C. C. LiTTLK 1>81 



Whiting, 1918, oonnidor lUv two roat rolourn allrlomorphic, thr h<*t4»n>- 

 /.ygwte IxMiig roininonly tortoiMo-Hlu'll. 



The t*Tiiuno|o^y umsl hy th«Mn Ih juh folloWH : 



/fejern, 1!U(J: Hhick li in (loininant to oninp* h which is iMirtu' in th«* 

 A*^ chnimtisoino. UiuhT onlinary r(»inlitiniis the factor lor nnm^«* h in 

 clcw«»ly linkiHi to T, a fact<>r fi»r torU>iw-Hhcll which acts only in the 

 pn*Si'nco o( Ii-hh\rk. The fiMualc is A' A', the luah' A'^ in fnrnjula. 



Wng/U, \9\H : Black is <hie to the action o| a factor A, while t-4»rtois4'- 

 sholl is jmniuccMl by hctorozy^<isis of an " exU'nsion " fartor p]. TorU»iw- 

 shell foniales arc thus AV, yellow males e-, and black males A*-, in 

 formula. The factor K is b(>rne in the A' chromosome. The female is 

 A' A', the male A'^ in ft>rmula. 



Doncaster, li)18 considei-s that yellow ami bhvck are allelomorphic, 

 and expri'sses yellow by V, and black by li. Where both an- present, a 

 YB or tortoise-shell animal is pHnluced. The female is A' A', and the 

 male A'^ in formula. 



W/nthifj, 1018 also nmsiders yellow Y to be allelomorphic with black 

 y, and supposes y to be borne in the X chromosome. The female is 

 homozygous, the male heterozygous for A"^. 



In 1912 I employed much the siime terminology {is that of I )onc<ister, 

 but in view of the production of blacks and tortoise-shells by two yellows 

 and the failure of blacks when crossed inter se to produce anything 

 except blacks, it is probable that the relationship between these two 

 colours may be more accurately expressed in somewhat the following 

 manner : B a factor for the production of black pigment which is found 

 in all X gametes. Y a factor for the restriction of black pigment from 

 the coat allelomorphic to y, a factor for the extension of black pigment 

 to the coat. One " dose " of Y is normally completely epistatic to one 

 "dose" of B, thus producing yellow individuals; but two "doses" of B 

 to one of Y produces a tortoise-shell. The factor Y and its allelomorph 

 y are also borne in the A"^ chromosome. Thus : 



This type of relationship will become clear us the crosses are taken up 

 in detail, and is further made use of in explaining the occurrence of 

 tortoise-shell males. 



