INHERITANCE OF WHITE-SPOTTING IN RABBITS. 



27 



carry both English and Dutch, the genetic properties of which would 

 be to produce a very wliite (low-grade) EngHsh. These two new kinds 

 of gametes would respectively be as numerous as the simple English 

 and simple Dutch gametes, if English and Dutch are inherited inde- 

 pendently (in different chromosomes, for example). If English and 

 Dutch are linked in inheritance (are borne in homologous chromosomes 

 but not at the same locus), these two classes of gametes would represent 

 the "cross-overs." The existence of the gamete which bears both 

 English and Dutch would be difficult to demonstrate, since one char- 

 acter is dominant, the other recessive; but the existence of gametes 

 bearing neither English nor Dutch would be easy to detect, either in 

 the straight Fj generation or in the back-cross generation. Such 

 gametes uniting with each other would produce self individuals, or 

 uniting with a Dutch gamete would produce a Dutch of grade 3 or 

 lower. Now, among 47 F2 young neither of these classes of individ- 

 uals has appeared. The 11 non-English F2 young are all Dutch of 

 grade 7 or higher. An even better test for the existence of gametes 

 transmitting neither English nor Dutch is the back-cross with Dutch; 

 for in this case any such gamete would produce one and the same kind 

 of zygote, viz, Dutch of grade 3 or lower. No such zygote has appeared 

 in a total of 88 Dutch and 105 English young obtained in back-cross 

 matings. This indicates that English and Dutch are either allelo- 

 morphs or closely linked. The grade distribution of these back-cross 

 young is shown in the table herewith. 



It should be of interest to compare this 

 distribution with that obtained in the orig- 

 inal English-Dutch cross (page 26) as indi- 

 cating whether the contrasted characters 

 have been contaminated in the Fi zygote. 

 The English back-cross young are of lower 

 grade (whiter) than the Fi English. The 

 respective means are 1.36 and 2.25. 



This whitening of the English is the result 

 of contamination from white Dutch in the 

 Fi zygote. No English individual produced 

 in the back-cross is darker than the English 

 produced in Fi. In both cases the darkest 

 English are of grade 3. This indicates ab- 

 sence of gametes transmitting neither English nor Dutch, for any 

 such gametes uniting with an English gamete should produce English 

 darker than grade 3. The F, Dutch (page 26) were self-white Dutch 

 heterozygotes. (Compare table 27.) None was darker than grade 3. 

 In this back-cross, any self gamete (neither Dutch nor English) which 

 might arise by cross-over should produce young equally dark, grade 

 3 or lower. But the lowest-grade Dutch recorded are of grade 6; 



