6 



INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



parents belong, whether both are of generation F or F^, or F3, or one parent 

 of one generation and the other of another. Other combinations of parental 

 characters should give the proportions in the progeny shown in table 1. 



On the second hypothesis, on the other hand, the proportions of the 

 different kinds occurring in the progeny will vary with the generation of 

 the parents. This hypothesis assumes the existence in each germ-cell of the 

 original parent of two comb allelomorphs, M and I in single-combed birds 

 and m and L in the Polish fowl, the capital letter standing for the presence 

 of a character (Median element or Lateral element) and the small letter for 

 the absence of that character. Consequently, after mating, the zygote of 

 Fj contains all 4 factors, MmLl, and the soma has a Y comb; but in the 

 germ-cells, which contain each only 2 unlike factors, these factors occur in 

 the following 4 combinations, so that there are now 4 kinds of germ-cells 

 instead of the 2 with which we started. These are ML, Ml, mL, and ml. 

 Furthermore, since in promiscuous mating of birds these germ-cells unite 

 in pairs in a wholly random fashion, 16 combinations are possible, giving 

 16 Fj zygotes (not all different) as shown in table 2. 



Tablb 2. 



* This convenient form of zygotic formulse. using a subscript 2 instead of doubling 

 the letter, is proposed by Prof. W. E. Castle. 



It is a consequence of this second hypothesis that, in F2, of every 16 

 young 9 should have the Y comb; 3 the I comb; 3 the 00 comb, and 1 no 

 comb at all. It follows further that the progeny of two F^ parents will 

 differ in different famiUes. Thus if a Y-combed bird of type a be mated with 

 a bird of any type, all of the progeny will have the Y comb. 



From Y-combed parents of various types taken at random 4 kinds of 

 families will arise having the following percentage distribution of the dif- 

 ferent types of comb : 



1. Y comb, 100 per cent. 



2. Y comb, 75 per cent; I comb, 23 per cent. 



3. Y comb, 75 per cent; 00 comb, 25 per cent. 



4. Y comb, 56.25 per cent; I comb, 18.75 per cent; 00 comb, 18.75 per 



cent; absent, 6.25 per cent. 



Again, mating two extracted I combs of F^ should yield, in F,, two types 

 of families in equal frequency as follows : 



1. I comb, 100 per cent. 



2. I comb, 75 per cent; no comb, 25 per cent. 



