Facts You May Not Have Known About Your Dna and Biological Connections to Ancestors

Updated on January 19, 2019
Rachel Aldinger profile image

"The true delight is in the finding out rather than in the knowing." -Isaac Asimov. Also I'm kind of a genealogy nerd.

Researching ancestry can be both a perplexing and intriguing endeavor! What are the pieces of DNA that make people who they are? What’s the difference between a first cousin and a first cousin once removed? What is a cousin, anyway? What does DNA really look like? How does one person’s genetic ancestry compare to another, and why is it this way? Want to know more? Read on for ten interesting facts you may not have known about your DNA and biological connections to ancestors!

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1. Dna Is Shaped like a Double-Helix

Ever wondered what DNA really looks like? The first X-ray image of DNA was perceived in 1952. DNA looks like a double-helix! This has been the standard image of DNA since, but as one might expect, DNA is best viewed through a microscope. Thanks to modern science, we now have a better idea of what we humans are made of.

2. Dna Can Stretch as Far as the Sun

Hypothetically. It is estimated that humans have about ten trillion cells. If you were to hypothetically “stretch out” your DNA, the range would be about ten billion miles. To put this into perspective, this is practically the equivalent to traveling to the sun and back 100 times!


3. Who Are Cousins?

Family labels are more complicated than one might think! Defining relationships to one’s relatives can be simple, (just call everyone family!) but specifically defining how each person is related is a little more complicated.

For instance, an aunt or an uncle’s child is to be defined as a first cousin. But, what is your first cousin’s child to you? While you might initially think second cousin, the truth is, the technical term to define this relationship is they are your first cousin once removed. Your second cousin is the child of your first cousin once removed.

You share a set of great grandparents with your second cousin.

4. Centimorgans Shared Across Relationships

Centimorgans are a unit of measurement with which to trace the links between genetics. The amount of DNA you share with each relative typically differs dependent on the closeness of the relationship. For example, on average, you share about 3400 centimorgans of DNA with each parent, this equates to about 50 percent inherited from each.

If you were to follow down this line of succession, you share approximately 1700 centimorgans of DNA with a grandparent, aunt or uncle, niece or nephew, and half siblings; that’s 25 percent of your DNA. Following this pattern, you share approximately 850 centimorgans, 12.5% DNA, with your first cousin.

5. The Amount of Dna You Share with Each Relative Varies but Can Be Similar

You share a similar amount of DNA with a relative depending on the relationship. For instance, you share the same or similar amount of DNA with an aunt or uncle as you do a great-grandparent, or a half-sibling. Another example is that you share a similar amount of DNA with your first cousin as you do a great-great grandparent, great-great-grandchildren, great-aunts and uncles as well as great-nieces and nephews and those in a similar category.

This information can be useful if you have taken a DNA test that links you to an online database, and are trying to determine how you are related to a specific match. The amount of DNA shared helps determine relationship categories on DNA testing websites, and ultimately, how you are related to a DNA match. If a relative matches as a first cousin, they may not be a first cousin, but a relative that shares the same amount of DNA equated to that of a first cousin genetic relationship.

6. Sibling Ancestry Genetics Are Not the Same (Unless Identical Twins)

Did you know full siblings may not share the same proportions of DNA? Except in the case of identical twins, genetics don’t necessarily pass down equally, even with full siblings. This is due to how genetic ancestry is shared from parent to child. Children who share the same parents can inherit vastly different traits. For instance, let’s say two siblings both come from an Irish and German descent. In this case, while the siblings both come from Irish and German ancestry, one inherited a larger proportion of German ancestry than the other.

Full siblings share 50 percent of their DNA. But, the DNA that is passed down through the generations won’t necessarily be passed down in one even proportion. While siblings descend from the same familial line, they may inherit different proportions from different regions. This is why, in some cases, full siblings receive different ethnicity percentages when it comes to the results from DNA testing services.



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7. Have a Parent Who Has a Twin? If Yes, Then Your Cousins Are Like Your Siblings.

Sort of. If you have a parent that has an identical twin, then genetically speaking, your cousins are practically half-siblings. While legally you would refer to them as your first cousins, genetically speaking you share nearly 25% of your DNA with them, the same amount of DNA typically shared with half-siblings. Identical twins’ DNA is nearly identical, so for this reason your cousins and you will share a closer genetic relationship than that of a typical cousin relationship. They are still your cousins, you just share more than the average amount of DNA typically shared with a first cousin due to the fact that one of your parents is a twin.


To take this one step further, it’s possible that you could share the same amount of DNA with your cousin as that of a full sibling. While this is generally uncommon, theoretically, if identical twins married identical twins, their children would genetically be like full siblings. This is because children inherit 50 percent of their DNA from each parent, but when each parent has an identical twin, both sets of children are inheriting two similar sets of DNA. This is when the hypothetical idea of “Identical Cousins” comes into play—the idea that cousins could potentially look “identical” or at least look very similar due to their closely related DNA.


Crazy, right!?

8. Females Have More in Common Genetically with Their Paternal Grandmother

Daughters share slightly more DNA with their paternal grandmother in comparison to other grandparents. Likewise, sons share slightly less DNA with their paternal grandmother. This has to do with the way genes are passed down from parent to child.

This is because of the X and Y chromosome. Specifically, females have two X chromosomes and males have both chromosomes X and Y. The Y comes from the father and the X from the mother. The X chromosome is passed down through the father, similar to that of his mother, tying back to the paternal grandmother.

Typically, the amount of DNA one shares with a grandparent is 25%. This is because a child inherits 50 percent of their genetics from each parent. Each parent has inherited 50 percent DNA from their parents as well. As a result, their children share half of that genetic link with grandparents. However, paternal grandmothers have about 26 percent in common genetically with their granddaughters, where as grandsons will share a slightly less amount.


9. Your Family History May Be Inaccurate

Many families have kept records of where their ancestors originated, but sometimes these records prove to be inaccurate. For instance, you may think your family has German heritage, then you take an at-home DNA test, and discover you’re not nearly as German (if any!) as you believed. Surprised!? Join the club. Many people are finding unexpected results thanks to modern science. These surprises are upending “outdated” genealogical research, and changing the way genetic history is viewed.

While one’s genetic ancestral history shouldn’t automatically be assumed incorrect, it’s always possible recorded family history may have some errors. This can happen for a variety of reasons, but the best way to find out your heritage is by researching it yourself. You can test this and learn more about your DNA through any credible DNA testing service! You never know, you may be surprised by the results.

10. You Have Approximately One Thousand Fourth Cousins

Fourth cousins share the same great-great-great grandparents and you may have nearly one thousand of them!

Think that’s a lot?

Take this one step further and it’s quite possible you have nearly five thousand fifth cousins. If you keep tracing back your lineage, you could discover you have a half million eighth cousins.


Talk about a family reunion!

Questions & Answers

  • How many cousins does each person have?

    The number of cousins each person has is going to vary by family and is dependent on the number of children born. If more distant cousins are included in this calculation, the number is likely to be in the thousands or more.

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