NONCHROMOSOMAL GENES 



237 



mutants. Two classes of streptomycin-resistant (sr) mutant strains have 

 been obtained: those which show typical Mendehan inheritance in 

 crosses, and those which do not. As an example of the latter class, Fig- 

 ure 9.1 shows a diagram of a cross between a resistant strain, sr-500, and 

 a sensitive strain, ss. 



When sr-500 cells of mating-tvpe plus {mt ) are crossed to any ss 

 strain of mating-tvpe minus (m^~), all progeny are found to be uniformly 



All sr 



All ss 



All sr 



All ss 



FIGURE 9.1. Nonchromosomal inheritance in Chlamydomonas. Plus and minus 

 signs refer to mating type which is inherited as a chromosomal gene diflFerence. The 

 cross sr mf X ss mf gives rise exclusively to sr offspring which segregate 2:2 for 

 mating type and other chromosomal genes. Fj clones of plus mating type backcrossed 

 to ss mt produce all sr offspring (4:0) but Fj clones of minus mating type back- 

 crossed to ss mf produce only ss progeny (0:4) (after Soger, 1954, Proc. Natl. Acad. 

 Sci. Wash., 40:356). 



