I have recently made contact with a putative (half)-first-cousin with whom I may share a grand-father. There is not a definitive documentary trail that confirms her grandfather and mine are the same, just some very suggestive evidence that our grandfather first married my grandmother and then married her grandmother (a marriage which has always been known to be bigamous in her family, which could explain the naming discrepancy that we're seeing).

I'm hoping she'll agree to DNA testing to resolve any lingering doubts, and want to understand how our relationship is likely to be predicted by a DNA testing company, so that if we find there is a match, we can assess how far back it goes. (It isn't, of course, impossible that we're related via a different route -- her grandfather could be related to mine rather than identical to him, for example).

Is there a reliable on-line reference for assessing the likely relationship between two individuals based on an autosomal DNA match?


There is a reference table of expected matching versus relationship on the ISOGG site here. This table shows the expected value (average value) of matching for a relationship; for half first cousins, it is 6.25% (or 425 cM). For different instances of half first cousins, it will randomly vary up or down from this value due to the randomness of genetic inheritance. There is other information on that page that suggests the range that a particular instance would likely fall within, but those are not exact boundaries and can't yield an absolute answer.

The number from that table applies to autosomes. For sex chromosomes different inheritance applies. Since both you and your potential cousin are female, if your putative shared grandfather is maternal for both of you, then you are likely to have X chromosome matching. However if the relationship is paternal, that is through either or both of your fathers, then you can't share X chromosome genes through the relationship (X matches could occur then only if there is some other relationship).

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