Theodosius Dobzhansky. Portrait of Theodosius Dobzhansky (1900-75), Russian-American geneticist. Dobzhansky linked Darwin's evolution theory with Mendel's laws of hereditary. He studied the genes of the fruit fly, Drosophila sp, and found a greater variability than expected. He found a high proportion of flies had debilitating genes which were masked by the presence of a dominant 'normal' gene. He showed that the presence of a masked debilitating gene was advantageous as it allowed a population to be more versatile in adapting to a changing environment. Dobzhansky wrote many influential books, including Genetics and the Origin of Species in 1937.