C. C. LiTTI.K 283 



or tortoist^-shell young, th«* pn»|K>rtion (lojK»iHling ii|)<)n whothcT the ydlow 

 nmlo or the foinalo t>r both wen* concemc<| in the fonnation of th<* yliX 

 ganieti'H. 



Thu» if the male was alone concerned, tortoim'-Hhell fenmleH. Iml lut 

 Hack males wouhl Ik* likely to apiKMir among the progmy. This appran* 

 to be the caiH» in the mating reeonled by Doncjuster ( HH.S) in wliirh two 

 yellows gave among their progeny three blue tenmlen with a cn*am 

 ooloureii patch (tortoise-shellH). If, (Ui the other hand, the fenmle {larent 

 was the unusual mut^itive individual, black males would f)ccur in aildition 

 to tortoise-shell females and yellows of both si'xes. This contlition was 

 realised in the c^use of female dilute yellow #23 (formerly owned by m<*) 

 whosi* breeding reconi is rejK)rted by Whiting, 1918. An explanation 

 of this sort would account for the aberrances note<l under Section II, 

 Heading 3, above. 



From the number of tortoise-shell and black young obttiined in the 

 twH> cases referred U^, and from the numerical relation of the black 

 females under headings 1 and 2 (Section II) to the expected colour chusses 

 (Doncaster, 1913), it seems probable that yellow animals forming yBX 

 gametes do so in approximately 50 °/^ of the gametes they form, as 

 would a normal heterozygote. 



In addition to yellow animals, certain tortoise-shell females might 

 theoretically be expected to show the same phenomenon. Such animals 

 would fonn an excess of, or jx)ssibly exclusively, yBX gametes, and, 

 in so far as they did so, would breed as blacks. Such an occurrence 

 would, however, give rise to no unexpected chisses of young in crosses, 

 but might result in the absence of some of those normally expected from 

 certain matings. Quite naturally this fact might, in a small number of 

 progeny, escape notice. 



There is no evidence to show that the appearance of any of the classes 

 above referred to is in any way connected with a break in se.r-linkage or 

 with the occurrence of tortoise-shell males, and lue may therefore, until 

 such evidence is presented, fairly consider them as independently pro- 

 duced. 



V. Criticism of existing hvpotheses to explain the appearance 



OF UNEXPECTED INDIVIDUAI^S OF NORMAL COLOUR TYPES. (Head- 

 ings 1, 2, and 3, Section II, above.) 



Attempts to explain the apiK*arance of the aberrant colour classes 

 referred to, have involved either {a) the breaking i»f .sex-linkage with 

 " crossing over " in the male, or (6) the (►ccurrence of a series of nKKlifying 



