DNA can provide information in different ways to help you identify your related family members and ancestors, depending on which part of the DNA molecule is used. Typically, four types of DNA are used to discover more about your ancestry:

atDNA
Autosomal DNA includes all the chromosomes (excluding the sex chromosomes). Autosomal DNA can help identify ancestors within the last 5–7 generations. However, because you only inherit half of each of your parents’ DNA, some of your ancestors “fall off” your genetic tree (meaning you did not inherit any of their DNA). You are also guaranteed to only share DNA with 2nd cousins or closer.

yDNA
The Y chromosome (one of the two paired sex chromosomes) is only carried by men and is passed from father to son. By comparing the Y chromosome of two males, you can determine whether they share a common patrilineal ancestor.

xDNA
The X chromosome (one of the two paired sex chromosomes) is carried by both males and females (females as “XX,” males as “XY”) but can only be passed down from a mother to her son or daughter. By comparing the X chromosome of two individuals, you may identify a common ancestor if that segment comes from an ancestor in your direct X path.

mtDNA
MtDNA, or mitochondrial DNA, is found in a cell’s mitochondria. Mitochondria are passed from a woman to all her children, regardless of whether they are male or female. By comparing the mitochondrial DNA of two individuals, you can determine whether they share a common matrilineal ancestor.

Haplogroup designations may be included in your test results for YDNA (father line) and mtDNA (mother line). Haplogroups are groupings of DNA similarities that reflect ancient migration patterns before genealogical records were created. YDNA haplogroups match with modern usage of surnames and can be helpful in solving problems on your surname line. mtDNA haplogroups reflect more variety and are loosely tied to your maternal ethnic origins.

Your family tree is what has been documented over time through family history records and often reflects relationships, some of which may not be biological. Your genetic tree consists of the relatives with whom you share DNA, which you mutually inherited from common ancestors within the last five to seven generations. Beyond five to seven generations, you may not share DNA with all your ancestors according to your family tree. Therefore, not all of your family tree ancestors will be revealed in your genetic tree from your DNA results.

DNA is inherited randomly, which means the DNA segments you inherited from your ancestors may be different than the DNA your relative inherited from the same ancestors. You are only guaranteed to share DNA with a second cousin or closer.

Yes, DNA can be used to solve family history problems. However, testing your DNA is just one step in a larger process. You will likely need a combination of DNA and traditional research to identify family history connections.

DNA test results may reveal unexpected information about your family history, such as adoptions, unexpected paternity, etc. The affected party may be yourself or a close DNA relative—perhaps someone you know nothing about. If this new information may be upsetting to you or to another family member, you may want to prepare family members in advance of possible results, or perhaps test at a later date. If you are not prepared to learn unexpected information, a DNA test may not be right for you at this time.

Multiple companies provide DNA testing, each offering different tests and resources to customers.

If you receive unexpected results, such as the identity of parents, grandparents, great-grandparents, or other previously unknown relatives, be sensitive and considerate of others involved who may be affected by the results. Approach family members with care and support as you discuss these previously unknown events.

Multiple community groups provide support and guidance on dealing with unexpected information:

NPE Gateway
DNA Secrets
Watershed DNA
Donor Conception Network
NPE Fellowship
ISOGG Unexpected Results wiki page

Most DNA companies will allow you to download your raw DNA results to preserve and potentially share with other companies. You can upload this raw data to some DNA companies for additional cousin matching. You will need to get instructions on how to do this from the company you tested with.

When your DNA test results are posted on a company website, you should have the option to share your data with everyone, with selected individuals, or to keep it private. Caution: Be aware that any time your results are stored by an organization, even if you opt not to share data, there is no 100% guarantee that someone will not be able to identify or use your data in another way. For some people, this is not an issue. For others, it is an important concern.