The Chazakah dictates that the husband of the mother is the father of the child.
A person's lineage regarding his Shevet is dependent on the father, not the mother.
A Revi'is of blood that is from two separate Mesim is Metamei b'Ohel.
If a Nazir becomes Tamei to a k'Zayis from a Mes or a k'Zayis Netzel from a Mes he must recount his Nezirus.
The same is true if he becomes Tamei to a spoonful of decomposition from a Mes or to the spine or skull or limb of a Mes, or a limb from a live person with sufficient flesh. (1)
Or if he becomes Tamei to a half of a Kav of bones or a half of a Log of blood. (2)
Or if he becomes Tamei by touching or carrying a bone the size of a barley. (3)
A person who is Tamei to a Mes must be sprinkled from the ashes of the Parah Adumah on the third and seventh day.
When a Nazir becomes Tamei to a Mes he must recount his Nezirus after he becomes Tahor.
A BIT MORE
1. Enough flesh that if the limb was still attached to the person the limb would heal.
2. The bones are only Metamei b'Ohel if the half of a Kav comes from the majority of bones in the body (125) or Rov Binyan (two thighs and a hip or two hips and a thigh) of the body.
3. All of the previously mentioned Tum'os are Metamei also b'Ohel, however bones that are more than the size of a barley and less than a half of a Kav are not Metamei b'Ohel.
If a Nazir becomes Tamei to a half of a Kav of bones or a half of a Log of blood he must recount his Nezirus. Tosfos asks that the Mishnah in Ohalos says that a quarter of a Kav of bones and a quarter of a Log of blood is Metamei b'Ohel. Tosfos answers that it is a Halachah l'Moshe mi'Sinai that a Nazir does not have to recount his Nezirus for Tum'as Ohel unless there is at least a half of a Kav of bones or a quarter of a Kav of blood even though less than that is needed to be Metamei b'Ohel.
There are certain Tum'os from a Mes that the Nazir does not have to shave for being Metamei to and he is not obligated to recount his Nezirus even though he is Tamei for seven days. The reason is because the Torah does not state "Tamei l'Nefesh" [which would imply to all Tum'os from a Mes]. Instead it states "v'Chi Yamus Mes Alav" which implies only Tum'os that are from the Atzmo Shel Mes. (Rambam Hilchos Nezirus 7:1)