50 INHERITANCE IN RABBITS 



black rabbit. Those with A and those without A are, however, visibly 

 different. 



Theoretically, if each factor is capable of independent variation, 256 

 different gametic combinations should be possible. In reality we are 

 acquainted with 18 visibly different color varieties, and we have evidence 

 that 48 different gametic combinations are capable of realization. This 

 leaves still a wide discrepancy between theoretical and known, and leads 

 to the conclusion either that many as yet unknown mutations are possible 

 in the rabbit, or that couplings may exist among these factors which pre- 

 vent their independent action. 



We have evidence of independent variation on the part of the factors 



A, C, I, U, and E, each of which has in one case or another either been 

 lost or been replaced by the alternative condition already described ; but 



B, Br, and Y are un variable; at least we have not ourselves seen evidence 

 among rabbits of independent variation on the part of these factors. 

 There can be no question, however, that both in the guinea-pig and in 

 the mouse such variation has occurred, resulting in the complete loss of 

 B from the gamete, and it is possible, as elsewhere stated, that such a 

 change has already occurred among European rabbits. Supposing, how- 

 ever, that B, Br, and Y are all constant constituents of the rabbit gamete 

 and that each of the five others may be either present or absent, the num- 

 ber of different gametic combinations theoretically possible becomes 32. 

 We have reason to believe that this entire assortment is produced and 

 that 1 6 other ones also occur owing to a second and different sort of vari- 

 ation in factor C. 



GAMETIC AND ZYGOTIC FORMULA. 



The diagram given on page 49 was intended to express the known aggre- 

 gate of independent factors which a pure gray rabbit transmits in each of 

 its reproductive cells (gametes). In producing a new individual each re- 

 productive cell must unite with another reproductive cell, the two together 

 forming a zygote. An individual resulting from the union of tw r o gam- 

 etes of like constitution will be double as regards each hereditary factor. 

 It is known as a homozygote (Bateson). This double condition we might 

 express by a subscript 2 following the symbol for each factor indicated. 

 We should then have a zygotic formula for the individual. 



But it sometimes happens that a gamete unites with another gamete 

 having a composition slightly different from its own --one which, for 

 example, lacks one or more factors found in itself. The zygote produced 

 is then a heterozygote and will be double as regards certain factors, but 

 single as regards others. But in sexual reproduction, as is well known, 

 there is a return from the double to the single condition. So that when 

 a heterozygous individual attains sexual maturity, it forms gametes each 

 of which contains the factor double in the zygote, but as regards those 

 which were single in the zygote, half the time they will be present, half 



