DIHYBRIDISM 



95 



every four smooth-coated, only one is white, on the average. 

 The Fo generation cannot be truly represented by a group 

 of four animals any longer, but only by a group of sixteen : 

 of these, nine show both dominant characters, blackness and 

 rough-coat ; three are black and smooth-coated, three are 

 white and rough-coated, and only one shows both recessive 

 characters, whiteness and smooth-coat. On the chromo- 

 some theory it is supposed that the two contrasting pairs of 

 characters are borne by different chromosomes, and further 

 that in the reducing divisions it is a matter of chance 

 whether it is the paternal or the maternal coat-deter- 

 mining chromosome that accompanies the paternal colour- 

 determining chromosome when it separates from its 

 partner. Thus it follows that in the Fj generation there 

 are four kinds of ova and four kinds of spermatozoa, and 

 consequently sixteen possible combinations, though many 

 of these are identical in effect. This may be represented 

 by a diagram, in which B represents black, W white, 

 R rough-coat, and S smooth-coat ; small letters are used 

 for the recessive characters where they are present, but 

 concealed by the presence of the dominant. 



When the chromosome theory was applied to Mendelism, 

 Boveri predicted that exceptions to Mendel's second law 

 would be found, for in cases where the genes for two 

 different sets of characters happened to lie on the same 

 chromosome they w^ould not be separated in the reducing 

 divisions, and the two characters would not be inherited 

 independently, but together. This was confirmed by 

 Bateson and Punnett, who, like Mendel, studied the 

 inheritance of characters in the pea, and subsequently by 

 many other workers on other subjects. The vinegar-fly, 

 Drosophila melanogaster and related species, has been 

 much used for such studies by Morgan and his colleagues, 

 for it has the advantages of being prolific and easily reared, 

 of being remarkably variable in very many characters, and 



