92 



PHYSICAL BASIS OF HEREDITY 



is back-crossed to a white miniature male the four classes 

 of offspring appear in the following proportions : 



Non-crossovers 

 White miniature Red long 



33.5 per cent 33 5 per cent 



67 per cent 



Crossovers 

 White long Red miniature 



1G.5 per cent 16.5 per cent 



33 per cent 



Here crossing over takes place in 33 out of 100 flies, 

 whereas in the former crosses between white eyes and 

 another mutant character (yellow) crossing over took 

 place only once in a hundred times. Based on these 

 numerically different ratios of crossing over, and on other 

 related phenomena, a theory of crossing over has been 

 formulated that will be discussed later. For the present 

 we are concerned only with the data. 



When more than two pairs of characters are involved 

 new phenomena of crossing over make themselves evident. 

 Some of these are more related to principles that are dis- 

 cussed in later chapters, but a few results may be pointed 

 out here. When, for example, three pairs are involved, 

 two may cross over, while the third does not. A female 

 with white eye color, miniature wings, and forked bristles 

 crossed to a wild-type male gives wild type in F^. An 

 Fj daughter back-crossed to a white miniature forked male 

 will give, in the next generation, not only the two original 

 combinations but recombinations also. As we have seen, 

 33 per cent, of all the offspring will be crossovers between 

 white and miniature; there will also be 20 per cent, of 

 crossing over between miniature and forked. In other 

 words, there will be both red miniature and white long 

 flies, and there wall also be crossovers between white and 

 miniature, i.e., miniature wings straight spines, and long 

 wings forked spines. It follows that there may also be 

 cases in which crossing over has taken place between both 

 of the above combinations at the same time (Fig. 38), 

 that is, there will be some flies that are white long- winged 

 and forked and others that are red miniature and straight 



