Cultural Ancestors and Genetic Ancestors
The value of identifying long-ago ancestors is not so much the genetic link: we have very little genetic link to our ancient ancestors. Rather, the value lies in establishing our family's specific geographic and ethnic origin. For example, the Mullins family has several branches that trace back to Scotland in the 1600s and early 1700s. We may not share many genes with the specific Scottish ancestors we have identified, but we know that our genes are drawn from the broad pool of Scottish genes, and further that we have a cultural link to Scotland.
So how many actual genes do we share with Scottish ancestors living in 1600, or with Charlemagne and other ancient ancestors? The following analysis shows that we are genetically related only to our most recent ancestors. We share specific genes with very few of our ancestors from 200 or more years ago.
Genetically, we are exactly one-half related to each parent. Each of our cells has 46 chromosomes (except ova in women and sperm cells in men – these sexual reproduction cells have 23 chromosomes each). When our existence begins with the fertilization of an ovum by a sperm cell, we start with 23 chromosomes from the mother and 23 chromosomes from the father. No more, no fewer. Every cell in our bodies has 23 chromosomes from our mother, and 23 from our father.
But how related are we to each of our four grandparents? One-fourth?
The answer is no: we are more related to some of our grandparents than others.
To understand how this can be, consider where the 23 chromosomes in each of our mother's ova came from: some came from her father, and some from her mother. The proportion varies in different ova. Some of a woman's ova will contain a large majority of chromosomes from her father, some a large majority from her mother, and others a more equal proportion. The same holds true for a man's sperm cells: they contain varying proportions of chromosomes from his mother and his father, respectively.
Thus, we are not equally related to all four of our grandparents. In fact we can't be: we get 23 chromosomes from our two maternal grandparents, so we must get more from one than the other. For example, it could be that we have 21 chromosomes from mom's dad, and just 2 from mom's mom. Even more strange: our brother or sister almost certainly has a different proportion of chromosomes from each grandparent. The odd result is that we are more related to some of our siblings than to others.
Going back one more generation, we get on average 5.75 chromosomes from each great-grandparent (46 divided by 8), but the actual distribution is that we get many more chromosomes from some great-grandparents than from others.
The 4th great-grandparent level is the first level where there are more ancestors than chromosomes: we have 64 4th great-grandparents, but only 46 chromosomes. It is almost certain that we are completely unrelated to at least 18 of our 4th great-grandparents.
Why "almost certain"? There is a phenomenon called genetic crossover that complicates things a bit. The bottom line of this event is that you can get a single chromosome from one parent that is partly from her father and partly from her mother. So, you may have a single chromosome that came from 2 different distant ancestors. Genetic crossover is a fairly rare event, but it does slightly increase the number of ancestors that we can be related to. But, only slightly.
By the time we consider our 8th great-grandparents, of whom we have 1,024, we are genetically related to fewer than 5% of them.
If we are not very, or at all, related to most of our long-ago ancestors, do we have any connection to them? The answer is yes, but the connection is not genetic: it is cultural. The value of knowing our ancestral heritage loses its genetic importance somewhere between 6 and 10 generations back, but it retains its cultural importance.
To go back to the original point, the Mullins family has deep Old World roots in Scotland. Because so many of our ancestors came from Scotland, we can each assume that many of our 46 chromosomes came from the pool of Scottish genes.