746 INHERITANCE 



and minus gametes from one with plus or minus from the other. 

 This phenomenon, known as relative sexuality, is accounted for by 

 Moewus (following Hartmann) by assuming that the gametes of the 

 different species differ in the strength, or "valence," of their sex tendency 

 and that if two gametes differ sufficiently in the strength of the sex 

 tendency they copulate, whether of the same or of different sex. 



Exemplifying this situation is the fact that any gamete of Polytoma 

 uvella may copulate with any gamete of P, pascheri. irrespective of the 

 sex of the gametes. Hence it is held that in one of these species the sex 

 tendency of the gametes is stronger than in the other species. 



The relations as to crossing over described in the preceding section 

 offer an opportunity for determining whether the sex factors present in 

 the chromosomes of the two species differ in the strength of the sex 

 tendency; also for determining in which species the sex factors are 

 stronger. By crossing over, the plus factor of one species may be brought 

 into the same chromosome with the minus factor of the other species. It 

 is then possible to determine which of these prevails, and thus to discover 

 in which species the sex factor is stronger. 



Moewus crossed plus P. pascheri gametes from a clone pure for sex 

 with minus P. uvella gametes from a race mixed for sex. According to 

 the hypothesis, the P. uvella gametes, being from a race mixed for sex, 

 contain a sex chromosome which carries both the plus and the minus 

 factors, P and M. The P. pascheri gametes have a sex chromosome con- 

 taining only the P factor. The situation as to sex chromosomes in the 

 zygote may therefore be represented as in Figure 172. From this cross 

 625 zygotes were obtained. Of these, 582 gave the results that are usual 

 without crossing over. Each zygote gave two clones that are pure plus, 

 as in the P. pascheri parent, two that are mixed, as in the P. uvella parent. 



The remaining 43, or 6.9 percent, gave exceptional results, presumably 

 due to crossing over. In these, two of the four cells from each zygote 

 yield pure plus clones, like the P. pascheri parent, while the other two 

 yield pure minus, unlike either parent. No mixed clones are produced. 

 How are these results accounted for? 



The original condition of the chromosomes is that shown in Figure 

 172. By crossing over (the break occurring between P and M), the con- 

 dition shown in Figure 173 is produced. One chromosome contains only 

 the plus P. uvella factor (P) ; it is obviously the gametes containing this 



