740 INHERITANCE 



adaptation to acid or alkaline medium, with ( 2 ) differences in the num- 

 ber of cells into which the zygote divides before the cells are set free. 

 Here, as in the former case, the results are such as to indicate crossing 

 over in the two-strand stage. Any zygote that yields crossovers yields 

 nothing but crossovers. Other cases of linkage involved factors for sex; 

 these will be mentioned in the account of sex inheritance. 



Certain general relations may be pointed out in the method of inheri- 

 tance of non-sexual characters thus far presented, particularly as illus- 

 trated by Polytoma: 



1. The inheritance is the typical Mendelian inheritance for haploid 

 organisms. The characters manifested in the haploid descendants are 

 combinations of characters that were manifested in the two parents. No 

 characters appear in the offspring that were not manifested in the two 

 parents, that is, no recessive characters occur in haploid organisms. In 

 them all characters for which factors exist are manifested. 



This is, of course, a consequence of the fact that in haploids the 

 chromosomes are not in pairs, but single. Hence heterozygotes cannot oc- 

 cur (except in the diploid zygotes) . In haploid inheritance, the combina- 

 tions of characteristics that occur in the offspring depend wholely on the 

 characteristics manifested in the immediate parents. 



2. The inheritance, like that in multicellular organisms, follows the 

 course that is to be expected if the different pairs of characters depend 

 on factors present in the different pairs of chromosomes. Independent 

 segregation, linkage, and crossing over are demonstrated in Protozoa. 

 Crossing over is, however, as before mentioned, of an exceptional type. 



SEX INHERITANCE AND SEX-LINKED INHERITANCE 



In any species or race of the flagellates examined by Moewus, there 

 are two sexes, or mating types. An account of these will be found in 

 Chapter XIV of the present volume, "Sexuality in Unicellular Organ- 

 isms." Moewus designates the two sexes on his earlier reports as plus 

 and minus, in later publications as male and female, those earlier called 

 minus being male, while the plus types are female (Moewus, 1937a). 

 Here the different types of sex inheritance will be summarized, the ac- 

 count being based upon the work of Moewus. 



In the flagellates investigated, some stocks are dioecious, others 

 monoecious. Both types may occur in different races of the same species. 



