Under neutrality all classes of mutation have an equal probability of becoming fixed in a population. In this article, we describe our analysis of the frequency distributions of >5000 human SNPs and provide evidence of biases in the process of fixation of certain classes of point mutation that are most likely to be attributable to biased gene conversion. The results indicate an increased fixation probability of mutations that result in the incorporation of a GC base pair. Furthermore, in transcribed regions this process exhibits strand asymmetry, and is biased towards preserving a G base on the coding strand. Biased gene conversion has the potential to explain both the existence of isochores and the compositional asymmetry in mammalian transcribed regions.