RECOMBINATION IN SEXUAL ORGANISMS 83 



TRANSMISSION GENETICS OF DIPLOIDS 



Mendels laws hold for all organisms but their consequences appear 

 differently according to whether the gametes may be analyzed directly. 

 Figure 3.9 contrasts the life cycles of some haploid and diploid organ- 

 isms. The baker's yeast, SaccJmromyces cerevisiae, is intermediate; part 

 of its life cycle is in a diploid and part in a haploid phase because both 

 the gametes and the zygotes multiply asexually. Hence in either phase 

 enough cells may be collected to observe phenotypic traits and to infer 

 the genotype. In haploid forms such as Chlamydomonas and Neuro- 

 spora only the gametes provide enough cells by multiplication to make 

 such analyses convenient. In multicellular forms it is the zygote which 

 divides asexually, the associated mitoses insuring a population of diploid 

 somatic cells. The gametes themselves do not usually divide. 



Nonetheless, meiosis in diploid organisms takes place essentially as in 

 haploids (Figure 3.3). As a result the alternative members of a pair of 

 alleles are distributed among the sperm or eggs in a 1:1 ratio. Usually 

 the genotypes of the gametes cannot be determined directly, but crosses 

 must be employed and deductions made from the phenotypes of the in- 



TABLE 3.3 



Plaque Morphology of Mutants of Bacteriophage 7*4 on Various Host Bacteria 



and Dependence of Genetic Map on Choice of Host 



(After Benzer, in McElroy and Glass, 1957, The Chemical Basis of Heredity. 

 Baltimore: The Johns Hopkins Press, p. 70) 



E. coli Host Strain 

 Phage Strain B S K 



Wild wild wild wild 



rl r r r 



rll r wild m 



rlll r wild wild 



Bacterial 

 host strain Genetic map 



rll rl rlll 



r 



m r 



K =^Z2 □ z= 



