DNA Basics: Are You an Equal Mix of Mom and Dad?

Do you inherit exactly half of your DNA from each parent?  As with so many questions in genealogy and in life, the answer is: it depends.  It depends on your genetic sex and whether you consider mitochondrial DNA.

A quick DNA primer is in order.  Most of our DNA is packaged into very, very long strands called chromosomes that are each made up of millions of subunits called base pairs (bp).  (For more on the chemistry of DNA, see the post DNA Basics: What Is DNA?)

Humans have 23 pairs of chromosomes, or 46 total.  We each inherit a full set of 23 from our mother and another full set from our father.  Twenty-two of those chromosome pairs are, for our purposes, identical; the one we get from mom is the same size and composition as the one we get from dad.  This is the autosomal DNA (atDNA) used the most in genetic genealogy.

HYanWong, CC0, via Wikimedia Commons


The 23rd pair is more interesting.  The so-called sex chromosomes, X and Y, are very different in size.  The X is much larger.  They also have different numbers of genes.  Genetic women have two copies of the X, one from each parent.  A genetic man, on the other hand, has one X, inherited from his mother, and one Y, from his father.  In fact, the Y chromosome is the one that determines genetic maleness.  If you have a Y chromosome, you’re genetically male; if you don’t have a Y chromosome, you’re genetically female.  And if you don’t have an X chromosome, you’re dead.

Because the X and Y are different sizes, men inherit slightly more DNA from their mothers than from their fathers.

Another exception is mitochondrial DNA (mtDNA), which is relatively short—only 16,569 bp—and circular rather than linear.  We refer to this as the mitochondrial genome, because it’s technically not a chromosome.  Mitochondria are inherited from the mother, so if we include mtDNA in our calculations, even women inherit slightly less DNA from their fathers.  You can read more about mitochondrial DNA here.

(Yes, there are exceptions to these rules; there are exceptions to all rules in biology!)

How Much More DNA Do We Inherit From Our Mothers?

The answer to that question depends on how you measure DNA.  Perhaps the simplest measure is base pairs.  One set of the autosomes (chromosomes 1–22) totals 2,875,001,522 bp for the human reference genome.  That number will vary somewhat between individuals, but not enough to affect our calculations.  The X chromosome is 156,040,895 bp and the Y is 57,227,415 bp.

With these numbers, we can calculate how much DNA we inherit from each parent for each sex.

Women inherit:

  • From Mom:
    2,875,001,522 bp atDNA + 156,040,895 bp X-DNA + 16,569 bp mtDNA = 3,031,058,986 bp
  • From Dad:
    2,875,001,522 bp atDNA + 156,040,895 bp X-DNA = 3,031,042,417 bp
  • Percent from Mom:
    (3,031,058,986 bp) / (3,031,058,986 + 3,031,042,417 bp) = 50.0001%

Men inherit:

  • From Mom:
    2,875,001,522 bp atDNA + 156,040,895 bp X-DNA + 16,569 bp mtDNA = 3,031,058,986 bp
  • From Dad:
    2,875,001,522 bp atDNA + 57,227,415 bp Y-DNA = 2,932,245,506 bp
  • Percent from Mom:
    (3,031,058,986 bp) / (3,031,058,986 bp + 2,932,245,506 bp) = 50.8%

As you can see, the bias is trivial for women, because the only difference is the mitochondrial genome, which is tiny in comparison to everything else.  Even for men, the imbalance is fairly small.


How Many More Genes Do We Inherit From Our Mothers?

Another way to look at inheritance is “genes”. Genes are the portions of our DNA that have a function.  Some genes are blueprints for making proteins (the workhorses of the cell), and some function in more subtle ways, like helping the cell make proteins.

Measuring inheritance using genes is less straightforward than simply adding up base pairs, because scientists still have not identified all of the genes in the human genome, and because there are stretches of DNA that look like genes, but are broken.  To simplify things, this set of calculations will consider only protein-coding genes and small non-coding RNA genes in the human reference genome.

Using these criteria, there are approximately 24,211 genes on the autosomes, 1,100 on the X chromosome, 101 on the Y chromosome, and 37 in the mitochondrial genome.  Modeling our calculations as above,

Women inherit:

  • From Mom:
    24,211 atDNA genes + 1,100 X-DNA genes + 37 mtDNA genes = 25,348 genes
  • From Dad:
    24,211 atDNA genes + 1,100 X-DNA genes  = 25,311 genes
  • Percent from Mom:
    (25,348 genes) / (25,348 genes + 25,311 genes) = 50.04%

Men inherit:

  • From Mom:
    24,211 genes atDNA genes + 1,100 X-DNA genes + 37 mtDNA genes = 25,348 genes
  • From Dad:
    24,211 genes atDNA genes + 101 Y-DNA genes = 24,312 genes
  • Percent from Mom:
    (25,348 genes) / (25,348 genes + 24,312 genes) = 51.04%

By this criterion, the maternal bias is a little more pronounced than with total base pairs, but it’s still pretty small.

Maternal Bias and Genetic Genealogy

Of course, the real question genealogists have is How does this impact our research?  For the most part, it doesn’t.  We already know that Y-DNA and mtDNA tests track the paternal and maternal lines, exclusively, so bias is not only a given for them, it’s necessary.

Autosomal DNA tests, which account for the vast majority of genealogical DNA tests, are by definition, based primarily on atDNA.  These atDNA tests do include data for the X chromosome, but it’s not used in either ethnicity estimates or matching.  In other words, you don’t need to worry about maternal bias when working with your atDNA results.  (The exception is 23andMe, which does include X-DNA in matching, so there will be a very slight skew toward the maternal side for matches who share X DNA with a male tester.)


13 thoughts on “DNA Basics: Are You an Equal Mix of Mom and Dad?”

  1. Hi,
    I have a comment, but it relates to 23&me. My son bought me the test for Mother’s Day of 2017. I was on the V4 chip. I just found out that 23&me wants us all to resubmit our DNA and pay an extra $99 or they won’t release the newest ethnicity and health reports. We would be on the V5 chip. I think they are playing dirty pool. I don’t like this. Ancestry does not make us redo our DNA. What’s up with that and what are their motives. This is just crummy. What are your thoughts?

    1. If there’s not much SNP overlap between 23andMe’s v4 and v5 chips, so ethnicity and health updates based on the v5 data chip wouldn’t transfer well to v4. It’s unfortunate, but I see where they’re coming from. $99 is half the list price of their Ancestry + Health Service, so at least they’re offering a discount. (Ancestry and all of the other companies will upgrade chips, too, and the different versions won’t always be compatible.)

    2. Kim, I was tested on the V5 chip, but I wanted to have access to the matches that went away. 23andMe wanted 125.00 for the upgrade feature. This is crazy when they were offering the complete kit at the time for 99.00.


  2. I don’t think Ancestry DNA updates correct when it comes to African Americans. My first DNA report was most accurate. My ancestors are from Eastern NC so the percentages were accurate for that geographical area, history and my family tree including 1 percent Indian. The update erased the Indian and added Somalia and increased me to 43 percent Nigerian but did not remove the 1 percent Jewish. My closest full Nigerian ancestor arrived in the 1600s. So how can I be 43 percent or even 33 percent Nigerian?

    1. Ethnicity estimates are an evolving field. Ancestry is updating theirs every year now, so hopefully the next update will align better with your family history.

      1. The DNA of ethnicity is not necessarily related to modern generations of DNA. The groups used to model against are evolving as said as more people who can prove all their ancestors (at least 3 generations back) came from one region become available. Your connection to those results is what will make this decision. In the US it is possible that the slave communities were enclosed images of the ethnicity of African regions where they were taken from and so the ethnicity may not have been diluted so easily? I have the same issue with Irish immigrants who were so numerous, could easily find each others common culture in the suburbs of New York with subsequent intermarriage?

  3. Hi Leah.

    Apropos, I think.

    I have a 3C2R match who shares 38 cM with me. That’s about average for the Bettinger dataset.

    But her mother (3C1R) matches me for 170 cM. That’s way above average and close to
    the Bettinger max.

    Any theories? Thanks.

    1. If there’s no other connection to the mom, it looks like your match to the mom is an outlier. The SCP tool puts the probability of 170 cM being a 3C1R at about 3%, so its definitely possible.

  4. Hi Diann,
    Surprise, surprise! 23&me updated my ethnicity report without forcing me to purchase the upgrade. Maybe, they got enough feedback that they decided to do it for everyone. Watch and see if yours changes. All of my family members got an upgrade, as well. I never saw an email regarding the upgrade, I just saw it when I was looking at 23&me. I hope yours upgrades, as well. Let me know if it does.

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