Y-chromosome DNA (Y-DNA) testing is used to trace the patrilineal line from son to father to grandfather, from the present to ancient times. The test won't tell you any names, places or dates, but will tell you to what degree you are related to another who also did a test. It also indicates the ancient ethnic groups and homelands of a patriarch from thousands of years ago.
The Y chromosome is exclusively inherited from father to son(s). In addition, the family name (surname) is usually inherited so that every male in a particular family will also likely have the same surname.
NOTE: Testing of Y-DNA is only performed on parts of the Y-DNA strand in areas that do not code for genes, therefore no health or disease information can be learned from a Y-DNA test.
The purpose of Y-DNA testing is to compare one's test results with another person's results. In particular, if someone with the same surname has test results very similar to yours, then you and they are probably related within possibly a dozen generations (or even less).
If some man with a different surname tests very closely to you, then it is possible an ancestor had a son and that son was given or took a different surname. The other possibility is that the common ancestor was so far back in time, over a thousand years, that the sons of a distant common ancestor took on different surnames.
A father's and son's test results can be off by one or two points due to mutations in quick-changing parts of the Y chromosome.
Females can have male blood relatives do a Y-DNA test in their place. Any sons will have the Y-DNA of the husband's patrilineage.
Females and males can test mitochondrial DNA (mt-DNA). That type of DNA test traces the matrilineage and ancient anthropological history of mother to grandmother to great-grandmother, etc. Mt-DNA is inherited by both sons and daughters from their mothers, but only daughters can pass it down. Mt-DNA is a bit less useful than Y-DNA because females tend to change their surnames upon marriage, and MT-DNA tends to mutate less so there are fewer differences to compare people to each other.
Notes on the human Y chromosome:
BIRTH YEARS FOR MILLER PATRILINEAGE:
Notes on human mitochondrial DNA:
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