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My 23andme Related Cousins



Chromosome Mapping - comparing two or more persons' genome segments located on a particular chromosome, or any expression of the outcome of such a comparison.


In Relative Finder, when testing the autosomal chromosomes and the Y and the X (sex indicator) chromosomes, the marker being tested is a SNP.


Think about it this way, you get 22 single autosomal chromosomes from your Dad you get 22 single autosomal chromosomes from your mom. they pair up at conception. Each chromosome has a certain amount of base pairs, SNPs, ect; if they were not numbered or named the same names you wouldn't be able to compare them.
So a Chromosome has two parts -  each a separate unit that has SNP markers tested that at 23 and me are number  (in the case of chromosome 3) from 1 to 180,000,000. So part one from Dad 1-180,000,00 and part two from Mom 1-180,000,000. which makes chromosome 3, which is a pair. 


I guess since the word chromosome seems to indicate a single unit we forget it is actually a pair inside that unit.


The Number Chart


Autosomal Chromosome 

pair - one

Autosomal Chromosome

pair - two

Autosomal Chromosome

pair - three

Autosomal Chromosome

pair - four

Autosomal Chromosome

pair - five

Autosomal Chromosome

pair - six

Autosomal Chromosome

pair - seven

Autosomal Chromosome

pair - eight

Autosomal Chromosome

pair - nine

Autosomal Chromosome

pair - ten

Autosomal Chromosome

pair - eleven

Autosomal Chromosome

pair - twelve

Autosomal Chromosome

pair - thirteen

Autosomal Chromosome

pair - fourteen

Autosomal Chromosome

pair - fifteen

Autosomal Chromosome

pair - sixteen

Autosomal Chromosome

pair - seventeen

Autosomal Chromosome

pair - eighteen

Autosomal Chromosome

pair - nineteen

Autosomal Chromosome

pair - twenty

Autosomal Chromosome

pair - twenty-one

Autosomal Chromosome

pair - twenty-two

Sex chromosome

Chromosome X

End of chromosome numbering


The size of the chromosome generally refers to the number of bases, and the chromosomes are numbered roughly in order of size, with one being the biggest. You may also see the unit Mb (megabases or million bases), where 118 Mb = 118,000,000 bases. The chromosomes do "hold their sizes", except for some messy details like Copy Number Variations (insertions and deletions of chunks of DNA) and the fact that we don't have a completely stable reference sequence for the whole genome. But for the purposes of Relative Finder / Family Finder, we can take the chromosomes to be constant in size. To find the size that 23andMe uses for chromosomes, you can go to Family Inheritance / Advanced and compare yourself to yourself. 

The cM unit measures the tendency for recombination, which is more important than the number of bases for relationship testing. If you match someone in a region that recombines a lot, that's more impressive than if you match someone where the chromosome seldom splits during recombination. 

The number of SNPs depends on how many variant positions have been discovered and how many of those are put on the chip. Some regions have more known SNPs (the HLA region on chromosome 6 is an example), but the cM unit is still the most important. At 23andMe, Version 2 covers about 550,000 SNPs; Version 3 covers closer to 1 million SNPs. Family Tree DNA's new Illumina chip includes a little over 700,000 SNPs in the download file for the autosomes (chromosomes 1-22). 

Ann Turner



CeCe wrote a blog post about investigating your small matches (~5 cMs) on 23andMe, using tools like HIR Search. If interested, go here: 



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