REVIEW OF MENDELISM 275 



Mendel had no knowledge of chromosomes or of the chromosomal 

 mechanism of maturation, which now seems to be the machine respon- 

 sible for the regularities seen in the various Mendelian ratios and for 

 segregation in general. It is remarkable, therefore, that Mendel fore- 

 saw a mechanism within the genetic apparatus of plants that coin- 

 cides in principle with that subsequently discovered. Arhong the great 

 discoveries that have resulted from the use of Mendelian methods 

 and procedures are the factor hypothesis, the chromosome theory of 

 heredity and of sex determination, linkage and crossing-over, and the 

 finer details of the heredity machine. 



The presence and absence theory. — In its original form the pres- 

 ence and absence theory implied that some gene was present in the 

 dominant individual that was absent in the recessive. It has been dis- 

 covered, however, that this theory fails to hold, especially in cases of 

 multiple allelomorphs, and probably does not hold at all. As an ex- 

 ample of multiple allelomorphs, we may cite the various eye-color 

 mutants of Drosophila (see page 294). A whole series of eye-color con- 

 ditions ranging from red to white are all known to be the result of mu- 

 tational changes in the same gene, with red, the wild type condition, 

 being dominant over any of the mutant conditions. Now these vari- 

 ous colors, such as vermilion, pink, salmon, cream, etc., are all reces- 

 sive to red, but dominant over white. This shows that the gene for 

 eye color is present in all these mutants but is merely modified in 

 various ways or in varying degrees. 



The presence and absence hypothesis, therefore, might then be 

 stated as follows: There is some positive element present in the 

 dominant gene that is absent in the recessive allelomorph, which pre- 

 vents the recessive gene from expressing itself. In this form the theory 

 is rather helpful and serves to rationalize the practice, now universal, 

 of representing any dominant gene by a capital letter, such as A , and 

 the recessive gene by the equivalent small letter, a. 



The factor hypothesis. — Mendel believed that for each character 

 there was a determiner; that each determiner produced a character 

 unaided. According to the factor hypothesis, as we have seen, such 

 simple characters as colors in plants and animals depend upon the 

 interaction of several genes, which are called "factors" because they 

 are not single causes but merely co-operative agents. It is now com- 

 ing to be believed that each character of the organism is the product 

 of the interacting of many, possibly all, of the genes in the organism, 

 but that some of the genes affect a given character more than do others. 



