286 Colour Inheritance in Cats 



If now the eggs of such a female are fertilized by sperm of a normal 

 male we have four possible types of zygotes. 



Eggs Sperm Zygote 



XX X XXX Dies 



X X- " Near male " always sterile 



XX 6 XXd Female with peculiar gametic condition 



d d- Dies 



Bridges has demonstrated that the XXX and 6- forms die, a7id that the 

 X- form although appearing like a male is always sterile. If now we 

 imagine a cross to be made between a tortoise-shell female cat showing 

 non-disjunction and a normal yellow male, we should have the following 

 condition : 



Non-disjunctional 

 Tortoise-shell female YBXijBX Normal Yellow male YBXe 



Forming gametes YBXyBX and — Forming gametes YBX and 6 



Zygotes (a) YBXyBXYBX Dies 



(6) YBXyBXd Tortoise-shell with peculiar gametic conditions 



(c) YBX- "Near male" always sterile 



[d] - d Dies 



If now one assumes that absence of the 6 chromosome allows the " near 

 male " class (c) to develop into a tortoise-shell, disturbing the normal 

 relation of yellow to black to produce a somatic mosaic, we could account 

 for the appearance at rare intervals of tortoise-shell " near males " which 

 were not fertile. It seems not unlikely that the absence of the 6 

 chromosome might well upset the somatic relationships of certain of 

 the characters whose factors are carried by the X chromosome. This 

 would account for the appearance of a tortoise-shell "male" from a 

 mating of yellow male x tortoise-shell female. (Doncaster, 1913.) 



Another mating which, according to Doncaster, has produced a 

 tortoise-shell male is that of yellow male by black female. Here, if the 

 black female showed non-disjunction, the following condition would be 

 found : 



Black non-disjunctional female yBXyBX Normal yellow male YBXd 



Forming gametes yBXyBX and - " Forming gametes YBX and d 



Zygotes (a) yBX yBX YBX Dies 



(ft) yBX yBX 6 Black female with peculiar gametic conditions 



(c) — YBX Tortoise-shell? "near male" always sterile (as in 



previous mating) 

 {d) — d Dies 



The third type of mating reported by Doncaster as having. produced 

 a tortoise-shell male is that of black male with tortoise-shell female. 

 Here everyone is in difficulty. If, as Doncaster suggests, the occasional 

 crossing over of F, the factor for yellow, to a ^ gamete is responsible for 



