INHERITANCE 743 



plus parent is pure for sex, it is the plus offspring that are pure; if the 

 minus parent is pure, the minus offspring are pure. 



Linkage and crossing over.- — Certain crosses yield a small proportion 

 of exceptional results, which are held to be due to crossing over. Such 

 are the following: 



Polytoma pascheri: pure plus clone by pure minus clone. Result, out 

 of 2,000 zygotes, 1,843 gave four cells each (as usual), two of which 

 were pure plus, two pure minus. 



The other 157 (7.9 percent) gave but two cells each, and these all 

 produced clones mixed as to sex. 



The results given by the 1,843 zygotes are those to be expected from 



P 



I 



^—^ i 



p n 



Figure 172. Diagram of the sex chromosome of Polytotna pascheri plus (upper line, 

 with factor P), and of P. uvella minus (lower line, with factors P and M). The two 

 together show the sex pair in the zygote of a cross between the two species. 



the principles thus far set forth. How are the 157 exceptional zygotes 

 to be accounted for? 



The exceptions might be produced either by non-disjunction of two 

 sex chromosomes, or by crossing over between them. If they were the re- 

 sult of non-disjunction, the 157 individuals would have received both 

 the plus-producing and the minus-producing chromosomes of the sex 

 pair; this would account for their mixed sex condition. There would be 

 a pair of sex chromosomes, in place of the usual single chromosome. 

 The exceptional individuals would therefore contain nine chromosomes 

 instead of the eight usual for Polytoma. But cytological observations 

 showed that only eight were present. The exceptional cases are there- 

 fore not the result of non-disjunction. 



The alternative explanation is that they are due to crossing over. But 

 how could crossing over between plus-producing and minus-producing 

 chromosomes yield clones mixed as to sex? 



Moewus concludes that the plus-producing chromosomes must con- 



