INTRODUCTORY. I9 



BALANCED INVIABILITY. 



The determination of the cross-over values of the factors was at first 

 hindered because of the poor viabiHty of some of the mutants. If the 

 viability of each mutant type could be determined in relation to the 

 viability of the normal, "coefficients of viability" could serve as cor- 

 rections in working with the various mutant characters. But it was 

 found (Bridges and Sturtevant, 1914) that viability was so erratic that 

 coefficients might mislead. At the same time it was becoming more 

 apparent that poor viability is no necessary attribute of a character, 

 but depends very largely on the condition of culture. Competition 

 among larvae was found to be the chief factor in viability. Mass 

 cultures almost invariably have extremely poor viability, even though 

 an attempt is made to supply an abundance of food. Special tests 

 (Morgan and Tice, 1914) showed that even those mutants which were 

 considered the very poorest in viability were produced in proportions 

 fairly close to the theoretical when only one female was used for each 

 large culture bottle and the amount and quality of food was carefully 

 adjusted. 



For the majority of mutants which did well even under heavy com- 

 petition in mass cultures the pair-breeding method reduced the dis- 

 turbances due to viability to a point where they were negligible. 



Later a method was devised (Bridges, 191 5) whereby mutations of 

 poor viability could be worked with in linkage experiments fairly accu- 

 rately and whereby the residual inviability of the ordinary characters 

 could be largely canceled. This method consists in balancing the 

 data of a certain class with poor viability by means of an equivalent 

 amount of data in which the same class occurs as the other member of 

 the ratio. Thus in obtaining data upon any linkage case it is best to 

 have the total number of individuals made up of approximately equal 

 numbers derived from each of the possible ways in which the experiment 

 may be conducted. In the simplest case, in which the results are of 

 the form AB : Ab : aB : ab, let us suppose that the class ab has a dis- 

 proportionately low viability. If, then, ab occurs in an experiment as a 

 cross-over class, that class will be too small and a false linkage value 

 will be calculated. The remedy is to balance the preceding data by 

 an equal amount of data in which ab occurs as a non-cross-over. In 

 these latter the error will be the opposite of the previous one, and 

 by combining the two experiments the errors should be balanced to 

 give a better approximation to the true value. When equal amounts of 

 data, secured in these two ways, are combined, all four classes will be 

 balanced in the required manner by occurring both as non-cross-overs 

 and as cross-overs. The error, therefore, should be very small. For 

 three pairs of gens there are eight classes, and in order that each of 

 them may appear as a non-cross-over, as each single cross-over, and as 

 the double cross-over, four experiments must be made. 



