|DEPARTMENT OF GENETICS - Prof Mark A. Jobling|
Deoxyribonucleic acid (DNA) is the molecule that encodes genetic information. Its structure resembles a twisted ladder known as a double helix, with the rungs of the ladder containing the DNA code. The code is written using four units known as bases: Adenine (A), Guanine (G), Cytosine (C) and Thymine (T), the order of these bases in DNA specifying the instructions to build a human being. Human DNA (our genome) is 3,000,000,000 bases long, which would measure about a metre in length if stretched out. It is compacted within our cells into separate structures known as chromosomes. Humans have 46 chromosomes.
Each of us inherits half of our DNA from our father, and half from our mother; thus, DNA is passed down from us to our ancestors, and variation arises along the way, producing a record of the past written in our genes. Using modern methods of molecular genetics, we can read this record and try to decipher events in human history such as the origins of our species, migrations, colonisations and mixtures between different populations. Most differences in DNA between individuals have no effect on health, and it is these neutral changes than we target when we undertake such studies.
Of our 46 chromosomes, 44 are shared between the sexes; the remaining two, known as the sex chromosomes, are called the X and Y. Females have two X chromosomes and men have one X and one Y chromosome. This is no coincidence - the Y chromosome contains information that determines the male sex and is inherited from father to son. As the Y chromosome is passed down from generation to generation relatively few changes occur in the DNA, because it escapes from a process called recombination that reshuffles information in the other chromosomes. This means that the record of the past written in Y-chromosome DNA is relatively simple to interpret - a history of male lineages. Women pass mitochondrial DNA, a small circular piece of DNA distinct from chromosomes, down to all of their children but men do not, so mitochondrial DNA can be traced back through generations of females in an analogous way to the Y chromosome (however, it has no link with patrilineal surnames).
In many societies a surname is passed down from a father to his children, in a similar way to the passage of the Y chromosome from a father to his sons. Thus it is possible that men sharing surnames may also share a Y chromosome type and therefore a common male ancestor. Studying sites of Y-chromosomal DNA variation known as markers can reveal how likely it is that men sharing the same surname have common ancestry.
Single Nucleotide Polymorphisms (SNPs) are binary markers that can be identified in the DNA of the Y chromosome; they occur very rarely, so that two men sharing a particular SNP state in their DNA almost definitely inherited this from a shared ancestor, who may have lived many thousands of years ago. We type sets of SNPs, and use these to define Y chromosome types called haplogroups, which in turn can be arranged into a 'family tree', or phylogeny. If two men belong to different haplogroups, this excludes them from sharing a recent common ancestor. An example of a SNP is shown in the diagram below.
The Y-phylogeny based on the markers we typed is shown below.
On the sheet that you received from us in 2004, your haplogroup, deduced from SNP typing data, is indicated; in the example below, the Y chromosome belongs to haplogroup I.
Another type of marker on the Y chromosome consists of short units of DNA (typically 3 or 4 bases long) that are repeated in tandem several times (see diagram above). These are called short tandem repeats (STRs), or microsatellites; variation in these markers occurs much more frequently than at SNPs. Different Y chromosomes can carry varying numbers of repeats at a set of specific STRs, called a Y-STR haplotype. Haplotypes can be compared to identify differences between men within the same haplogroup. Related men will show the same or very similar patterns in terms of the numbers of repeats seen at a set of markers, while unrelated men will tend not to. In this study 17 Y-STRs were examined and a 17-locus haplotype was defined; in the example data sheet above, the haplotype is given under 'Microsatellites', and each is given a DYS number.
Disclaimer: Please note that, while every effort has been taken to ensure that typing is complete and accurate, every piece of scientific research has a non-zero error rate, and we can give no guarantee that every one of the approximately 80,000 marker typings returned to volunteers is correct.
Last updated: 9th February, 2009
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