Autosomal DNA is inherited equally from both parents. The amount of autosomal DNA inherited from more distant ancestors is randomly shuffled up in a process called recombination and the percentage of autosomal DNA coming from each ancestor is diluted with each new generation. [Source: ISOGG Wiki for Autosomal DNA]
The following screenshot from FamilyTreeDNA shows how much DNA that A shares with her top matches.
A's matches are sorted by relationship. Her closest relationship is to her sister, M.
Her next closest DNA matches are:
to my brother, S, sharing 1970.09 cM, with the longest shared block being 163.73 cM.
to me, Elizabeth, sharing 1837.98 cM, with the longest shared block being 101.44 cM.
to my brother, R, sharing 1805.56 cM, with the longest shared block being 108.96 cM.
The Shared cM Project, conducted by The Genetic Genealogist, Blaine Bettinger, shows the range of shared cM (centimorgans, a measurement implying genetic distance) that can be found between people of a known genealogical relationship. Hundreds of genealogists responded to his request for information and Blaine has generated some interesting charts to display the information.
In the case of aunt/uncle - niece/nephew relationship, the average amount of DNA reported to be shared is 1703.45 cM, with the greatest amount of DNA shared at 2226.60 cM and least amount of DNA shared at 121.34 cM. So it appears that my brothers and I share more than the average amount of DNA expected between an aunt and a niece or nephew.
The next step is to compare us in FamilyTreeDNA's chromosome browser.
|FamilyTreeDNA Chromosome Browser|
The 23 chromosomes are numbered and the X chromosome is at the bottom. The orange lines show where my brother S shares DNA with my aunt A. Blue is where I share DNA and green is where R shares DNA.
You can see on chromosome 1, on the right, where there is an orange bar and a blue bar: this is one location where S and Elizabeth (but not R) share DNA with A.
Look at chromosome 9 or 15 or 18 or 19 or X for locations where me and both my brothers share defined segments of DNA with A.
That largest segment that S matches A is found on almost the entire length of chromosome 10. The largest segment that R matches A is found on chromosome 16 and the largest segment that Elizabeth matches A is found on chromosome 12.
DNA recombines from generation to generation, but at this close a relationship, it's pretty easy to tell an aunt / niece or nephew relationship.
For other cousins who have tested at AncestryDNA or 23andMe, they can upload their DNA results to GEDMatch.com, a free online tool that has a chromosome browser so everyone doesn't have to test at all the companies to see their genetic relationships. If you're related, look for me there!
See Sisters' Ethnicity Results to see where my aunt's distant ancestors were from.