Is the Y chromosome ALWAYS passed down from father to son?

Question

Is it possible that 2 brothers that have the same biological but yet their Y chromosome does not match.

How are parternity tests performed and based on what?

Answer 1

Yes, the Y chromosome is only father to son. Some very minor changes can occur but it will still indicate being from the same line. A bug difference is a non-paternal event. That is, a different father. Females don’t have a Y chromosome so it has to come from the father.

Full paternity tests look a bit broader than just the Y chromosome. See https://en.m.wikipedia.org/wiki/DNA_paternity_testing for more info. 

Answer 2

It is certainly possible that the Y chromosome of a father and son aren't an exact match because a mutation occurred, but they'll still be extremely similar. The chances of so many mutations in one generation that father and son don't match at all is vanishingly small.

If two brothers do Y-DNA testing at FTDNA, and they don't show up in each other's matches, they don't have the same father.

Comments

If they have the same mother but different fathers, the brothers will share approximately 25% of their DNA. Full siblings share half.

---

Yep! To clarify, half brothers will show up in each other's autosomal matches. But if they don't share a father, they won't show up in each other's Y-DNA matches. (And if they share a father but not a mother, they'll show up in each other's Y-DNA matches but not in each others' mtDNA matches.) (And all this is assuming that their fathers or their mothers aren't related. Half brothers whose fathers have the same father or otherwise share a reasonably recent male-line ancestor will still share Y-DNA.)

Answer 3

Blame it on a mix-up at the lab.  It can happen.

Comments

Even in a male that has  XYY chromosome syndrome,  the two Y chromosomes are both from one father.... just a  "mistake" in meiosis which creates an extra Y ..... I.E.  they didn't get Y chromosomes from two different fathers.   (WikiPedia says about 1 in 1000 males have this syndrome.)

---

In my time following genetic genealogy forums, I've only seen ONE genuine lab mix-up. It's either NPE, sperm donor situation, or a hidden adoption.

While two brothers may have a small mutation in their YDNA, one will not be an R1b and the other G2 unless they have different fathers. It's not magic.

---

If it's a mix-up at the lab, a retest will show it.

---

Yep, that's right Sharon, and honestly (even if this makes me unpopular on Wikitree) people need to stop giving others false hope over stuff like this. I understand that discovering that you have a NPE in your line is upsetting. But that's something one needs to be prepared for before one chooses to do DNA testing.

There was a case on another genealogy forum, where a young man did a DNA test that showed 0% South Asian DNA. The problem here is that this young man's father was Indo-Fijan of Bengali descent. If absolutely ZERO South Asian DNA is showing up, there's a problem here. The other forum members started telling this young man not to worry, that his Native American DNA was "really" Bengali and there was some mix-up at the lab and all this. They think they're helping but all they're doing is delaying the inevitable! Sure enough, a month or so later the same young man came back with the follow-up to this tale. Further DNA testing revealed that his father was NOT his biological father, which was clear from the first test he did. My point is there's no sense in dragging something out and giving people false hope. If one cannot handle a DNA test result that is not what they hoped for, then they should not do DNA tests. Just live the rest of one's life in a happy fantasyland.