Note that it should be 'either of the two mammary glands' - if you say 'either of the two' it makes the subject plural. 'Which' is for objects. 'Who' is for people. Your sentence is actually an interesting one, because either 'who' or 'which' is possible, depending on what you want to say. If the pronoun is referring to the glands, it should be 'which' i.e. 'either of the two mammary glands ... which produce milk.' If the pronoun is referring to the women, it should be 'who' i.e. 'in women who produce milk' As for the pronunciation, for a native English speaker, the two words 'which' and 'witch' have exactly the same final consonant. It's a 'ch' sound as in 'church', which is composed of a 't' followed by 'sh'. But, if it helps you to think that there is a 't' sound in this word (to prevent you pronouncing the 'ch' as a 'sh', as it would be in French), then that's fine. English speakers don't deliberately place 't' before the 'sh' sound in this word, but it's OK if you imagine that it's there.

Who is correct in this sentence because you are talking about somebody a person. "whitch" is the correct pronunciation but not the correct spelling ..