48 Inheritance of (Joal-Colonr in J^ahhifs 



When the agouti factor is present the three homozygous forms 

 XX, X'X', X"X" form what may be regarded as a series of diminishing 

 intensity for the melanic pigment, viz. black, agouti, yellow. In the 

 absence of the agouti factor the series is black, black, tortoise. Though 

 the first term is now in appearance indistinguishable from the middle 

 one, it is theoretically possible to regard it as potentially of a more 

 intense black, the difference between the two being rendered visible 

 when A is inserted as an indicator. So far the manifestation of pig- 

 ment is not at variance with the view that the throe allelomorphs 

 X, X' and X" form a series of diminishing intensity. A difficulty 

 however arises when we come to consider certain of the heterozygous 

 forms. XX' AA is agouti-black, and since X' must be regarded as a 

 factor for higher pigment intensity than X" we should expect XX"AA 

 to shew less intensity of melanic pigment than XX'AA. It, should be 

 something lighter than agouti-black, whereas it is actually full black. 

 We cannot therefore regard the factors X, X', and X" in this case as 

 forming a series of diminishing intensity for melanic pigmentation, 

 and Sturtevant's argument, applicable in the cases of the Himalayan 

 rabbit, the eye-colour of Druso])hila, and the variegated Aqmlejia, 

 here foils to the ground. 



Apart from the difficulty of interj)reting the experiments dealing 

 with the D factor in rabbits in terms of the " Multiple Allelomorph " 

 hypothesis, there seem to me to be other reasons for treating all of 

 these cases as examples of complete coupling and retaining the inter- 

 pretation in terms of the " Presence and Absence " hypothesis. In this 

 connection some recent work of Tanaka on silkworms is much to the 

 point. In his account of the inheritance of the two larval characters 

 striping (S) and yellow (Y) as opposed to non-striping (s) and white (y), 

 Tanaka shews that the experimental data can only be interpreted on 

 the supposition that there is partial coupling in the male gametes and 

 complete coupling in the female gametes. The male gametic series is 

 of the form 3SY : ISy : IsY : 3sy (or 2SY : ISy : IsY : 2sy), while the 

 female series is of the form SY : sy. Unless we are to suppose that 

 the hereditary mechanism is of a different order in the two sexes, we 

 can scarcely avoid the conclusion that the complete coupling exhibited 

 in the female is but a particular form of the partial coupling found 

 in the male series of gametes. And if we accept the hypothesis of 

 complete coupling in this case it seems not unreasonable to extend 

 it to other cases where partial couijling for the factors concerned is 

 hitherto unknown. 



