Autosomal DNA refers to the DNA in the "autosome" chromosomes, the non-sex chromosomes. Humans have 2 copies of 23 chromosomes (1 from the mother and 1 from the father) and one chromosome determines a person's sex - 2 Xs for female and 1 X and 1 Y for male. The other 22 are the autosome chromosomes.
The DNA from each parent's 2 copies of the 22 chromosomes is randomly combined to create 1 new set of 22 chromosomes. And each parent passes his and her new set of the 22 autosome chromosomes to a child. So a child receives 1 copy of each autosome chromosome from his or her mother and 1 copy from his or her father.
The DNA in these autosome chromosomes is passed unchanged from both mother and father to the child and so 1/2 of a child's DNA should be the same as 1/2 of the mother's DNA and 1/2 of a child's DNA should be the same as 1/2 of the father's DNA.
Since the selection of the father's and mother's DNA for a child's 22 chromosomes is a random process siblings will receive some DNA from each parent that is the same and some that is different. So on average siblings should share about 50% of DNA from their parents.
We know that the DNA from the autosome chromosomes is passed unchanged from both mother and father to each of their children but from scientific studies we also know that there are occasional mutations or changes in the DNA when it is passed from a parent to a child.
If a child receives the mother's or father's DNA with a small mutation or change then the child will pass on that mutated or changed Autosomal DNA to his or her children who will pass on that mutated or changed Autosomal DNA to the next generation.
So when mistakes occur and some sections of the DNA are passed from parent to child with changes or mutations genealogists can use each of these changes or mutations as new branches of a DNA family tree that can be used to help understand the relationships among ancestors and descendants over time.
Scientists know which sections of the Autosomal DNA have changed or mutated most often over the years and so the Autosomal DNA Test uses these most changed or mutated DNA sections on the 22 autosome chromosomes.
Today the major Autosomal DNA testing companies evaluate a person's DNA at approximately 700,000 to 800,000 different locations and they determine what nucleotide or base pair is at each location.
The 4 nucleotides or base pairs are adenine (A), thymine (T), guanine (G) and cytosine (C). For more information about these nucleotides see our discussion Use DNA Testing to Determine Your Ancestry.
The results of the DNA testing at the different locations are usually reported as A, T, G or C to represent each of the 4 nucleotides or base pairs. Then these detected base pair results can be used for DNA comparisons.
Most testing companies will compare a person's DNA test results to the DNA test results of all the previously-tested people in the company's database. In general the more DNA matches 2 people have the closer they are related as "genetic cousins".
The testing company will usually provide a "tested individual" with a list of his or her DNA matches with individuals in its database. Based on the amount of Shared DNA or DNA Matches the individuals from the database might be listed under categories like "Immediate Family", "Close Family", "2nd Cousin", "3rd Cousin", "4th Cousin" and "Distant Cousin". The basis for including people in these categories will usually be explained by the testing company.
Most testing companies will also provide information so that you can contact any of your matches or "genetic cousins". With communication by email and/or notes tested individuals can try to determine their actual relationship with any of their "genetic cousins".
Most testing companies will also compare a person's DNA test results to the DNA of a "Reference Group" to estimate a person's ethnicity. Each company's "Reference Group" contains what the company believes to be good estimations of all the major ethnicity groups - perhaps 150 or more groups. Based on DNA matches the company will estimate a person's ethnicity.
An individual's ethnicity estimate might be something like this: