Lands of our fathers: Y-chromosome diversity and the histories of human populations 24/6/03. By Mark A Jobling Mark Jobling explores the Y chromosome and human history.

We inherit many strange things from our parents, biological, behavioural and material, but the males among us inherit from our fathers perhaps the strangest thing of all – a Y chromosome.

Ninety-eight per cent of our genome is diploid, with the chromosomes coming in matching pairs, and these pairs engage in recombination, the reshuffling of segments when eggs and sperm are made. The Y chromosome is different: specialised for male sex-determination, it stands aloof and haploid, and, for most of its length, fastidiously avoids the messy business of recombination.

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This is why the Y chromosome is so widely used in human population studies. As it passes from father to son down paternal lineages, the only changes occurring are due to mutation, and the molecular record of the past is therefore relatively easy to interpret compared to the recombining X chromosome and autosomes.

Findings support the hypothesis that modern humans originated in Africa about 150 000 years ago.

The simplest and slowest kind of mutation alters a single base in DNA, for example from a 'C' to a 'T'. A rich resource of these binary DNA markers has now been discovered on the Y chromosome, and in combination these define signatures of different Y-chromosomal types called 'haplogroups'. Haplogroups can be arranged into a well-resolved genealogical tree, representing the branching order in which the different mutations occurred since all modern Y chromosomes had a common ancestor (sometimes known as 'Y-chromosomal Adam', although he was only one of many men in a population, rather than the solitary forefather of the Book of Genesis).

Armed with this tree of haplogroups, we can tour the world and ask how they are distributed among human populations. There are two striking observations. First, we find haplogroups on the deepest-rooting branches of the Y-chromosome tree only in Africa, which supports the hypothesis that modern humans originated in Africa about 150 000 years ago.

Second, most haplogroups have geographically restricted distributions, and the haplogroup profiles of individual populations are quite distinctive. This is unlike most other parts of the genome, which often show little variation between populations. Why the difference? It comes down to the inertia of men. In most societies, if a man and a woman marry but are from different places, it is the woman who moves, and the man who stays put. The effect is to sharpen the differentiation among Y chromosomes.

This strong geographical patterning means that the Y chromosome is an excellent tool for looking at population histories – from the migrations out of Africa about 50 000 years ago, to agricultural expansion from the Near East into Europe beginning 10 000 years ago, the colonisation of the Pacific Islands (completed only 1500 years ago), and the dramatic reshaping of genetic diversity in some parts of the world since the time of Christopher Columbus.

Comparison of Y-chromosome diversity with that of mitochondrial DNA (which is passed down from mothers to children, so recording a maternal history) is particularly illuminating. European colonists of the Americas and the Pacific took with them all of their genes, but contributed almost none of their mitochondrial DNA to the local populations; in contrast, they spread a generous number of their Y chromosomes. This 'sex-biased admixture' is a striking reflection of the sexual politics of colonisation and slavery.

Politics of another kind have left a remarkable imprint in the gene pool of Central Asia: 8 per cent of the Y chromosomes over an enormous geographical area belong to a very closely related cluster with a time of origin only about 1000 years ago. Comparison of this distribution and its likely date and place of origin with those of the Mongol Empire suggests that the ancestors of this multitude of chromosomes were Genghis Khan and his dynasty of male-line descendants.

Family historians researching their own dynasties will take heart from this, since men sharing surnames (passed from father to son in many societies) might also share Y chromosomes. This raises the possibility of tracing surnames back to single founders, and of linking together branches of family trees by the information written in DNA, rather than in birth, marriage and death records.

The recent announcement of the DNA sequence of the Y chromosome, and continuing advances in differentiating between Y chromosomes in populations, promise new insights into this strange part of our genome, as well as the histories of our forefathers.

Professor Mark Jobling is at the Department of Genetics, University of Leicester .