Understanding the genetic cause of inbreeding depression is crucial to the conservation of endangered species, and to the understanding of genetic diseases encountered in certain human populations. Inbreeding depression is an overall reduction in the fitness of a population due to mating between close relatives. There are two prevailing hypotheses for the genetic basis for inbreeding depression; the partial dominance theory and the overdominance theory. The partial dominance theory essentially states that the reduction in fitness associated with inbreeding is due to an increase in frequency of homozygosity for deleterious alleles. The overdominance theory states that the reduction in fitness is due to a decrease in the frequency of heterozygotes, …show more content…
Each paper empirically shows that inbreeding depression is taking place by demonstrating that fitness is declining with increased inbreeding, and sets up an inbreeding experiment using an outcrossing control to determine the cause of the inbreeding depression. Roff’s and Fox’s papers show that fitness of the offspring of crossed inbred lines is higher than outcrossed lines, a major component of the partial dominance theory. Fox et. al.s’ paper supports the purging component of the theory by showing that genetic load decreased across the generations, and fitness increased. Purging was also displayed in their data because the low larval survival in the first generation of inbreeding is essentially high extinction rate, and this rate decreased as inbreeding continued. Husband and Schemske’s paper supports the hypothesis in a more indirect way by predicting that diploid organisms will be more greatly affected by inbreeding depression than tetraploid organisms, testing this prediction, and determining that the prediction holds true. However, their paper did not go into whether or not purging was taking place. Roff’s experiment contained data in the form of levels of extinction of inbred and control lines, that would be useful for determining if purging was taking place, that is, whether inbred lines experienced a higher level of extinction. However, the data showed no significant difference in the extinction rates of control vs. inbred lines. The fact that purging was not seen in this data, and that fitness in the inbred lines did not increase conflicts with the partial dominance hypothesis. Based on the information provided by these three papers, I reject the hypothesis that the underlying genetic basis for inbreeding depression is partial dominance. While I believe that partial dominance plays