Alexandria Science
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Alexandria Science

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Table 1. Multiples to use after adding up the distinct segments that you and your siblings have with a match. Yes, these multiples could overestimate the amount of DNA your applicable parent would share with a particular match, but they’re also just as likely to underestimate that amount. Not using the multiple is even more likely to underestimate the amount. The multiples in the table get us the expected value of shared DNA.
Equation 1. Formula for finding how many generations back an ancestor would be, on average, in order to give you a particular percentage of ethnicity.
Figure 1. Probabilities relative to other relationship types for AncestryDNA data. Note the large differences between groups that have been considered to have the same curves (and do have the same averages). The sometimes large differences between maternal and paternal relatives, which are included in the relationship predictor, are not shown here. I’ve had very intelligent scientists tell me that there’s a problem with my curves — that grandparent/grandchild histograms only have one peak. My response has been that I regard this to be one of the most important discoveries ever made in genetic genealogy. My histograms also have only one peak. But for probability curves that are relative to those of other relationship types, there can be no gaps in between. All probabilities for a given cM value must add to 1.
Figure 2. Probabilities for a somewhat randomly chosen match of 2,255 cM. I bet you can see that the in-group differences (grandparent/grandchild vs. half-sibling vs. aunt/uncle/niece/nephew) and the paternal/maternal differences are significant enough to pay attention to.
Figure 3. Probabilities relative to other relationship types for 23andMe data. Compared to the AncestryDNA curves in Figure 1, the results for 23andMe show even more striking differences between relationship types that were considered to be in the same group.
Figure 4. A comparison of very low cM probabilities between relationship predictors at DNA-SCI (left) and DNA Painter (right). The DNA Painter tool is very well-built. However, the data come from an AncestryDNA simulation about which no methodology has been released. It also isn’t clear if the AncestryDNA data include essential population weights.
Figure 5. Relative probability curves for full-siblings, 3/4 siblings, double 1st cousins, and other relationships possible over that interval of cM at 23andMe.
Figure 6. Relationship prediction for 2,555 cM that indicates 3/4 sibling is by far the most likely relationship. The cM value was chosen somewhat randomly, but with the intent of showing a value with a high probability of being 3/4 siblings.
Figure 7. Relative probability curves for AYPR results at GEDmatch. The relationship type listed in the legend indicates a DNA tester’s father’s relationship to the mother.
Figure 8. Shared cM frequencies between a sibling and a 3rd to 6th cousin when the other sibling shares the cM range listed in the subplot title. Subplot e, with yellow bars, answers the question posed above about a full-sibling who shares 35 cM with a cousin and another sibling who shares none.
Figure 9 . HIR cM vs. FIR cM for full-siblings and 3/4 siblings. The red line shows the best IBD cutoff value to use in order to differentiate between full-siblings and 3/4 siblings.



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