INHERITANCE 747 



that yield the pure plus clones. The other chromosome contains both the 

 P from P. pascheri and the M from P. uvella. The gametes that contain 

 such chromosomes act, as above mentioned, like pure minus gametes. 

 Therefore the effect of the M P. uvella factor completely overcomes the 

 effect of the P factor from P. pascheri. The sex factors of P. uvella are 

 thus stronger in sex tendency than those of P. pascheri. 



From this the further conclusion is drawn that the gametes of P. uvella 

 are stronger in sex tendency than the gametes of P. pascheri. Since P. 

 uvella has only monoecious races (mixed as to sex), these gametes, ac- 

 cording to the assumption, contain both a strong P and a strong M 

 factor. It might be anticipated that the two would partly or entirely 

 counteract each other, leaving the gametes weak or neutral in sex tend- 

 ency. This, however, does not occur; if Moewus's assumption is correct, 

 one of the two fully prevails, the other having no effect, so that the 

 gametes are strong. 



Moewus carried out numerous other crosses of these two species, which 

 gave results that are in accord with those just set forth. They all indicate 

 that the P. uvella factors have a stronger sex tendency than the P. pascheri 

 factors. They agree further with the idea that the P and M factors are in 

 different loci, and that by crossing over they may be brought into the 

 same chromosome. 



One further result of these experiments is of special interest. In cer- 

 tain cases there were crosses of two dioecious races (produced by hy- 

 bridization), in which the gametes were of the same sex (both plus or 

 both minus). In such crosses between plus gametes, according to the 

 theory, each gamete contains only the P factor; no M factor is present. 

 All the offspring in such cases are then of the plus sex, as the theory 

 would lead one to expect. Similarly, if the two gametes each contain 

 only the minus factor, all the offspring are of the minus sex. 



An elaborate investigation of these matters was carried out by Moewus 

 (1935a, 1935c) on diverse races of the unicellular alga Protosiphon 

 botryoides. The relations here, while differing much in details, are con- 

 cordant with those above set forth for Polytoma in the matters of cross- 

 ing over of sex factors, different strengths of sex tendencies in different 

 races, segregation at the reduction division, and the like. The results will 

 therefore not be taken up in detail here. It is to be remarked that in this 

 species, as in the others, crossing over (as the data are reported by 



