INHERITANCE 739 



of the crossover combinations (large cells with short flagella), 50 per- 

 cent of the other crossover combination (small cells with long flagella) . 

 The same relations hold for the 30 crossover zygotes out of 600 in the 

 recrossing of the new combinations; they yield 100 percent crossovers. 



Such results can occur only if crossing over takes place between the 

 two entire chromosomes that are in synapsis, that is, only if crossing 

 over occurs in the so-called two-strand stage of the synapsed chromo- 

 some pair. In other organisms, so far as the matter has been analyzed, 

 crossing over occurs only after the two synapsed chromosomes have 

 split, so that there are four strands instead of two. Crossing over, then, 

 occurs between but two of the four strands, with the result that two 

 strands remain without crossovers. If this were the case in Polytotna, the 

 zygotes that yield crossovers would yield but two crossover cells and 

 two that were non-crossovers, in place of yielding only crossovers. The 

 same situation is found in all the accounts of crossing over in Flagellata 

 given by Moewus, up to his article of 1938. In this publication he states 

 that he has observed in Chlamydonwnas the occurrence of crossing over 

 in accordance with the four-strand schema (details to be given later). 



None of the four pairs of characters thus far considered was found to 

 be linked with sex. That is, any of the alternative characters occurs 

 equally frequently with either sex. If, therefore, sex depends on a 

 chromosome pair, it is a fifth pair, not one of the four that carry the 

 characters above discussed. 



In an article of 1936, Moewus presents the results of extensive studies 

 of inheritance in crosses of Chlamydomonas eugametos with C. paupera. 

 Here eleven pairs of characters were distinguishable, some morphologi- 

 cal, others physiological. In addition, there were sex differences, making 

 twelve pairs of characters in all. 



In the species of Chlamjdojnonas there are ten chromosomes, so that 

 some of the twelve pairs of characters must have their factors in the 

 same chromosome, and in fact Moewus discovered that some of the char- 

 acteristics are linked. He reports that in the many crosses made, he 

 analyzed the 8,000 haploid individuals derived from 2,000 zygotes re- 

 sulting from crosses, and that he obtained 1,024 diverse types in such 

 proportions as to show that each of the ten chromosomes bears the fac- 

 tors for one or more pairs of characters. Of most of these combinations 

 no detailed accounts are given. 



Linkage was found to exist between two physiological characters, ( 1 ) 



