I am fortunate to have a number of relatives with whom to compare this cousin. The expected percentage of DNA in common with a known second cousin is 3.125%. He and my mother share 3.27%, while he shares about 3.1% with her sister and 2.77% with their first cousin V. It is interesting to see in the chart below that each of them inherited mostly unique segments with only a few segments in common. The chart compares my mother in dark blue, my maternal aunt in light green and their first cousin V in light blue to their second cousin (KnownC). All three women have the same familial relationship to him, but due to the random inheritance of autosomal DNA, they each share unique patterns and amounts of DNA with him. Notably, all four only share at one small spot on Chromosome 11.
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| Click on the chart to enlarge and get a closer look |
I am also able to compare our known cousin to descendants of the women above to demonstrate what occurs when the DNA is passed to successive generations. Second cousins once removed would be expected to share half as much DNA as a second cousin (1.563%) and a second cousin twice removed would share half of that (.781%), the same percent as a third cousin. My sisters and I are all second cousins once removed from our known cousin. I share 1.08% and my sisters share 1.84% and 1.38%. For second cousins twice removed, the actual percentages found in this study are .38%, .73%, .84% and .57%. Below are four charts comparing our cousin to various family members, demonstrating how the DNA inheritance changes as it is passed down. All of the charts are laid out in the same way - comparing one of the second cousins and then two successive generations. All charts can be enlarged by clicking on them.
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| Note: On Chromosome #8 there is an unaccounted for small match. This match does not show in any other family member and may be a psuedo-segment |
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| Note: On Chromosome #22 there is an unaccounted for match, This may be from inherited from the paternal side. |
**Update - In the comments below, Dr. Ann Turner makes an important point about these four charts. She writes, "In addition to showing how shared segments vary in size and position, the multi-generation diagrams also show how the segments are inherited in an all-or-nothing fashion much of the time, especially as the segment size grows shorter. The longer segments naturally have a higher probability of being split by recombination."
