Chromosomes and Genealogy

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Sometimes people ask about how chromosomes relate to genealogy. Every single one of us started with that question, right?

Are chromosomes different sizes, and does that matter? What are the mystery terms, cMs and SNPs? How does all of this intersect with genealogy? Do I care?

These are all great questions, and of course, there are different ways to answer. Let’s start with some basics.

Chromosomes 1-22

First, you have two copies of each of chromosomes 1-22.

The karyogram above, a photo taken through a microscope, courtesy of the National Human Genome Research Institute, shows the chromosomes of a human male. I’ve added the numbering and labeled the X and Y chromosomes (23).

You inherit one copy of each chromosome from each of your parents. You can see the two halves of each chromosome, above. One half of each chromosome is contributed by the person’s mother, and the other half is contributed by the father.

That’s why DNA matching works, and each match can be designated as “maternal” or “paternal,” depending on how your match is related to you.

Each match will be related either maternally, paternally, or sometimes, both. Of course, that’s presuming the matches are identical by descent, and not identical by chance, but that’s a different discussion. For this article, we’re referencing valid matches with whom you share common ancestors – whether you know who they are or not.

Your 23rd chromosome is different than chromosomes 1-22.

Chromosome 23 Determines a Child’s Sex

Your 23rd chromosome is your sex-determination chromosome and is inherited differently.

You still inherit one copy of chromosome 23 from each parent.

  • Males inherit a Y chromosome from their father, which is what makes males male.
  • Males inherit an X chromosome from their mother.
  • Females inherit an X chromosome from both parents, which makes them female.
Chromosome 23 Father Contributes Mother Contributes
Male Child Y chromosome X chromosome
Female Child X chromosome X chromosome

Because males don’t inherit an X chromosome from their father, X chromosome matching for genealogy has a unique and specific pattern of descent which allows testers to immediately eliminate some potential common ancestors.

The Y chromosome can be tested separately for males and follows the direct paternal line. You can read about the 4 Kinds of DNA for Genetic Genealogy, here.

The X chromosome is quite useful for genealogy due to its unique inheritance path and is included by both FamilyTreeDNA and 23andMe in matching.

Picture This

Three of the four major vendors, plus GEDmatch, provide a visual match depiction of your chromosomes using a chromosome browser:

Unfortunately, Ancestry does not provide a chromosome browser or segment location information.

Using your chromosomes as the canvas, matches to your father and mother are shown using the chromosome browser at FamilyTreeDNA, below.

You can see that a tester matches both parents on the entire covered region of all of their chromosomes. The beginning and the end tips of each chromosome sometimes aren’t covered, and neither are some other regions that are very SNP-location-poor. Omitted regions are shown by hashes. Regions that are light grey, but not hashed, are covered, but the match’s test didn’t produce results in that region.

This is why you may have a slightly different size match with one parent versus the other, especially if they both didn’t test at the same vendor at the same time.

The chromosome browser graphic visually answers the chromosome size question, but there’s more to this answer. It’s easy to see that there’s a significant difference in the physical chromosome size, but there’s more to the story.

SNPs – Chromosome Street Addresses

SNPs, known as Single Nucleotide Polymorphisms, are mutations recorded at specific addresses on chromosomes. Each chromosome holds a specific number of addresses that are read during sequencing and used for match comparison.

All of your other matches that are not parent-child and not your identical twin will match on some subset of these locations.

The Rest of the Answer – Centimorgans and SNPs

Centimorgans (cMs) are units of recombination used to measure genetic distance. You can read a scientific definition here.

For our conceptual purposes, think of centimorgans as lines on a football field. They represent distance on the chromosome.

SNPs are locations that are compared between two people to see if a match occurs.

Think of SNPs as addresses for blades of grass on that football field where an expected value occurs. If values at that address are different, then they don’t match. If values are the same, then they do match. For autosomal DNA matching, we look for long runs of SNPs that match between two people to confirm a common ancestor.

Think of SNPs as blades of grass growing between the lines on the football field. In some areas, especially in my yard, there will be many fewer blades of grass between those lines than there would be on either a well-maintained football field, or maybe a manicured golf course. You can think of the lighter green bands as sparse growth and the darker green bands as dense growth.

If the distance between 2 lines on the football field is 8 cM, for example, and there are 700 blades of grass growing there, you’ll be a match to another person if (almost) all of your blades of grass between those 2 lines match, assuming the match threshold is minimally 8 cM and 700 SNPs.

For purposes of autosomal DNA, the combination of centimorgans (distance,) and the number of SNPs (locations) within that distance measurement determines if someone is considered a match to you. In other words, you’re listed as a match if the shared DNA is over the minimum or selected thresholds. Think of track and field hurdles. To get to the end (a match), you have to get over all of the hurdles!

For example, a threshold of 8 cM and 700 SNPs means that anyone who matches you equal to or greater than both of these cumulative thresholds will be displayed as a match. Centimorgans and SNPs work in tandem to ensure valid matches.

A Second Yardstick

So, the second measure of chromosome size is the number of cMs from the beginning to the end of the chromosome, and the number of SNPs on that chromosome.

Different vendors, and different DNA testing chips cover slightly different regions. This is my match with my mother, which shows:

  • Total matching cMs on each chromosome
  • Total matching SNPs on each chromosome
  • SNP Density, which is a calculation (cM/SNPs) showing how “thick” the SNP grass is on each chromosome

The higher the matching number of cMs, especially in a row (longest segment,) the higher quality the match, and the closer the relationship.

Note that endogamous, or intermarried populations, may need separate interpretations. I discussed the signs of endogamy in this article.

Calculating Matches

Some vendors provide the ability to select your match cM and SNP thresholds, and others make those selections for you. Most vendors no longer display the number of matching SNPs, given that SNP-poor regions are, for the most part, automatically eliminated, although you can view them in your matching segment download file. In other words, the vendors simply take care of this for you. The accepted rule of thumb has always been that 500 (some said 700) or fewer SNPs was too small to be genealogically relevant, regardless of the cM match size.

Vendors include numerous and varying factors in determining match quality and potential relationships, including:

  • Total shared DNA, meaning total matching cM
  • Longest shared, meaning contiguously matching DNA block
  • X matching
  • Sex of tester (especially with respect to X matching)
  • Endogamy flags
  • Half versus fully identical DNA regions (to positively identify relationships such as half vs full siblings)
  • Triangulated segments
  • Family Matching (maternal and paternal bucketing) at FamilyTreeDNA
  • Tree matching

Not all vendors include all factors, and each vendor utilizes proprietary algorithms for features like triangulation.

The question isn’t chromosome size or even match size alone, but the quality of the match plus additive genealogical features like Theories of Family Relativity at MyHeritage to identify common and even previously unknown ancestors.

Be sure to test at the primary vendors or upload for free to MyHeritage, FamilyTreeDNA and GEDmatch to receive as many matches as possible. You just never know where that match you really need is hiding!

Enjoy!

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