A couple years ago, I asked a 7th cousin to test his YDNA to 37 markers.
The Y chromosome is passed from father to son without combining with the mother's DNA. This means that my Y chromosome is identical to my father's Y chromosome, whose Y chromosome is identical to his father's Y chromosome, and so forth, back through the generations, through millions of years, back to the beginning of the Y chromosome and sexual reproduction.
Of course, this isn't strictly true. While DNA copies itself with amazingly high fidelity, small copying errors do sneak in over time. Sometime a small part of the DNA copies incorrectly. Sometimes a small part of the DNA copies correctly but then instead of copying the next section of DNA, it repeats the copying the previous section, like a record skipping back.
Those parts of the DNA which repeat are prone to repeat again, sometimes resulting in dozens of repeats. When this happens in wrong part of the DNA, serious problems can occur but it often occurs in non-critical areas, without causing serious problems.
All human males have several locations in their Y chromosome prone to repeat (short tandem repeats) again and again. Since these repeats are inherited by their sons, over many generations, some men have more or fewer repeats than other men at these locations (STRs). By studying how many repeats are in each STR, we can determine how closely these men are related. Since these repeats happen every few generations, if you have an identical number of repeats at each STR then you are related within a very few generations.
New repeats aren't regular, however; they are random. You may get multiple additional new repeats in a generation or none for several generations. So if you have a different number of repeats at may STRs, your common male ancestor might still be recent but he may have also lived hundreds or even thousands of years ago.
There are hundreds of STRs on the Y chromosome. The simplest test looks at only 12 STRs; those most likely to have repeats. However, this really doesn't tell you much; it's simply too few STRs. A more detailed test looks at 37 STRs. If you match on that many, you are likely to be a very close relative. However, several differences does not mean you are not closely related. Some of those STRs change repeats very quickly, adding or dropping a repeat.
My 7th cousin, whose surname is Simpson, whose paper trail points to William Simpson, who died in Delaware in 1757, as our common ancestor. After testing his DNA on 37 STRs, he had a difference in repeats on 3 STRs. This, along with the paper trail and the common surname, pretty much proved that we were 7th cousins. However, the DNA evidence alone was not overwhelming.
We recently tested my cousin's at 67 STRs. When compared to mine, he matched on all additional 30 STRs, meaning we still only differ on 3 STRs. However, that difference is now out of 67 STRs, not 37. This is much stronger evidence that we're indeed 7th cousins. I suspect if we upgraded to 111, he would still only differ on 3 or 4 STRs.
So if you are YDNA testing, it pays to test at least 67 STRs and perhaps 111 might even be better. If you don't, you risk missing relationships that you can't see with the less detailed test. If my cousin didn't know his genealogy or an ancestor had changed surnames then we never would have recognized the relationship.
I have several genealogical brick walls that could be broken down with just a record or two. Sadly, it almost appears as if some people were deliberately trying to hide them on me!
Even when you can find them, censuses are not the most accurate genealogical source in the world: names are badly misspelled, ages are wrong by several years, and birthplaces are inaccurate. These errors are usually not the census takers fault. When faced with a difficult name, a heavy accent, and an illiterate family who has no idea how their name is spelled then you get what you get. Also, people lie about their ages or other information, such as where their parents were born; sometimes because they simply have no idea and simply guess.
However, sometimes, the census taker were simply muddled. Or, worse, the family wasn't home and they asked the neighbours,
A while back, I posted about my efforts to prove Sadie Simmons father. Sadie's great granddaughter, Linda, had a strong DNA match with an English family, whose roots went back past 1800 in London. This family was Jewish, as was Sadie, Linda's only known Jewish ancestor. Sadie was born in Maryland and reported multiple times that both her parents were English. Lo and behold, this family had Simmons ancestors and one of them had a son or sons who were in Maryland. So far, so good.
However, while I knew Abraham Simmons was in Maryland for many years, including the time Sadie was born, he had a large family recorded on several censuses and Sadie was not there. In fact, there was no record of Sadie at all prior to the 1900 census, when she was married and in New York. And there was no record of why she converted away from Judaism to Christianity.
My theory was that Abraham was not her father but Aaron, who also migrated to America. Aaron appears on a ship's' passenger manifest to New York in 1859 and then appears on the 1870 census in West Virginia but I had no record of him in Maryland. I guessed he was in Maryland because his brother was there but that was pure conjecture.
This 1860 census in Maryland has everyone we would hope to see:
Census records almost always, 99.9% of the time, list the nuclear family from oldest to youngest, with any household inhabitants, including extended family, at the end. So the order is as we would predict, in accordance to their actual ages, but not the recorded ages.
In fact, Abraham was probably 25, Rachel was 27, Sarah was 1, and Aaron was 20. The name was also Simmons, not Simons. This name is correctly spelled on pretty much every record, probably because the Simmons were literate and could provide the correct spelling. Aaron's name is also misspelled.
My guess is that the census taker collected the information from neighbors and botched things badly. However, this is certainly them, as the address aligns with city tax records for Abraham.
So Aaron was in Maryland. He could have still been Sadie's father. However, Abraham also had a daughter named Sarah. While she is the wrong age, and Sadie is very consistent about her age, people have lied about their age before. Since I can't find any record of Sadie of the correct age, perhaps Sadie did lie about her age.
Or perhaps John Simmons is really her father. Abraham and Aaron did have a brother, John. And a John Simmons of the correct age, cigar maker, did migrate to America from London about the right time. Is there another Simmons in the mix?
My hunt for records continues!