A Quick Tour of Your 23 Chromosomes

Your entire genome is packaged into a tidy set of chromosomes - 23 pairs in all. It is one of the first things people learn about DNA and one of the easiest to picture, so here is a friendly tour of what each set does and how it shows up in your raw file.

Twenty-three pairs, one from each parent

You carry 23 pairs of chromosomes - 46 in total - and the “pair” part matters. For each one, you inherited a copy from each parent. That is why nearly every position in your raw file lists two letters - one allele on each copy of the pair. Line them up and your genome reads as two parallel sets of instructions, working together. Having two copies also builds in redundancy: if one copy carries a variant, the other is often there to balance it, which is part of why a single letter rarely tells the whole story.

The 22 autosomes

Twenty-two of those pairs are the autosomes - the numbered chromosomes, from 1 through 22. They are ordered roughly by size, with chromosome 1 the largest and the higher numbers generally smaller, though the ordering is approximate rather than exact. The autosomes carry the great majority of your genes and do the bulk of the everyday genetic work, from metabolism to eye colour.

Because both copies of each autosome are present, autosomal DNA is where most trait and ancestry analysis happens - there is simply the most material to read.

The sex chromosomes

The 23rd pair is the sex chromosomes, and it comes in two arrangements: two X chromosomes, or one X and one Y. The X is a sizeable chromosome carrying many genes; the Y is much smaller and carries far fewer. This pair is also why the X and Y follow their own special inheritance paths, unlike the freely mixing autosomes. In a raw file the sex chromosomes are labelled X and Y, and you may notice single letters rather than pairs at some of their positions, because those chromosomes are not always present in two matching copies.

The extra one labelled MT

There is a small bonus outside the 23 pairs: mitochondrial DNA. It lives in the cell’s mitochondria rather than the nucleus, it is tiny compared with the chromosomes, and it passes down the direct maternal line. In raw DNA files it is often listed under its own label, usually MT, so you may see rows like this:

rs2853490	MT	7028	CC

If you spot MT in the chromosome column, that is your mitochondrial DNA making a cameo alongside the numbered chromosomes.

Gene density varies

Not every chromosome is equally crowded. Gene density varies from one to the next - some pack genes tightly, others are relatively sparse - so a chromosome’s size does not neatly predict how many genes it holds. It is one of the quiet quirks that makes the genome more textured than a simple ranking by length.

How your raw file uses all this

Every variant in your raw file is stamped with the chromosome it sits on, which is what lets a tool organise hundreds of thousands of scattered rows into a coherent, chromosome-by-chromosome picture. To see exactly how those columns are laid out, walk through our anatomy of a raw DNA file. And to see your autosomes put to work estimating where your ancestry signals come from, our origins analysis reads across many positions and chromosomes at once.

All of it runs privately in your browser with on device DNA analysis, so your raw file never leaves your device.

This article is educational and is not medical advice.