198 Outline of Genetics 



strains are more strongly plus (female, as was later determined by 

 indirect means) and others less strongly plus, and the same was 

 true of the minus (male) strains. This strongly suggests a quanti- 

 tative interpretation of sex. 



Recently Burgeff (9) has discovered some startling facts in 

 connection with sex in some of the other genera of Mucorineae. 

 Absidia shows the customary plus and minus strains, as does also 

 Parasitella. This latter genus is a parasite upon other genera of 

 the same family, and in connection with this parasitic habit there 

 appears a remarkable situation. The plus strain of Parasitella 

 will parasitize the minus strain of Absidia but not the plus strain 

 of Absidia; while the minus strain of Parasitella will parasitize 

 the plus strain of Absidia but not the minus strain of Absidia. 

 The author concludes that the hypothetical sexual substance which 

 distinguishes the plus and minus mycelia of Absidia is identical 

 with the substance that induces parasitism, and that the parasitic 

 relationship here has arisen as the result of an unsuccessful attempt 

 at hybridization between the two genera. 



The discussion to date leaves the interpretation of 

 sex determination in a distinctly unsettled condition. 

 We find that in a great many animals and a very few 

 plants a very definite sex chromosome mechanism oper- 

 ates to determine sex; and that sex is a qualitative 

 proposition, only the two conditions of strictly male and 

 strictly female being possible. On the other hand, it is 

 suggested by the sexual behavior of some animals and 

 quite a number of plants that the general physiological 

 condition is important in determining sex; and that sex 

 is a quantitative matter, intersexes or sex intergrades 

 being possibilities that are frequently realized. 



There are three possible conclusions with reference 

 to these contradictory theories: (i) an acceptance of 

 one and rejection of the other; (2) the claim that both 

 amount to the same thing, that they express the same 



