6 MASS. EXPERIMENT STATION BULLETIN 396 



and the same chicks were again classified as + or — for back feathering at 8 

 weeks. The particular point of interest in these data is the percentage of + 

 and — birds in the si and SI groups, respectively. Table 4 presents the data. 



Table 4. — Relation of Chick Feathering at 12 Days to Back 

 Feathering at 8 Weeks. 1938 and 1939. 



Chick Back Males Females 



Feathering Feathering 



at 12 Days at 8 Weeks Number Percent Number Percent 



Control Line 



+ 21 95.4 108 74.5 

 si, 



1 4.6 37 25.5 



+ 274 23.9 117 12.3 

 SI, 



873 76.1 838 87.7 



Line 1, Bred for Complete Back Feathering at 8 Weeks 



+ 39 97.5 70 78.6 

 si. 



1 2.5 19 21.4 



+ 117 49.2 58 34.3 

 SI. 



121 50.8 111 65.7 



Line 2, Bred for Absence of Back Feathering at 8 Weeks 



f + 0.0 0.0 



si i 



[- 0.0 1 100.0 



+ 6 8.3 1 1.8 

 SI. 



66 91.7 55 98.2 



Almost 24 percent of the males in the control line that had not exhibited rapid 

 feathering at 12 days of age showed back feathering at 8 weeks of age. In line 1 

 about half of the males that were classified as slow feathering when 12 days old 

 developed back feathering when 8 weeks old. Only 8 percent of the males in 

 line 2 showed back feathering when 8 weeks old. These facts suggest that the 

 presence of the recessive gene si in heterozygous condition causes the develop- 

 ment of some back feathering in males at 8 weeks. The fact that almost all of 

 the male chicks that had shown themselves to be homoz^'gous for gene si at 12 

 days later developed some back feathering, points to the sex-linked gene si as of 

 major importance in the development of back feathering at 8 weeks. 



About 75 percent of the females showing rapid feathering at 12 days had 

 complete back feathering at 8 weeks of age. As has been previously pointed out, 

 every female will show some feathering on the back at 8 weeks, making a classi- 

 fication for back feathering difficult and of questionable reliability. For this 

 reason, more stress has been laid on the male side of the population. Since 

 females can carry gene si in simplex state only, its influence on back feathering 

 may be less in this sex. In all lines, females that lacked gene si were much less 

 likely than males of the same line to develop back feathering at 8 weeks. 



These earlier data point to the importance of the sex-linked gene si in its re- 

 lation to feathering in the back region. Careful attention is given later to its 

 relation to degree of feathering and to other inheritance phases of the problem. 



