|by Stephen Oppenheimer
Analogous to the maternally transmitted mtDNA residing outside our cell nuclei, there is a set of genes packaged within the nucleus that is only passed down the male line. This is the Y chromosome, the defining chromosome for maleness. With the exception of a small segment, the Y chromosome plays no part in the promiscuous exchange of DNA indulged in by other chromosomes. This means that, like mtDNA, the non-recombining part of the Y chromosome remains uncorrupted with each generation, and can be traced back in an unbroken line to our original male ancestor.
Y chromosomes have been used for reconstructing trees for less time than mtDNA has, and there are more problems in estimating time depth. When these are solved, the NRY method may have a much greater power of time and geographical resolution than mtDNA, for both the recent and the distant past. This is simply because the NRY is much larger than mtDNA and consequently has potential for more variation.
Yet Y chromosomes have already helped to chart a genetic trail parallel to the mtDNA trail. At the major geographical branch points they support the story told by mtDNA: they point to a shared ancestor in Africa for all modern humans, and a more recent ancestor in Asia for all non-Africans. In addition, because men’s behaviour differs in certain key ways from women’s, the story told by the Adam genes adds interesting detail. One difference is that men have more variation in the number of their offspring than women: a few men father considerably more children than the rest. Women, in contrast, tend to be more even and ‘equal’ in the number of children they have. The main effect of this is that most male lines become extinct more rapidly than female lines, leaving a few dominant male genetic lines.
Another difference is in movement. It has often been argued that because women have usually travelled to their husband’s village, their genes are inevitably more mobile. Paradoxically, while this may be true within one cultural region, it results in rapid mixing and dispersal of mtDNA only within that cultural region. For travel between regions, or long-distance intercontinental migrations, by sea for example, the burden of caring for children would have limited female mobility. Predatory raiding groups would also have been more commonly male-dominated, resulting in increased mobility in the Y chromosome.