Sunday, September 26, 2010

Known Relative Studies with 23andMe: Great Grandchild DNA Inheritance

Great Grandchild Inheritance Pattern (click to enlarge)

I am very fortunate to have the opportunity to see the inheritance pattern of a great grandchild in my known relative studies at 23andMe, so I decided to share it. For those of you who are not familiar with the 23andMe user interface, the above chart is an illustration of the 23 pairs of chromosomes that we all possess. For simplicity, only one chromosome is displayed to represent each pair, thus the 23 bars. Using the Family Inheritance Advanced tool one can choose to compare selected individuals and a chart is generated to illustrate the shared DNA between them.

In this chart, the light blue is the shared DNA with the mother, the light green is the shared DNA with the grandmother and the dark blue is the shared DNA with the great grandmother. Of course, the parent and child (light blue) share DNA across all 23 of the chromosomes, as would be expected. This represents the 50.18% of shared DNA between the child and parent (expected ~50%). The grandparent and child (light green) usually would share approximately half as much DNA with the stretches being broken up into smaller blocks. In this case, the child inherited a larger than expected amount of shared DNA with the grandparent at 31.54% (expected ~25%). The great grandparent and child (dark blue) should share approximately half as much DNA as the child and the grandparent, with the blocks broken up into even smaller fragments of shared DNA. In reality, the percentage is less than half at 14.75% of shared DNA, which is still rather high compared to the norm (expected ~12.5%).

As you can see, the percentages will vary from the expected values. Notably, 31.54% is the highest sharing I have seen between a grandparent and child in my research so far. Since the child inherited 50.18% from the maternal side, there is only ~18.64% that could have been inherited from the other maternal grandparent, which is well below the expected 25%.

Notice that as the relationship gets more distant, the shared blocks of DNA get smaller, disappearing completely on some of the chromosomes. On Chromosome 13, there are no stretches of shared DNA (that meet the threshold for this tool) between the grandparent and child. This means that on Chromosome 13, this child has inherited significant DNA from the other grandparent's ancestors (in this case, the maternal grandfather). Following this same pattern, on Chromosomes 6, 13 and 14, the great grandparent and child do not have any significant blocks of DNA. Also notice on Chromosome 10, the grandchild inherited the entire maternal grandmother's chromosome (light green across the entire bar). That means that Chromosome 10 has no (significant) DNA from the maternal grandfather.

Please remember that, in this case, we are only looking at half of the child's chromosomes. The child has another set of 23 chromosomes inherited from the father. Since we are not comparing the child with any paternal relatives, none of those chromosomes are represented in this chart or analysis.

**For more posts on my family studies, please see here. **

7 comments:

  1. Thanks for posting, that's very interesting. It's kind of strange when one chromosome comes completely from one grandparent, as with chromosome 13.

    It's also interesting how many of the segments that the child shares with the great-grandparent and the grandparent are either approximately the same size, or lost all together (as with chrom 5). I kind of wish 23andme had the option to view segments under than 5cM, it might shed more light as to whether the segments are truly lost or simply just smaller than 5cM.

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  2. Good point, Angela. I wish I had access to the raw data files to send to Jim McMillan. That would give us the answer.
    Thanks for commenting!
    CeCe

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  3. Fascinating! This is a great example of how much of DNA was actually passed along compared to how much is predicted theoretically. I guess it is to reasonable that if more (or less) is passed in one generational transfer, the amount actually passed becomes the basis for the approximate halving of that DNA in succeeding generational transfers. I hadn't thought about it in that light. Thanks for sharing.

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  4. Very nice explanation and study.

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  5. I have a similar case. The female child inherited the following:

    Mother - 49.7% shared, 24 segments

    Maternal Grandmother - 19.6% shared, 22 segments

    Maternal Grandfather - 30.2% shared, 34 segments

    In this case, the Mother had the following breakdown with her parents who are the maternal grandmother and grandfather referenced above:

    Mother - 50.3% shared, 25 segments
    Father - 49.7% shared, 23 segments

    A male sibling of this child has pending results at 23andme. I'm anxious to see how his results will turn out. I also have pending results for the child's maternal great grandmother as well.

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