How Many Segments are in DNA?

Humans have 46 DNA segments on their 23 chromosomes, 23 inherited from each parent. In this article, learn more about these DNA segments, how many segments we have, and what it means to share these in common with someone.

DNA segments are an important part of understanding how we are related to our DNA matches. When we see that we share a few, or even several, segments with our genetic relatives, most of us usually wonder how significant this really is.

In other words, how many DNA segments does a person have that they could share with other people? As I mentioned above, a person does have a total of 46 DNA segments, 23 passed down from each of their parents.

The total of 46 segments includes segments on the 22 numbered chromosomes. In addition, it also counts the two copies of the X chromosome that females inherited from both their mother and father, or the X and Y chromosome that males inherit from their mother and father, respectively.

But, there is more to the story. If we only have 46 segments, how is it possible to share more than 46 segments with a relative, such as a half-sibling?

In addition, how big are these total DNA segments that we got from our parents? There are so many questions – but don’t worry.

Below, I’ll explain everything you need to understand about this topic.

How many DNA segments does a person have in their DNA?

As you now know, a person has 46 DNA segments on their 23 chromosomes. They have two copies of each of their 23 chromosomes, each passed down from their mother and father.

The copies of their chromosomes that they were inherited from their mother and father are made up of “recombined” (basically, mixed-up) versions of the two copies of each chromosome that they got from their parents.

While you don’t need to understand much about that just yet, it’s an important detail to know. The fact that the copies of your chromosomes that you got from your dad, for example, play an important part in understanding how you share DNA segments with other relatives.

More on that topic further on in this post.

How big are our DNA segments?

Each chromosome, or DNA segment, that we inherit from our mother and father is the same length on each chromosome. In other words, the copy of Chromosome 1 that we got from our mother is the same length as the copy of Chromosome 1 that we got from our father.

Even so, it is also important to note that each chromosome has a different total length. Some are significantly longer than others.

Even though the length of each chromosome varies, every human has the same number of chromosomes and they are all the same length. We are all humans of the same species, after all!

So, how long are those chromosomes? We measure chromosome length, especially in the world of genealogy and DNA matches (or genetic genealogy) using centimorgans (cMs).

In the table below, you can see how many centimorgans are on each chromosome. We have two copies of each of the below chromosomes, which means that the total amount of DNA that we have is equal to twice as much as the totals you see in the chart.

Chromosome Number	Approximate Length (range) in cMs
1	267-284
2	253-269
3	219-223
4	206-214
5	199-209
6	189-194
7	180-187
8	160-168
9	160-167
10	174-181
11	155-161
12	167-176
13	126-131
14	111-125
15	118-141
16	131-134
17	124-138
18	119-129
19	99-111
20	104-114
21	58-70
22	53-79
23	182-196

There is a range in the length of the chromosome because each DNA testing company tests slightly different positions in our genome causing a variation in length. If you try to perform a comparison yourself using a chromosome browser, you will notice that your numbers are likely slightly different than what you see above.

Even so, it does serve as a good general reference.

What does it mean to share DNA segments with someone?

When you share a DNA segment with someone, it means that you and your relative have both inherited a piece of DNA that was passed down from a shared ancestor. The shared ancestor could be as recent as a parent, or very distant.

The size of a shared DNA segment can provide some clues as to how closely you are related to your match. As you know, you inherit the entire length of a chromosome from each of your parents.

If you recall, towards the beginning of this article, I mentioned that the chromosomes that your parents passed down to you are made up of a combined, brand-new, completely unique chromosome made from bits and pieces of the two chromosomes that they inherited from both of their parents.

This process is called recombination. Every generation, recombination takes place, leaving about 50% of DNA from any given ancestor behind.

Remember, your chromosomes are a set length and so we can only inherit 50% of each parent’s DNA.

The DNA that you do inherit, however, contains small pieces of DNA that match other relatives. These relatives, despite the effects of recombination over a number of generations, have managed to inherit DNA from the ancestor(s) that you share in common.

How many DNA segments should relatives share?

As a general rule, we will share fewer, shorter segments with distant relatives. With closer relatives, we expect to share a higher number of longer segments.

We can often see the size of the DNA segments that we share with our matches using a chromosome browser. Alternatively, we can see the size of the longest shared segment on Ancestry DNA.

There is no exact number of DNA segments that we should share with any given relative, however.

I’ve written a few posts about shared DNA segments that you might find interesting and helpful if you are trying to research shared segments with a relative of a particular relationship:

• All About DNA Segments
• Total Centimorgans vs. Shared Segments
• How Many DNA Segments Do Siblings Share?
• Does a Long Segment Always Mean a Recent Ancestor?
• If you’ve noticed that different testing sites report different chromosome positions for shared segments: Comparing Chromosome Positions Across Testing Sites

Conclusion

I hope that this post has helped you learn more about how many DNA segments a person has in their genome, as well as what it really means to share DNA with someone. Plus, I hope you now know more about how many DNA segments you should share with your relatives.

If you have any questions about something that you read in this post, or if you have a specific question about the number of DNA segments that you share with a DNA match, I would love to hear from you in the discussion below.

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