184. TECHNICAL BULI.F/JIN 11 



straight line gives a less abrupt negative slope. An examination of tliese 

 frequency distributions has brought to light \aluabl(' information concern- 

 ing the inheritance of winter clutch size. 



Goodale (1918) recognized the fact that characteristic types of rhythm 

 in laying exist in Rhode Island Reds. He applied the time element and 

 classed hens as one-half, two-thirds, three-fourths, etc. with respect to 

 rhythm. He fully recognized the importance of rhythm in egg laying. 



Riddle (1925) presents data to show that the common pigeon lays the 

 greatest percentage of single eggs during January and February and the 

 smallest percentage during July and August. Since the characteristic 

 clutch of the pigeon consists of two eggs, it seems probable that adver.se 

 weather conditions tend to reduce the rate of laying and in consequence 

 the mean clutch size. 



Daily egg records of the wild ancestors of the domestic fowl are not 

 available for study in comparison with the records of improved flocks. 

 Information on the question of clutch size must for this reason be obtained 

 on flocks of domestic fowl largely unimproved in fecundity and from some 

 information on wild species of birds. 



The mean winter clutch size of the foundation birds hatched in 1912, 

 from which the flocks reported upon are descended, is 1.9 for the 119 birds. 

 The mean for the 276 birds hatched in 1916 is 2.5, and for the 541 birds 

 hatched in 1925, 3.1. The normal clutch size for the common pigeon is 2. 

 but this normal may be modified by weather conditions as Riddle (loc.cit.) 

 shows. Data on the Massachusetts flocks of Rhode Island Reds hatched 

 from 1916 to 1925 indicate the existence of two modal classes for winter 

 clutch size and probably a third modal class higher than these two. The 

 first mode occurs at a clutch size of 2 and is very distinctly separated 

 from the second mode, which is about 2.2. The third mode probably is 

 at the 2.6 class. What then is the behavior of clutch size in inheritance? 



Proposed Theory. 



Extensive studies of available data on Rhode Island Reds indicate that 

 the normal clutch size for the domestic fowl is two and that adverse 

 weather conditions may operate to reduce some of the clutches to one. 

 Further bearing on this point is the fact that the hen not infrequently 

 liberates two ova almost simultaneously making a double-yolk egg, but 

 that the occurrence of more than two ova in the same shell is an extremely 

 rare phenomenon. It seems probable, therefore, that the hen ordinarily 

 ovulates twice either on the same day or on successive days, and that a 

 greater length of clutch than two represents a modification of the normal. 



The first modal class appears to consist of the normal individuals that 

 ovulate twice in favorable environmental conditions and thus have a char- 

 acteristic clutch size of two or less than two. The second modal class 

 In the flocks studied is made up of birds modified for clutch size, so that 

 their mean is greater than two. The third modal class occurs at a clutch 

 size of 2.6. 



On a Mendelian factor basis the following seems warranted after very 

 extensive studies of clutch size in families of sisters: That the normal un- 

 improved hen is a recessive for clutch size. That a gene I added to normal 

 gives a clutch size greater than 2. That a second gene I' makes a clutch 

 size of 2.6 or more possible. That genes and I and I' together give the 



