BROODINESS IN DOMESTIC FOWL. 



105 



Several factorial explanations of the observed ratios between broodies 

 and non-broodies in the several famihes can be developed, but choice 

 among such explanations cannot be made because of the small size of the 

 individual famihes, i.e., the offspring of a single mother. Nor are any 

 of them of value save as working hypotheses. The one on which Table 

 II is based is presented simply to show that a close agreement between 

 theory and fact is possible, and this theory was chosen for presentation 

 because it gives a slightly better agreement between observed and theo- 

 retical rations, with one partial exception, than the others. This theory 

 assumes that the appearance of broodiness in Rhode Island Reds requires 

 the simultaneous presence of two factors, designated A and C, in either 

 homozygous or heterozygous condition. A better fit in the case of the 

 partial exception can be secured by assuming that there is also a dominant 

 factor (presumably a modifier) for non-broodiness, which may be desig- 

 nated as N. Non-broodies, therefore, may be of numerous genetic types, 

 the homozygous forms being NNAACC, niiAAcc, nnaaCC; and nnaacc, 

 where A and C, respectively, represent the factors (condition of germ 

 plasm) necessary for broodiness. A broody bird, then, in homoz3'gous 

 form must be nnAACC. As shown by Table III, which gives the theoreti- 



Table III. — The Theoretical Ratio>^ resulting from Matings of Different 

 Types, airanged by Ratios. 



[A ratio appears only once under its respective theory.] 



cal ratios expected on both the NNAACC and AACC theories, it will 

 be seen that matings between birds of the' same phsenotype may give 

 several different ratios, including those in which the proportions between 

 broody and non-broody birds are reversed. Thus (NNAACC theory) 

 broody x broody may give 3 broody to 1 non-broody, or it may give 1 



