We have all heard the phrasing “half-brother” or “half-sister.” Many know what that means: Two siblings share one common parent.. But what about half-first cousins, half-second cousins and beyond? What is a half-cousin? And how much DNA do half-cousins share? Let’s break them all down.
To define half-cousins, think in the same terms as defining siblings and cousins: If full first cousins share a common pair of grandparents, half-cousins share one common grandparent rather than the couple. Second cousins share great-grandparents; half-second cousins share one great-grandparent. This logic applies to each relation as the family tree extends.
In the most general sense, the average amount of shared DNA between half-first cousins compared to full first cousins is halved, just like the number of grandparents. Blaine Bettinger, a New York-based genealogist who started the Genetic Genealogist blog in 2007, likened the DNA math to a bowl of Skittles: You’ll find far more matching Skittles when pouring from two bags (or two shared grandparents) than if you pour just one bag of Skittles into the bowl.
If first cousins share, on average, 866 centimorgans (a genetic measurement unit), half-cousins share an average of 450 centimorgans. In percentages, first cousins share an average of 12.5 percent of their DNA, while half-first cousins average 6.25% of common DNA.
What is a half-first cousin?
Half-first cousins share a single grandparent. For instance, Sarah’s father and Tom’s mother are half-siblings who share the same father but a different mother. Therefore, Sarah has the same grandfather as Tom but a different grandmother.
Research shows that half-first cousins range from 2%-11.5% in shared DNA. The average amount of shared DNA is 6.25%. That is more than enough common genetic information to link two people.
“The amount of DNA you share with a half-sibling or half-cousin is a huge amount, so you’re always going to find them if they use the same ancestral DNA services,” Bettinger said. “There has never been a case of second cousins or closer where you don’t share your DNA.”
What is a half-second cousin
Following the same principles as above, just add a great to the common familial relation: Second cousins share great-grandparents, so half-second cousins share one great-grandparent.
As Bettinger said, this is still within the range for DNA detection to find relatives. Half-second cousins range between 0.6% and 2.5% of shared DNA, averaging 1.5%, more than enough to tell if someone is related to you. Half-second cousins average a share of about 120 centimorgans, or roughly 7/50s of the average half-first cousins.
What is a half-third cousin?
This is where the family tree continues to expand and the bloodlines extend. Third cousins share common great-great-grandparents, so half-third cousins share one great-great-grandparent. In these cases, the range of common DNA can actually span anywhere from zero to 168 centimorgans and a miniscule percentage.
What is a half-fourth cousin?
Our tried-and-true formula helps here, too: Just add a great. Fourth cousins share great-great-great grandparents, and half-fourth cousins share one great-great-great grandparent. Bettinger’s centimorgans chart doesn’t even calculate half-fourth cousins, but full fourth cousins can share anywhere from zero to 139 centimorgans, averaging 35.
Ultimately, any detection of relatives using DNA matching techniques will be a lot easier the closer two people are on the family tree. As Bettinger said, genealogists are confident in matching up to and including second cousins and half-second cousins. After that, a DNA match to determine two people are related isn’t impossible, but the potential for similar genetic data significantly lowers.