Having members of your family take the AncestryDNA® test can help you fill in information about your family that may not otherwise be available. For more information on AncestryDNA kits, see AncestryDNA® Resources.

Full siblings

Many people believe that siblings’ ancestral origins are identical because they share parents, but full siblings share only about half of their DNA with one another. Because of this, siblings’ ancestral origins can vary.

All the genes passed on to full siblings come from the same gene pool (that is, the genes of both parents), so each region passed on to children must be present in one or both parents as well. However, some siblings may inherit regions from their parents that others don’t, and it’s likely that each sibling will inherit different amounts of regions from one another. Children inherit 50 percent of their DNA from each parent, but unless they’re identical twins, they don’t inherit the same DNA as each other.

If your parents’ genes combined were a deck of 52 cards, you'd receive 26 of them. If you had a younger sibling, he or she would also receive 26 cards, and (due to probability) about half of that sibling's 26 cards (13 cards) would be the same cards you received. If a third sibling were born, that sibling would share about 13 cards with each of you as well (but not the same 13). This pattern would continue with each sibling born.

Testing your siblings is useful when one or more parent can’t be tested (to help fill in the parents' ancestral origins) and to learn which regions were passed down to the sibling.

Half-siblings

Half-siblings share 25 percent of their DNA. 50 percent of each half-sibling’s DNA comes from the shared parent, and they inherited about half of the same DNA from that parent as one another.

Testing a half-sibling can help you fill in that sibling’s non-shared parent’s ancestral origins and show you the regions inherited by that sibling.

Parents and children

Like siblings, parents and children share 50 percent of their DNA with one another. While the shared DNA between full siblings includes 25 percent of the mother’s DNA and 25 percent of the father’s DNA, the DNA shared between a parent and child is 50 percent of that parent’s DNA.

Having your own offspring tested will show which regions the child inherited and provide clues regarding the second parent’s ancestral origins and biological connections (if unknown).

Testing a parent can help you deduce which of your own regions and DNA matches come from which parent. When a parent is tested, a button appears on your DNA match list that enables you to filter by matches you share with that parent.

Aunts, uncles, nieces, and nephews

You share about 25 percent of your DNA with a biological aunt, uncle, niece, or nephew. Your aunt or uncle shares 50 percent of their DNA with your parent (their sibling), who shares 50 percent of their DNA with you. The reverse is true with a niece or nephew; you share 50 percent of your DNA with their parent, who shares 50 percent of their DNA with their child (your niece or nephew).

Testing an aunt or uncle can help you determine which side of the family some of your DNA matches are on. To see which matches you share, click on your aunt or uncle, then click the Shared Matches tab. An aunt or uncle’s DNA can also confirm more distant matches of yours (since they may be more closely related to the match) and provide a fuller picture of your grandparents' ancestral origins, if your grandparents haven’t been tested.

Testing a niece or nephew can also help you determine which side of your family your shared matches are on.

Grandparents and grandchildren

You share about 25 percent of your DNA with a grandparent or grandchild. Your grandparent shares 50 percent of their DNA with your parent, who shares 50 percent of their DNA with you; likewise, you share 50 percent of your DNA with your child, who shares 50 percent of their DNA with your grandchild.

Testing a grandparent can help you determine which side of the family some of your DNA matches are on (using the Shared Matches feature) and confirm distant matches, since they may be more closely related to distant matches than you are. A grandparent’s DNA can also provide a picture further back in history of the regions passed down to your family.

Testing a grandchild can help you determine which side of your family your shared matches are on and provide clues regarding the regions the grandchild received from your son- or daughter-in-law.