Experimental Hybridizing 75 



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cells the three kinds of individuals of the second generation. A 

 white spermatozoon meeting a white egg, as one may say for 

 brevity, produces a "pure" recessive individual ; a gray sperma- 

 tozoon meeting a gray egg gives a "pure" dominant individual; 

 but when a white spermatozoon meets a gray egg, the off- 

 spring will be mixed, a heterozygote, and like the first 

 hybrid (FJ. 



If this hypothesis is true, we ought to be able to determine 

 which groups of characters where several are involved - 

 are contained in a given chromosome, for the number of chro- 

 mosomes is often small, and therefore the actual number of 

 combinations is limited. All characters contained in the same 

 chromosome should "Mendelize" together. 



If we consider two characters, the principles stated above 

 wiU apply. Let us take the case of the round and yellow, 

 angular and green peas. The round form, A, may become 

 combined with the yellow, B, or with the green color, 6, giving 

 AB or Ab; the angular form a with B or b, giving aB or ab. 

 To work this out we must suppose the color to be contained 

 in a different chromosome from the form, for otherwise they 

 could not shift over. When the chromosomes unite in synapsis 

 the ^4-bearing chromosome can only unite with a, and B can only 



unite with b. Hence since \ and ] , may be turned either wav 



[a po 



on the spindle, A may pass into a cell with B or b, and also a 

 into a cell with B or b. 



Simple and logical as Mendel's assumption appears to be on 

 the hypothesis of each character being contained in only one of 

 the chromosomes, yet I do not think it can be accepted in this 

 form because the primary assumption that each character is 

 contained in a single chromosome and not in others is highly 

 arbitrary, and also because there are certain actual results that 

 are difficult to explain on the assumption of "pure" germ-cells. 

 I In fact, it has not been shown beyond question that the chro- 

 | mosomes are the bearers of the hereditary qualities. The evi- 

 dence that is generally supposed to establish the assumption of 



