 2,125, gives 1.2 i>er 

 cent of crossing-over. 



In the females there were two classes where crossing-over occurred 

 between Notch and ruby, containing 51 -f 18 = 69 cross-overs (omitting 

 the rejected females). Dividing these 69 by the total number of 

 females, viz, 2,125, gives 3.25 per cent of crossing-over. 



In the females there were three classes where crossing-over occurred 

 between Notch and forked, viz, 859-1-18 + 7 = 884. Dividing the.se by 

 the total number of females, viz, 2,125, gives 41.6 per cent of crossing- 

 over. 



THE IDENTIFICATION OF THE MODIFYING GENES. 



The following method, which has come into use in this laboratory 

 as the best and quickest method to identify modifying genes in the 

 second or third chromosome, takes advantage of two dominant genes, 

 one in each of these chromosomes, as well as of the fact that there is no 

 crossing-over between the members of any pair of chromosomes in the 

 male. 



The three chromosomes of the Notch female that are in\-ol\'ed are 

 represented in the left top line in figure 93. The gene for Notch is in 

 one X chromosome and the genes for eosin and ruby in the other X. 

 The second and the third chromosomes are supposed to carry the 

 modifying gene or genes, whose presence there this experiment is 

 designed to test. 



The chromosomes for the Star Dichaete (S' D') male are shown in the 

 second line. The X chromosome carries only normal genes, while the 

 second chromosome carries the gene for Star (S') in one member of the 

 pair and its normal allelomorph in the other member; the thin! chromo- 

 some carries the gene for Dichaete in one member of the pair and its 

 normal allelomorph in the other. Neither Star nor Dichirte are \i:il)le 

 in homozygous condition; hence, as stated, one member of each of the 

 pairs of chromosomes that carry these dominant genes is Star or 

 Dichaete respectively, the other normal. 



Therefore, when such a male is crossed to the selected Notch fenmle, 

 all the Star Dicha;te sons have received the Star and Dicluete g(Mies 

 (and their respective chromosomes) from the father and the homologous 

 chromosomes from the mother. The single X chromosome that the 



