Degrees of Relation and Number of Genes Shared
Estimates of how many human genes exist range from 25,000 to 100,000. As the sequencing of the human genome nears completion, the consensus is closing in on the suprisingly low figure of around 30,000. The following
table uses this number to estimate how many of an individual's genes are shared by their relatives.
Of course, any individual's mother and father share over 99% of their autosomal
[2] DNA (the DNA not carried on the X and Y
[3] chromosomes
[1]). This means that the individual shares much more of each parent's DNA than the 50% indicated by the table.
Thus the table shows us that 50% of our DNA is directly received from each parent and not that the genes inherited from one parent are entirely different to those inherited from the other.
What the table also shows us is that, beyond 7-degrees of relation, the number of genes shared in this manner is rather few. By extension, an individual's sixteenth-century, 13 times great grandfather has less than a half chance of providing a single autosomal gene to that individual, assuming that there is no other relationship between the two.
In terms of genes, there isn't much significance in taking your family tree back to Norman times for, at that distance, our genetic heritage will long ago have merged into the population's gene pool
[5]. In fact, ignoring the DNA that all human's share, it's possible to be genetically unrelated to a direct but distant ancestor even when hanky-panky can be ruled out!
There are, however, three genetic lineages which can be traced back much further.
The first is your pure maternal line, i.e. your mother's, mother's, mother's etc. lineage. This is because you inherit a small fragment of DNA, called mitochondrial DNA
[4], solely from your mother.
The second is a man's pure paternal line. This is because a man inherits his Y-chromosome
[3] solely from his father. This does not apply to women as they do not have a Y-chromosome.
The third are specific genetic markers that can be pinned down to specific populations. An example of this is the gene associated with cystic fibrosis, the origin of which can almost certainly be traced back to European populations. Most such genetic markers will not be associated with disease although, to date, these have been the best studied.
The amount of DNA involved with first two of these lineages is rather little compared to the rest of our DNA. However, because of its unique status, it is often analysed for genealogical purposes. The third group of lineages represent a much larger, more diverse, number of genes, but, as genetic typing advances, I believe it will play a much larger role in genealogy.
