102 MASS. EXPERIMENT STATION BULLETIN 199. 



Germinal Basis of Broodiness. 



The normal or wild condition is the presence of broodiness, otherwise 

 the race could not survive. Hence, any changes from the normal con- 

 dition of those parts of the germ plasm which are responsible for the 

 existence (as distinguished from amount) of broodiness will probably 

 result in a failure in the appearance of the broody instinct. Broodiness 

 may also, of course, fail to become manifest from non-genetic causes, 

 and hence some genetically broody birds are recorded as non-broody. 

 Non-broodiness, therefore, is a comprehensive term employed to describe 

 the pha?notypic condition resulting from several sorts of genetic differences. 

 The situation is parallel, in certain respects, to that of eye color in Dro- 

 sophila. Red eye is the normal or wild color. Changes in the germ plasm 

 result in a host of other eye colors, which are all non-red. In like manner, 

 from the genetic standpoint, all broody birds are presumably alike in 

 their fundamental broody mechanism, except for the homozygous or 

 heterozygous condition, and, of course, the presence of modifiers discussed 

 in the next paragraph. Non-broody birds, on the other hand, may belong 

 to ciuite different genotypes. It is, therefore, improbable that any one 

 scheme can be applied to the inheritance of non-broodiness in domestic 

 fowl. Indeed, the data given in Tables II and III indicate clearly that 

 several types of non-broodies exist in the Rhode Island Reds. The presence 

 of these types suggests that still different genetic types of non-broodies 

 will be discovered in other breeds. 



It seems clear on general grounds that a distinction must be made 

 between the primary factors concerned with broodiness and modifying 

 factors. The latter may act in various ways on the primary mechanism 

 for the manifestation of broodiness, but they cannot act unless that 

 mechanism is present. We may expect that such modifying factors wHl 

 control the intensity of broodiness in either a plus or minus direction, 

 and, extending this reasoning to its logical conclusion, such modifying 

 factors may prevent entirely any manifestation of broodiness. Non- 

 broodiness, therefore, may result from some genetic change in the second- 

 arj^ mechanism, as well as in the primary. 



There are three possible sorts of matings (the male being treated as 

 though capable of giving phsenotypic expression to his genotypic con- 

 stitution), viz., both parents may be broody, both non-broody, or one 

 broody and the other non-broody. From each of these three possible 

 matings there are three possible groupings of the female offspring, i.e., 

 (1) all may be broody, (2) all non-broody, and (3) part broody, part 

 non-broody, as shown in Table I. 



