What happens when you do not have a living grandparent from one or both sides of your family tree? Is it still possible to create a chromosome map? In situations where you do not have two living grandparents available to test, it may still be possible to create a complete or nearly complete chromosome map by testing other relatives. The key to successfully creating a grandparent chromosome map is testing several additional relatives whose combined DNA tests cover the majority of the DNA that you likely inherited from each of your grandparents.
If one of your grandparents has an identical twin who is still living, then they will have the same DNA as your grandparent. Their DNA can stand in for the DNA of your grandparent. In other cases, where your grandparent has no twin, then even more relatives need to be tested.
One strategy for being able to perform chromosome mapping is testing 2-3 of your own siblings. Each individual inherits exactly 50% of their autosomal DNA from their mother and 50% from their father. At any given site on their paternally inherited DNA they can only have inherited DNA from one of their grandparents. The same can be said for the maternally inherited DNA. Most individuals inherit approximately 25% of their DNA from each of their grandparents. Siblings will share some of the DNA they inherit from their parents, and not share other portions. By testing multiple siblings, researchers can achieve the following levels of approximate coverage of their parent’s DNA:
| Number of siblings tested | 1 | 2 | 3 | 4 | 5 |
| Percent coverage of parental DNA | 50% | 75% | 87.5% | 93.75% | 96.875% |
At any given site of DNA, there are three possible ways that the DNA of siblings will compare to each other. Either they will share DNA on both their maternal and paternal copies (a fully identical region), they will share DNA on a maternal or a paternal copy (a half-identical region), or they will not share DNA (a non-identical region). In the first case, they share DNA with the same grandparent on both their maternal and paternal side (50% coverage of the possible inherited DNA at that location). In the second situation they share DNA with the same maternal or paternal grandparent, but they inherited DNA from a different grandparent on the other side (75% coverage of the possible inherited DNA in that location). For example, both individuals might have inherited DNA from their maternal grandmother, but on the paternally inherited DNA, one sibling inherited DNA from the paternal grandfather and the other sibling inherited DNA from the paternal grandmother. In the last scenario, the siblings share no DNA with each other on a particular segment meaning that they inherited DNA from different grandparents (100% coverage of the possible inherited DNA in that location) One of them inherited DNA from the maternal grandmother and the other inherited DNA from the maternal grandfather in the same region. Also, one of them inherited DNA from the paternal grandfather and the other inherited DNA from the paternal grandmother in the same region. By testing your own siblings, you can identify larger segments of DNA that likely belong to deceased grandparents.
Through comparison of the half identical and fully identical regions that siblings inherit from their parents, it is often possible to create nearly complete chromosome maps based on the segments of DNA that they inherited from a single grandparent in common, two grandparents in common or no grandparents in common. However, in order to assign these segments to specific grandparents, it is necessary to test additional relatives of an individual’s grandparents and compare against them as well.
If any siblings of a grandparent are still living, they should be invited to perform DNA testing. If not, then their children (first cousins of the subject’s parents), should be invited to test. By testing 2 siblings of each of your grandparents, you will likely be able to identify most of the segments of DNA that you inherited from each of them. Rather than testing all of your relatives at once, consider testing them in step-wise fashion. This way you can determine if they already have close relatives who have tested.
When you test yourself and your siblings and when you test the siblings of your grandparents and their relatives, you can frequently determine the origin of your DNA at the grandparent level. Any segments where a known relative of your paternal grandfather matches a sibling but does not match you has to have come from your paternal grandmother. Likewise, any segments where a known relative of your maternal grandmother matches you but does not match your sibling, means that you inherited DNA from your maternal grandmother in that region, and in the same region, your sibling inherited DNA from your maternal grandfather.
These conclusions assume that each of your grandparents share no large segments of DNA in common with each other. These assumptions may not necessarily hold true if your parents are related to each other, or if you are a member of an endogamous population.
Admittedly, there is most often no simple way to reconstruct the DNA of a deceased grandparent. However, by testing their living relatives and descendants it may still be possible to identify many of the segments of DNA that they passed on to you.
Are you interested in extending your family tree through chromosome mapping? Our team of professional genealogists are experts at genetic genealogy and can help you create a DNA testing plan, or perform additional analysis on existing DNA test results. Contact us today for a free estimate!
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