Please note that this is a very simplistic view of DNA and the Y Chromosome. There are a number of good sources for more detailed information such as Wikipaedia.
Every person’s DNA is inherited from both parents. The mother’s mitochondrial DNA together with twenty-three chromosomes from each parent combine to form the genome (genes) of the offspring. As a result, most human cells contain 23 pairs of chromosomes. The mitochondrial DNA is always inherited from the mother by both boys and girls.
One of the 23 pairs of chromosomes determines the gender of the offspring. These are called the X and the Y chromosome and they are different. The Y chromosome is much smaller for one. Human females have 2 X chromosomes whilst males have one X and a Y chromosome. Obviously, then, if a child is a boy, he can only have inherited the Y chromosome from his father, as his mother does not have one.
Daddies pass this present to their sons relatively unchanged, that is it is an exact copy of his own yDNA (short form of the Y Chromosome DNA). However, on very rare ocassions, a mutation occurs in one of the genes (enviromental??) which means that the son recieving the mutated gene will pass this onto his own sons. It is still possible to identify that both chromosomes are related. Over generations, different individuals will inherit a particular mutated gene that becomes a bit like the family signature and as more mutations occur, distinct groups emerge giving rise to the differences that scientists use to detemine which paternity line an individual belongs to.
Despite the mutations, scientists have been able to identify that all living human males are related and are the descendants of Y-chromosomal Adam who lived between 60,000 and 90,000 years ago in Africa. That shows how little the yDNA has changed! Y-chromosomal Adam is a fictional person and changes as some male genes die out! One guaranteed way to ensure this is to have no children or that you and all your brothers have baby girls! As an observation, is this why over the centuries having male offspring was so important that the reasoning was hidden behind the social/religious structures that were created? For example, the passing on of the surname to the son. If so, isn’t fascinating that these rituals were created before we knew about DNA? A conversation for the blog, me thinks.
As an additional note, can a particular individual be identifed via the yDNA? The answer is no, as your forefathers, your brothers and their sons and you could all have the same yDNA. In fact, so may your sixth cousin removed if no mutations have occured in your paternal line for some generations. DNA identification requires more than the yDNA, much more.
In the area of genealogical research, just having you yDNA analysed is useless. It only becomes useful when it is compared to that of another individual usually to someone who may be related. Analysis can confirm 100% if the two individuals are not related. Where mutation has occurred, the certainty diminishes depending on the test but can be as good as 100% but usually over 99%. If a relationship is established, an educated guess to how far back the connection existed can be made.
If you have understood this, then you will now see why this can be a powerful tool in the genealogist arsenal. For example, consider that two Ellis families on both sides of the Atlantic were researching their family tree but are stuck in determining whether the Joseph Ellis born in Wakefield in 1583 is a common ancestor as documented evidence is sparse. If a male from the USA family and a male for the English family had there yDNA analysed, it can be determined whether the two families are connected or not. If not, then someone’s research is incorrect or two Joseph Ellises where born that year! If so, a reasonable deduction will determine whether the connection was indeed Joseph Ellis or the connection was earlier or later than that event. If so, you still have a connection and may be able to pool resources. Brilliant, result!
yDNA tests are not cheap yet, though I suspect as interest grows, the competition will lower prices in the near future. Carefully considering who gets tested will reduce your costs though it may be possible by grouping together with other interested parties, a negiotiated price may be possible.
Another simple explanation of the use of genetic genealogy is here.