HETEROZYGOUS CHARACTERS 111 



hybrid ratio (9:3:3:1) we might expect to see modified in a 

 similar way, if a cross were made involving simultaneously 

 two Mendelian characters imperfectly dominant. The num- 

 ber of distinguishable classes, as shown originally by Mendel 

 (see Appendix) would then be 9, numerically as follows: 1:1: 

 2:2:4:2:2:1:1. For three factors all imperfectly dominant 

 the modified trihybrid Mendelian ratio would be expressed 

 by (1 + 2 + 1) ' and for n factors by (1 +2 + 1)°. Hetero- 

 zygous characters must from definition always be unfixable. 

 In the foregoing cases comparison of their behavior in breed- 

 ing experiments with that of the corresponding homozygotes 

 has shown this to be true, but there exist cases in which 

 only one type of homozygote has been found to occur, the 

 other being apparently impossible of production. 



The first case of this sort to be demonstrated is found 

 among yellow mice and to Cuenot (confirmed by Little) we 

 owe its demonstration. If certain strains of yellow mice are 

 crossed with black ones, the offspring produced are of two 

 sorts equally numerous, yellow and black. From this result 

 alone it is impossible to say which is the dominant character, 

 but breeding tests of the offspring show that yellow is the 

 dominant character. For the black offspring bred together 

 produce only black offspring, but the yellows bred together 

 produce both yellow offspring and black ones. The curious 

 feature of the case is that when yellows are bred with each 

 other no pure yellows, that is, homozygous ones, are obtained. 

 Hundreds of yellow individuals have been tested, but the 

 invariable result has been that they are found to be hetero- 

 zygous; that is, they transmit yellow in half their gametes, 

 but some other color in the remaining gametes — it may be 

 black or it may be brown, or gray. Non-yellows obtained by 

 mating yellow with yellow mice never produce yellow off- 

 spring if mated with each other. This shows that they are 

 genuine recessives and do not contain the yellow character, 

 which is dominant. 



Now ordinary heterozygous dominants, when mated with 

 each other, produce three dominant individuals to one reces- 



