Connections and Differences Between Genealogy and Genetics

The theoretical number of one’s ancestors can be easily derived mathematically. With each preceding generation, the number of ancestors doubles: one has two parents, four grandparents, eight great-grandparents, and so on, following the simple formula 2 to the power of ⁿ, where n is the generation number. One’s own generation is counted as 0 (zero), meaning 2 to the power uf ⁰ = 1, which represents oneself. The parental generation (generation 1) leads to 2 to the power of ¹ = 2, meaning two parents. When calculating back to the tenth generation, we arrive at 2 to the power of ¹⁰, which equals 1,024 ancestors.

The average age at which a new generation arises is approximately 30 years (more precisely, 33 years for men and 29 years for women). This means that the tenth ancestor generation lived around the times of Newton, about 300 years ago, and theoretically consisted of 1,024 members. This impressive number of over a thousand ancestors already seems quite implausible. The calculation becomes even more bizarre when we consider the number of ancestors one would have had in the High Middle Ages: using the formula 2 to the power of ⁿ and assuming roughly three generations per century, each of us should have had 2²⁸ = 268,435,456 ancestors.

However, if one—like myself—claims Ashkenazi heritage, this calculation contradicts the historically plausible number of approximately 800 Jewish families living in the Rhineland in the early 12th century. Historians refer to this modest founding group as a “bottleneck,” which represents the origin of the Ashkenazi Jewish population that later grew to 16 million before the Shoah.

The explanation for this enormous discrepancy between the mathematically calculated and historically assumed number of ancestors lies in the fact that overlaps occur within the family tree and must be considered. This is particularly true for highly endogamous communities such as the Ashkenazim. Most people likely do not know the names of their ancestors beyond their great-grandparents. If these ancestors were known by name, one would realize that the same individuals appear multiple times within the family tree. The further back one traces lineage, the more frequently this phenomenon occurs, eventually becoming the dominant pattern. This also explains why, as one looks further back in time, the number of ancestors first stagnates and then, in even more distant past generations, decreases—leading to the “bottleneck” of the 12th century.

As a result of this circumstance, most of us share the same ancestors. Furthermore, today’s Ashkenazi Jews are likely fifth cousins or more distantly related. The available gene pool (i.e., the total genetic material present within the endogamous community) among Ashkenazi Jews is relatively small and not very diverse. This, in turn, explains the frequent occurrence of “typical” physiognomic traits, certain so-called Ashkenazi genetic diseases, and possibly even the talent for abstract thinking, which has contributed to achievements in linguistic, scientific, and artistic fields.

Another important point is that not every genealogical ancestor is represented in our genetic makeup. Genes are sequences in the genome that code for specific traits, such as eye color. Everything that defines a person morphologically is determined by their genes. Identical twins, for example, resemble each other so closely because they share the exact same genetic material. But here’s the crux of the matter: We have approximately 30,000 genes, which are encoded on 3 billion base pairs in DNA. This means that an average gene consists of about 100,000 base pairs—though with significant variations, as some genes are much larger. We inherit half of our genes from our parents, a quarter from our grandparents, and an eighth from our great-grandparents. After just seven generations, the mathematical share of inheritable genetic material is already smaller than the size of larger and more complex genes. This means that certain distant ancestors could not have passed on these large genes to us, while others have contributed a slightly larger share of our genetic makeup than would be expected purely mathematically. However, this effect is less pronounced among the Ashkenazim because their endogamy results in a smaller number of genealogical ancestors. In the non-Jewish population, the number of ancestors is significantly larger, meaning that there were more cases in which genetic material—due to the space limitations—was not passed down to all genealogical descendants.

Finally, it is important to note that all people today are descendants of the entire population. Therefore, genealogical research efforts to trace ancestry as far back as possible are primarily of genealogical significance. Genetically, this information is far less relevant. The only reliably traceable lines of genetic inheritance are the patrilineal haplotypes on the Y chromosome and the matrilineal haplotypes in mitochondrial DNA. However, as genealogical distance increases, the genetic contribution of these haplotypes to present-day descendants becomes progressively smaller. They are primarily of general population-genetic significance and provide only limited insights into the hereditary traits of an individual.