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Chromosome Basics

Chromosomes are tiny structures found within your cells. They contain the DNA information and instructions that define who you are - what you look like, how your body works, and even what genetic diseases you might have.

Humans have 46 chromosomes. But chromosomes come in pairs, so we typically think of them as 23 pairs of chromosomes. The first 22 chromosome pairs (called autosomes) are numbered 1 through 22. We'll primarily focus on these autosomal chromosomes. The 23rd pair are called the sex chromosomes - men have an X and a Y sex chromosome and women have two X chromosomes.

Chromosome Inheritance

One autosomal chromosome from each pair comes from your mother and the other comes from your father. This means you get half of your DNA from your mother and half from your father. Each chromosome they pass on to you is a combination of their own pair of chromosomes which they got from their parents (your grandparents).

The image above depicts how one pair of chromosomes may be passed from your parents to you. The colors don't mean anything special - they simply depict the individual chromosomes and chromosome sections.

You'll notice that the chromosome passed to you from each parent may not be an exact 50/50 combination of their own chromosomes. This means that you might have a bigger portion of one of their chromosomes than the other - you might be more related to one of your grandparents than another on that chromosome. In fact, you might have an exact copy of one of your parent's chromosomes, and thus you'll get no portion of their other chromosome.

If you add in another generation, things get a bit more complex. This depicts just one chromosome pair. Remember that you have 22 pairs that will be various combinations of your grandparent's chromosome pairs. In this example, one of the mother's chromosomes (the one she got from her father) was passed on directly to the child. This child will not match his maternal grandmother on this chromosome. I'm not sure how often this non-recombination occurs, but of my 44 autosomes, 6 were not recombined from my parents to me. While lop-sided chromosomes or non-recombination may occur on a particular chromosome, across all 46 chromosomes, things tend to average out - you'll get around 25% of your DNA from each of your grandparents.

For each generation you go into the past, you will get less and less of that ancestor's DNA. The chromosome segments they pass on will become smaller or lost due to recombination. This is why autosomal DNA analysis is usually only useful to at most 6 or 7 generations back - you have so little DNA from very distant ancestors that it becomes difficult to analyze it reliably.

Some Definitions


Centimorgan (abbreviated cM) is a measure of genetic linkage. Think of it as a measure of DNA information within a chromosome. Each chromosome contains different amounts of information. Chromosome 1 contains 281.5cM of information. Chromosome 2 has 263.7cM. Chromosome 21 has only 70.2cM.


SNPs, or single-nucleotide polymorphisms, are tiny pieces of a chromosome that contain distinct blocks of information. There are thousands of them per chromosome. SNPs are compared between two people to see if they match. The amount of information in matching SNPs is measured in cM.

The cM values for SNP matches are sometimes referred to as "chromosome length" or "match length". However, information is more densely packed in certain areas or SNPs within chromosomes, so there's not a direct correlation between number of SNPs and cM amount. When you view GEDmatch's graphical depiction of chromosome matches, a bigger matching block does not always mean a higher cM value.


A "segment" refers to a section or block of contiguous SNPs. A "matching segment" is a section that is the same between two people.

Start and End Location

Individual markers (called base pairs - the things that SNPs are made of) within a chromosome are numbered. There are millions of these markers per chromosome. A segment of a chromosome can be identified by these location numbers.


Sometimes SNPs marker values match between two people simply by chance. This is called IBS or Identical By State. And sometimes they match because they were passed down from a common ancestor. This is called IBD or Identical By Descent.


This is Most Recent Common Ancestor - the ancestor from which you and a DNA match received your common DNA segments.

Each child gets an entirely different combination of their parent's chromosomes (unless they are identical twins). This means that you can match with someone that a sibling (or other relative) does not.

In this example, you can see how the children match different segments of their grandparent's chromosomes. A match with a distant cousin in the dark blue section for Child #1 would not be present at all for Child #2. And Child #1 wouldn't match any relatives on the maternal grandmother's line on this chromosome due to lack of recombination.

* * * * *
If you don't match someone, this does not always mean you are not related. It's possible that matching segments from an ancestor simply weren't passed down to each of you. A match to one of your known relatives may still be your cousin, even if you don't match them - especially if their segments align on your chromosome map to your ancestor. Testing additional relatives can be helpful in finding additional cousins and unknown ancestors. They will establish a family baseline with which to triangulate to determine genetic ancestry lines.

Testing older relatives (especially parents and grandparents) will get you an extra generation (or 2 or 3) further back in time - enough to discover cousin connections that would otherwise be impossible!

* * * * *

This information was taken from Genetic Genealogy using GEDmatch
An Absolute Beginners Guide
by Jared Smith

Edited by: STLOUISWOMAN at: 11/19/2017 (17:46)
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