BACK-MUTATION AND PROGRESSIVE MUTATION 17-5 



analysis in pedigrees in which regular Mendelian segregation oc- 

 curs. They are apparently non-reverting cultures, but they may 

 simply have a more complex and therefore a slower adaptation 

 mechanism than their allelic sibs which ferment more readily. 



THE ORIGIN OF LACTOSE FERMENTERS 



There are many Saccharomyces capable of fermenting lactose. 

 Kluyver pointed out that no lactose fermenter is capable of fer- 

 menting maltose, and vice versa. The fact that these two charac- 

 ters are mutually exclusive is strong evidence that they are con- 

 trolled by the same gene. Many lactose fermenters are Zygosac- 

 charomyces and the Zygosaccharomyces are now accepted as or- 

 iginating from Saccharomyces. The original Saccharomyces was 

 a maltose fermenter incapable of fermenting lactose; lactose fer^ 

 mentation probably originated by the transformation of the mal- 

 tose-fermenting gene in Saccharomyces into a lactose -ferment- 

 ing gene, rather than by the de novo production of a lactose-fer- 

 menting locus. It is possible to make a rough estimate of the time 

 involved in the perfection of this mutation. Yeasts are not abun- 

 dant in the intestinal tract of mammalian infants and this suggests 

 that lactose -fermenting yeasts did not develop lyitil humans began 

 to collect milk in containers. Chance contamination of milk with 

 wild yeasts may have resulted in the germination of haplophases 

 whose maltose -fermenting genes mutated to lactose fermenters 

 and became established in the new environment. The collection 

 of milk by humans probably began between twenty and fifty thou- 

 sand years ago and sometime during that long period in which 

 lactose was available as a substrate, yeasts have been able to 

 alter the gene controlling maltose fermentation into one which was 

 capable of fermenting lactose. Up to the present time, however, 

 no Saccharomyces has evolved capable of fermenting both lactose 

 and maltose. This step in evolution would require the duplication 

 of the lactose and the maltose alleles by unequal crossing over. 



UNEQUAL CROSSING OVER AND EVOLUTION 



Alexander and Bridges (1928) supposed that chromosomes be- 

 come longer by duplicating small sections repeatedly through un- 

 equal crossing over. Fig. 17-3 shows how unequal crossing over 

 (originally discovered by Sturtevant, 1925) would result in pro- 

 ducing a slightly longer chromosome with two genes in place of 

 the original one and a corresponding chromosome deficient for the 

 gene is question. Bridges (1935) finally found in the salivary glands 

 of Drosophlia dramatic evidence supporting his original view con- 

 cerning the mechanism by which chromosomes increase the num- 

 ber of their genes. Fig. 17-4 is a reproduction of a part of the 



