No intermediate hybrids are produced. Regarding 

 the shade factors in most of the colors, it is impossible at 

 present to determine their inheritance. ^ 



(d) — Abnormal Cases of Mendelism 



In the foregoing examples the factors representing the 

 characters of zygotes are independent and normally dis- 

 tributed. From the outset abnormal distributions were 

 observed and these were looked upon as cases that did not 

 follow the laws of Mendel. Later, however, these apparent 

 exceptions were explained and they too fell into line in 

 harmony with the Mendelian laws. 



The disturbing causes may be grouped as follows: — 



1. the absence of or imperfect dominance; 



2. the presence of multiple factors when several differ- 

 ent factors produce similar effects ; 



3. the presence of compound factors when certain factors 

 interfere, or do not act unless aided by others; 



4. Linking of two or more factors, and crossing-over. 





J 



Fig. 17. — Diagram sliowing the course 

 of color heredity in the Andalusian fowl, 

 in which one color does not completely 

 dominate another. P parental generation. 

 The offspring of this cross constitute 

 Fl, the first filial or hybrid generation. 

 F2, the second filial generation. Bottom 

 row, third filial generation. 



1 — Imperfect Dominance 



Sometimes the offspring (FO are unlike either parent; 

 in such cases the dominance is said to be imperfect. A 

 classic example is the Blue Andalusian fowl, which is 

 heterozygous. The homozygous forms are the black and 



(1) — For other theories of coat color see Babcock and Clausen, Chap. 

 XXIX. 



101 



