MEASURING FITNESSES OF DROSOPHILA GENOTYPES IN  THE LAB

     It is possible to measure the fitnesses of Drosophila genotypes in a laboratory environment by performing a series of crosses.  As an example, let us assess the viability component of fitness for genotype AA relative to Aa. To do so, one performs the following experiment:

     1.  Cross AA individuals to Aa individuals.
     2.  Rear the offspring in the environment in which fitness is to be measured (i.e. a Drosophila population cage).
     3.  Score the numbers of surviving adults that are AA and Aa.

     One then compares these numbers to the numbers expected if there is no difference between the genotypes in viability.  If there are no viability differences, then the number of AA individuals should equal the number of Aa individuals.  Whether this is the case may be tested using a standard Chi-square test of observed numbers against expected numbers.

     If the numbers differ significantly, then one can estimate the viability of AA relative to Aa.  Let us assign a viability of 1 to Aa, and V to AA.  Then, assuming that Mendel's laws have produced the expected 50:50 ratio of the two genotypes in the offspring of this cross,  the following relationship should hold:

         N_AA/ N_Aa = V/1

Consequently, we may estimate V by the ratio   N_AA/ N_Aa.

Similarly, if we let the viability of aa relative to Aa be U, then we can estimate U by the ratio N_aa/ N_Aa .Thus, if differences in fitness among the genotypes are cause only by viability differences, the relative fitnesses of AA:Aa:aa are V:1:U.

     More complicated crosses can yield information on fecundity/fertility components of fitness, which can be combined with the information on viability to estimate the total relative fitnesses of the three genotypes.
 

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