744 INHERITANCE 



tain a factor ("realisator") for the plus sex condition, the minus-pro- 

 ducing chromosomes a factor for the minus sex condition, and that by 

 crossing over, both must come into the same chromosome, which there- 

 fore produces the mixed sex condition. The cells containing such a 

 chromosome may become either plus or minus, depending on external 

 conditions. 



But if the plus and minus factors may by crossing over come into the 

 same chromosome, these factors are not alleles; they are not at the same 

 locus. The condition of the chromosomes may then be represented as in 

 Figures 172 and 173, in which P represents the plus-producing factor, 

 M the minus-producing factor. 



If crossing over occurs between the P and the M chromosomes, half of 



_E » 



Figure 173. Diagram of the sex chromosomes produced by crossing over betv/een the 

 chromosomes of Polyloma pascheri plus (light line) and P. uvella minus (heavy line). 

 The upper chromosome has the factor P from P. pascheri, the factor M from P. uvella. 



the resulting chromosomes in this case contain both P and M, while the 

 other half contains neither. Moewus holds that the cells carrying neither 

 P nor M die. Thus is accounted for the fact that each of the 157 ex- 

 ceptional zygotes produces but two cells in place of the usual four. It is 

 to be noted that here, as in former cases, the results are such as would 

 be given by two-strand crossing over; a zygote produces either no cross- 

 over cells or exclusively crossover cells. 



From these considerations and others of similar character, Moewus 

 concludes that in clones mixed as to sex (monoecious races), the sex 

 chromosome contains both the sex factors — the factor P for producing 

 the plus sex, and the factor M for producing the minus sex. 



Moewus (1936) reached similar results in a cross of Chlamydomonas 

 paradoxa and C. pseudoparadoxa. Both species are dioecious, any clone 

 being pure for one sex or the other. When plus clones of one species are 



