Can You Inherit Zero DNA from a Great-Grandparent?

Genetic inheritance is a fascinating subject, yet it occasionally leads to perplexing scenarios that test the limits of our understanding. One such scenario is the possibility of inheriting no discernible genetic material from a great-grandparent. This article explores the extremely rare circumstances where this might occur, delving into the technical aspects of genetic inheritance and the more common scenarios that might result in a similar outcome.

The Technically Impossible

Firstly, it is essential to clarify that technically, inheriting no DNA from a great-grandparent is impossible. However, in extremely rare circumstances, the amount of genetic material shared might be so minimal as to be virtually unmeasurable with current technology. This article delves into these rare situations and discusses why it is more likely that the exact nature of a great-grandparental relationship might be unclear without additional information.

Rare Circumstances and Accelerated Segmentation

Genetic inheritance over many generations tends to result in smaller and smaller shared segments of DNA. This phenomenon is accelerated in certain scenarios, such as frequent random mutations or mismatches in genetic information. For example, segments shared by direct descent tend to get smaller with each generation, eventually becoming too small to be distinguished as coming from a specific ancestor, particularly over eight generations. This is not due to changes in DNA but rather because these segments are so small that they resemble many other people's DNA.

Measurement of Shared DNA

The shared inheritance between two individuals can be measured precisely by the length of shared segments of each strand of a chromosome, with units of measure being centimorgans (cM). On average, one shares about 850cM with a great-grandparent or great-grandchild, although the normal range can range from 450 to 1500 cM. This wide range makes it challenging to determine the exact relationship based solely on cM match.

Genealogical Considerations

Some genealogists take segments smaller than 9–10 cM less seriously. In most cases, segments smaller than 7 cM are considered and are often false positives or "identical by state." Chances are, a large minority of such matches are not truly "identical by descent" (the expected way DNA is inherited) but rather represent segments that have similar characteristics due to pile-ups (long segments sticking together over many generations).

Factors Influencing Shared DNA

Several factors can also diminish the amount of cM shared with biological relatives, including gender-specific variations. Women typically share slightly fewer cM with their maternal grandfathers than statistically expected, while they often share more from their maternal grandmothers. This is because the cM shared with the father of one's maternal grandfather is usually slightly less than that shared with other great-grandparents.

Other Scenarios

While the absence of genetic material from a great-grandparent is rare, there are other common scenarios that can lead to similar outcomes. These include:

Unofficial and undocumented adoption of a grandparent or parent Sperm or egg donation, including the "DIY" forms practised by previous generations Newborn babies being accidentally swapped at a hospital, a phenomenon that seems to have occurred more frequently than previously thought Variations of polygamy, which is commonly practiced in some cultures Extramarital liaisons or sexual assault leading to pregnancy and a child being born

Conclusion

In summary, while inheriting no DNA from a great-grandparent is technically impossible, the possibility of inheriting extremely small amounts of genetic material exists. Understanding the technical complexities and the rare scenarios that might lead to such outcomes can help in clarifying the nature of genetic relationships. If you find yourself in a situation where no discernible genetic material is shared with a great-grandparent, it is advisable to consult a geneticist or a specialized genealogist for a more thorough analysis.