Comma before a relative pronoun?

It depends what type of relative clause it is - defining or non-defining.

- A defining relative clause tells you which person/thing etc I'm talking about, and no comma is used : I have two brothers and a sister. One of my brothers lives in France and one in the States. The one who lives in France is a famous neurosurgeon. - the relative clause specifies which of the two brothers the new information refers to.

- A non-defining relative clause just gives you an extra piece of information about the person/thing (etc) : I have a brother and a sister. My brother, who is a famous neurosurgeon, lives in the States. here, commas are used.

Your example is almost certainly non-defining - the speaker is just giving us information about the deer - and so the comma could be used. Compare :

Synthetic musk oil would help conserve a certain species of deer, whose gland is the source of musk. (Non-defining)
Synthetic musk oil would help conserve the species of deer whose gland is the source of musk, but would do nothing to help the other species at risk. (Defining)

Thank you.I got it 🙂