The value of identifying long-ago ancestors isn’t so much the genetic link: We actually have very little genetic link to our ancient ancestors. Rather, the value lies in establishing our family’s specific geographic and ethnic origin.
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’ve identified, but we know our genes are drawn from the broad pool of Scottish genes, providing a cultural link to Scotland.
So, how many actual genes do we share with Scottish ancestors living in 1600, or with our earlier ancestors from the 11th century such as Queen Anne of France, or the 9th century such as Charlemagne? We’re only genetically related to our most recent ancestors, and share very few specific genes with our ancestors from 200+ years ago. Let’s dig into the specifics . . .
Genetic Relation To Parents
Genetically, we’re 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 (in each cell). No more, no fewer.
[Note: one of these chromosomes, the Y-chromosome, is incredibly important in genealogy research because it’s the only chromosome that you can trace back through your family tree: It’s passed along the paternal line. See the related article about Y-DNA.]
Genetic Relation To Grandparents
But how related are we to each of our four grandparents? One-fourth? The answer is no: We’re more related to some of our grandparents than others.
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 majority of chromosomes from her father, some a 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’re not equally related to all four of our grandparents. We get 23 chromosomes from our two maternal grandparents, so we must get more from one than the other. For example, we could have 17 chromosomes from mom’s dad, and just 6 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 others.
Genetic Relation To Great-Grandparents
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 more chromosomes from some great-grandparents than others.
Genetic Relation To Great-Great-Grandparents And Beyond
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’s almost certain that we’re completely unrelated to at least 18 of our 4th great-grandparents.
By the time we consider our 8th-great-grandparents (of whom we have 1,024), we’re genetically related to fewer than 5% of them. That is, even if we get exactly one chromosome from each of 46 8th-great-grandparents, that still leaves 978 of them from whom we got no genetic material.
Genetic Crossover
Why “almost certain”? There’s a phenomenon called genetic crossover that complicates things a bit. The bottom line is that you can get a single chromosome from one parent that’s 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 we can be related to.
The Cultural Connection
If we’re not very—or at all—related to most of our long-ago ancestors, do we have any connection to them? Yes! But the connection isn’t genetic: it’s 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.
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 broad pool of Scottish genes.
© 2013 W. Mullins