The Genetic Factors in the Development of the Housemouse, etc. 117 



transmitted and segregated, this need not be an argument against 

 the possibility of a polygenic origin of our poultry. 



In mice, there happen to be at least two factors present in some 

 types, v\'hich are absent from the wild Mus inusculus. One of these 

 factors, which is unknown in the rabbit or the cavy produces by its 

 presence yellow colour, apparently independently from the further 

 gametic constitution. This factor I have called I. Not to complicate 

 the formulae I have not included it in them. This factor has been 

 studied by nearly all those authors who have worked with yellow 

 mice, Miss Durham, Castle, CuEnot, Haacke. (Plate's mice, as 

 I have elsewhere stated, were not yellow). The striking fact about 

 this factor, is that, until now, there has never been produced an 

 individual homozygous for its presence. All such yellow mice are li 

 and they produce gametes with and such without I in equal numbers. 

 From investigations of Miss Durham, and Castle it seems that the 

 zygotes formed of two gametes both having i fail to develop. Two 

 of these yellow mice will therefore, when mated together produce 

 twide as many yellowyoung as such of other colours. I have recently 

 bred some of those yellow mice, and get the same results. 



It seems to me not impossible that this factor i has been acquired 

 by a cross of Mus muscuhis with some other Mus having this factor. 

 It is at present quite out of the question to find out which animal 

 tliis can have been. Mns inusculus is however distributed about ever^^- 

 where on earth where people live, and it has ample opportunity to 

 come into contact with the most diverse rodents. I know of no 

 yellow wild species of the genus Mus, and it is even quite possible, 

 that if this factor I came originally from another species, this other 

 animal was not yellow at all. The fact that this factor, when present 

 in a germ, with all the other genetic factors which cooperate to make 

 a mouse, makes the animal yellow, does not prove that in quite 

 another combination of genetic factors its presence must have had 

 the same results. It is believed by some fanciers that Mus inusculus 

 and Mus sylvaticus have been crossed, and that from the descendance 

 of this cross some differently coloured mice have been derived. 



This is not impossible. By artificial insemination I once succeeded 

 in ferilizing a female ^^us inusculus with the sperm of a Mus sylvaticus 

 male, but the female either aborted or ate her offspring, for I never 

 saw any, though he was undoubtedly pregnant. Mus musculus and 

 Mus sylvaticus do not mate naturally, neither do ^fus decuvianus and 

 Mus ratlus, as far as Morgan's and my own experience go. 



