INHERITANCE 745 



mated with minus clones of the other, the majority of the zygotes yield 

 four clones, of which two are pure plus, two pure minus. But in a par- 

 ticular case, out of 1,000 such zygotes, 117 gave exceptional results, all 

 the four clones from each zygote being mixed as to sex. These excep- 

 tions are held to be due to crossing over. One of the haploid parents had 

 the factor P, the other the factor M in the sex chromosome. By crossing 

 over, the two factors are brought into one chromosome; the cells that 

 receive this chromosome yield clones that are mixed as to sex. Half of 

 the sex chromosomes in which crossing over occurred would lack both 

 the factors P and M. Moewus holds that the cells that receive such 

 chromosomes die, while the cells that receive both P and M divide twice 

 before escaping from the zygote. Thus the zygote produces four cells as 

 usual, all having the crossover combination P and M. Here, as in former 

 cases, the results are those characteristic for two-strand crossing over. 



Many other cases of crossing over of the sex factors (with results that 

 require two-strand crossing over) are described by Moewus (1936) in 

 crosses of C. eugametos and C. paupera — these all indicating that clones 

 which are mixed as to sex carry a chromosome which has both sex fac- 

 tors, P and M. Other cases will be mentioned in later paragraphs. 



Sex-linked inheritance. — In crosses of the two species of Chlamydo- 

 monas just mentioned, Moewus observed sex-linked inheritance. The 

 species C eugametos has an eyespot, while C. paupera has none. When 

 C. eugametos of one sex is crossed with C. paupera of the other sex, in 

 the descendants the gametes that are of the same sex as the C. eugametos 

 parent have the eyespot, while those that are of the same sex as the 

 C. paupera parent have none. The eyespot is thus linked with sex. 



There are, however, a few exceptional cases, due to crossing over, in 

 which the eyespot no longer goes with the parental sex. The results are 

 complex and will not here be presented in detail. In all cases the re- 

 sults, as given by Moewus, are those that would be characteristic for two- 

 strand crossing over. By analysis of the results, Moewus believes that he 

 is able to establish the order in the chromosome of the two sex factors 

 (P, M) and that for eyespot (S), as P-M-S. 



Relative sexuality, in crosses hetiueen different species. — In crosses 

 between different species, in some cases gametes of like sex may copulate 

 and yield descendants (see Chapter XIV). Plus gametes from one spe- 

 cies may unite with either plus or minus gametes from the other species, 



