HEREDITY IN SOMATIC CELLS 365 



type, paba y w ad , does not fall in this class; it is white rather than yel- 

 low because the white gene is epistatic to yellow: 



X — > yellow pigment — > green pigment 



w 



The other 85 per cent of the sectors form diploid conidia which are 

 homozygous for either w or y. The white diploid sectors are either Ad 



or ac/,- the latter are doubly homozygous, ,, and the former, upon 



w ad 



analysis of sexual spores from fruiting bodies, turn out to be recom- 

 binants of composition — —. The yellow diploid patches are either 

 w ad 



paba y paba y 



These homozygous conditions and the recombinations between the 

 genes can be accounted for by somatic crossing over. Not only can the 

 reciprocal recombination products be found in sectors corresponding to 

 the twin spots of Drosophila, but both reciprocal products of exchange 

 have been recovered from one nucleus. The experiment went like this. 

 A heterozvgote, which was heteroallelic at the adg locus, was prepared 

 and selection of adenine-independent asexual spores was made on mini- 

 mal medium. Only two of the four possible products of an exchange 

 between the two mutated sites at the ad^ locus would be expected to 

 be adenine-independent and to grow on the selective medium (Figure 

 12.6). By selecting haploid segregants from these adenine-independent 

 strains, it could be discovered which contained the two crossover strands, 

 b and c. This was possible because the ad^^ allele allows growth on 

 minimal medium at about half of the wild-type rate, while the allele 

 containing the combination, ad^^ ad^, allows no growth at all. Thus, the 

 possession of strands c and d could be distinguished. The presence in a 

 haploid of strand b resulted in yellow color. The correctness of the 

 identification of strand c was checked by crossing the haploid thought 

 to contain it with wild type. Recombination during meiosis separated 

 the allele, arfjg ad^, into its components, ad^Q+ and +adg. There can, 

 therefore, be no doubt that somatic crossing over yields reciprocal prod- 

 ucts, which arise by exchange within a cistron. 



Somatic crossing over can be used to map genes in cells that are 

 diploid and not undergoing sexual reproduction. The principle is based 

 on the fact that homozygosis can be achieved only if an exchange occurs 

 between a locus and a centromere. Therefore, among homozygotes for 

 a distal marker there should be some for markers between it and the 



