GENETICS OF BACTERIOPHAGE 



In the cross hr7 X wild (distantly linked markers) the great majority of parti- 

 cles heterozygous for h are not heterozygous for r. 



In the cross hrl3 X wild (closely linked markers) the singly heterozygous class 

 is smaller, but the proportion of heterozygotes yielding recombinants is large 

 compared to the proportion of recombinants among the progeny as a whole. 



In both crosses the total frequency of h-h~^ heterozygotes is roughly two per 

 cent. 



We conclude that the formation of particles heterozygous for h and for r obeys 

 identical rules. 



A method of somewhat similar principle is applicable to crosses between 

 pairs of r mutants. This is important in that effects of linkage can be 

 tested in different regions of the genetic map; unfortunately, not with very 

 great precision. The principle of the test can be illustrated by the example 

 rS X r4. Heterozygotes resulting from this cross should segregate to yield the 

 following pairs; r3 + rSr4, r4 -f r2r4-, r2 + r4, r2 + wild, r4 + wild. This list 

 includes all possible pairs excepting the recombinant pair, which is assumed to be 

 absent. The first two and the last two classes will all be of the same size for 

 reasons of symmetry, and the sum of the frequencies of the last three classes will 

 amount to two per cent of the population. If the two factors are not linked, heter- 

 ozygotes belonging to the last three classes, and only these, will yield mottled 

 plaques. If the factors are linked, only the last two classes will yield mottled 

 plaques, and the difference between the proportion found and two per cent will 

 measure the size of the doubly heterozygous class yielding r2 -\- r4- The assump- 

 tion that particles doubly heterozygous for linked factors do not produce mottled 



TABLE 4. YIELDS OF r+ AND MOTTLING PHAGE IN r CROSSES 



The results shown are means and their standard deviations computed from counts of about 

 4000 plaques on 20 plates. The duplicate counts of the crosses marked with an asterisk were 

 made from the same population on different days; the other duplicate counts represent in- 

 dependent experiments. The per cent r+ in the left half of the table measures the equality of 

 infection with the two parental viruses; in the right half it msasures the linkage between r loci. 



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