Understanding Double First Cousins in Genealogy

The term “double first cousin” sounds like something out of a complicated family tree puzzle. But it’s real, and more common than you think. This unique relationship raises eyebrows and questions about everything from shared DNA to family gatherings.

Want to know if sharing grandparents with the same couple is as weird as it sounds? We’ve prepared everything you need to know about this fascinating familial bond.

» Reveal double first cousin connection using a DNA kit

What are double first cousins?

Double first cousins share both sets of grandparents. Let’s say your mother had a sister and both of them married two brothers. Then, you and your aunt’s kid would be double first cousins.

Many people mistakenly assume “double first cousins” means incest. But, there’s no direct marriage between first cousins involved — their parents aren’t related.

» Explore the different family relationships

Double first cousins are a unique yet helpful occurrence in geneology.

How much DNA do double first cousins share?

Double first cousins may have varying degrees of genetic similarity:

• Non-twin siblings: Share about 25% of their DNA, like half-siblings.
• Fraternal (non-identical) twins: Also share about 25% of their DNA.
• One set of identical twins and one set of non-twins: Share approximately 37.5% of their DNA.
• Two sets of identical twins (double identical cousins): Share approximately 50% of their DNA, like full siblings. Identical twins occur in about 0.3% of births, so the chance of two pairs marrying and having children is exponentially rare — but it does happen.

DNA percentage isn’t everything

Double first cousins have some specific genetic features that you can use to differentiate between the two:

• Identical DNA segments: Parts of their DNA might be identical, reflecting the close relationship between both parents — unlike half-siblings who only share DNA from one parent. These shared segments can increase the likelihood of shared physical and health-related traits.
• Shorter DNA segments: Double first cousins’ twin DNA segments tend to be shorter than half-siblings’ due to more “genetic shuffling” — recombination — occurring over generations.

So how would you apply these concepts in your research? Let’s say you do a DNA test and find a match with a high percentage of shared DNA. The results suggest you two are half siblings. But, after looking through historical records you identify them as your first cousin.

This discrepancy might hold more clues. So, you can look at DNA segments to find unusually long ones that may point to a double first cousin relationship. The discovery could lead you to a previously unknown marriage.

» Discover this comprehensive introduction to genetic genealogy

How double first cousins can enhance research

Double first cousins share a distinct genetic signature compared to regular first cousins. While regular first cousins inherit genes from two shared grandparents, double first cousins inherit DNA from all four. This difference is crucial for accurate interpretation.

You can use it to precisely find which segments you got from each of the four shared grandparents. How? With a technique called phasing, which can help confirm the parental origin of DNA segments. This way, you can precisely trace your ancestral line and even confirm parentage.

As we’ve said, double first cousins share about 25% of DNA. But, the other 75% are also valuable. They represent genes you inherited from ancestors other than shared grandparents.

You can use these 75% to find people who share smaller segments of DNA with you, suggesting a common ancestor further back in time. This can help find more distant relatives and expand the family tree.

» Discover how to optimize your DNA matches in genealogy

How to optimize genealogy through double first cousins

A well-defined research methodology is essential for maximizing the benefits of this unique relationship. Here’s a breakdown of the steps involved:

1. Identify double first cousins

DNA testing kits or labs can reveal potential double first cousins based on shared genetic percentages. When you encounter a genetic match sharing about 25% to 50% of your DNA, you’re likely looking at a double first cousin.

Make sure you’ve ruled out other close relationships like aunt, uncle, or half-sibling.

» Read about how a first cousin match helped solve a 12 year mystery

2. Confirm the relationship

Confirm the DNA connection using traditional methods. This involves reviewing family trees and birth certificates or interviewing relatives to establish the ties between potential double first cousins.

Because this relationship stems from two siblings marrying two other siblings, focus on identifying sibling pairs within family lines. These sources can be beneficial:

• Census records: The U.S. Federal Census, beginning in 1850, is a valuable starting point. While you may need to look at multiple census records to account for family growth and movement, these documents often list children by name, making it relatively easy to identify potential sibling groups.
• Wills and probate records: These legal documents frequently outline family relationships, including children and their inheritances. They can provide concrete evidence of sibling bonds and clues about parental generations.
• Birth, marriage, and death records: These vital records show direct proof of parentage and familial connections. By compiling information from multiple family members, a comprehensive family tree that clearly illustrates sibling relationships can be constructed.

Note: The biggest challenge might involve sorting through names if both fathers in the double first-cousin connection have the same surname — if they’re brothers. This relation can lead to confusion, especially in earlier records with limited details.

» Explore our top tips for searching historical records

3. Explore non-matching DNA

Once you’ve established shared ancestral lines, compare your ethnicity results with your double first cousin to identify unique genetic contributions in the non-matching DNA.

By comparing your non-matching DNA segments with large genetic databases like MyHeritage, you can identify distant relatives who share smaller fragments of your DNA. These matches often represent connections from more remote branches of your family tree:

• Could you look for clusters of matches that share common ancestors or geographical origins?
• Pay attention to matches with unusual surnames or from unexpected locations, as these might lead to previously unknown family lines.
• Use triangulation techniques to confirm relationships and pinpoint common ancestors.

DNA analysis of the non-matching segments can reveal unexpected ethnic backgrounds that aren’t apparent in the shared family lines with your double first cousin. This can lead to fascinating discoveries about your ancestral origins:

• For example, someone with predominantly European ancestry might uncover a small percentage of Native American, African, or Asian heritage.
• These genetic traces can inspire targeted research into potential tribal affiliations, historical migration patterns, or long-forgotten family connections.
• Consider how historical events, such as colonial expansions or forced migrations, might have contributed to these unexpected genetic markers.

» Learn more about the ethnicity estimates in DNA tesing

4. Combine evidence

Use this knowledge to target your traditional genealogical research efforts. As you uncover new facts about your family history, ask yourself if they fit the double first cousin relationship scenario and if it’s represented in the earlier generation’s facts.

Cross-verification can help ensure the accuracy of your research and reveal errors or inconsistencies in historical records.

Because there is one less set of grandparents in a double first cousin relationship, there is less information to find when taking your family tree back another generation. This allows for more in-depth research into known ancestral lines.

Throughout this process, maintain meticulous records of your research process and findings. Genealogical software like MyHeritage can map out complex relationships and identify patterns. Regularly review and update your research plan based on new information and insights.

» Explore these interview questions to ask your family

Double the cousins, double the clues

Double first cousins are defined by the sharing of both sets of grandparents. Despite people usually mistaking it for incest, there’s no blood relation between the parents of the people who share both sets of grandparents.

With DNA testing, you can find potential double first cousins, and get a starting point for further research. You can also confirm relationships and build more accurate family trees. Ultimately, combining this analysis with traditional research leads to a detailed and accurate family trees.

» Could you have double first cousins? Find out with a DNA test