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Robert has a big family with four full siblings, and two much older half-siblings that his mom had conceived with her high school boyfriend that she married just after graduation but later divorced. Robert and his four full siblings were conceived following a second marriage, with Robert being the youngest child. Robert’s oldest full sibling is interested in ancestry and had already submitted a DNA test to 23andMe. Robert’s brother was really excited to learn that Robert registered for a first-year college seminar course that provided an opportunity to obtain genetic test results from 23andMe. Robert had seen his brother’s DNA test results and was interested to see how his own results would compare. Robert didn’t think much about spitting into the tube and sending off for a DNA test. After a month when Robert’s test results were available on the 23andMe website, he was shocked by the results. Robert was clearly able to see the match with his brother at the top of the DNA Relatives list, but instead of appearing as a full-sibling, his brother was identified as a half-sibling with 29% shared SNPs in 50 segments. They both share the same maternal haplogroup, but have different Y chromosome haplogroups.


What are plausible causes of this lower than expected relationship between Robert and his brother?

What are potential consequences of Robert sharing this information with his still married parents?

If you were Robert, what would you do with this information?


It has been a week after Robert’s results have appeared on the 23andMe website and he has not communicated with his brother. Robert calls his brother and it is apparent that he hasn’t seen the DNA matching results. After a bit of discussion with his brother, it’s pretty clear that his brother rarely logs into the 23andMe website and has even forgotten his login information. Robert decides not to share his discovery with his brother at this time.

Should Robert tell his brother about this discovery?

Robert could change his account to opt out of DNA Relatives matching, so that his brother never learns of their genetic relationship. Should he do so?

Should Robert involve his brother in a conversation with their mother about this discovery?

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