What makes of a certain gender?

Hi!
This is sort of a serious issue to me…
I was in a discussion with my sister about what makes a person of a certain gender…
What makes you a woman?
What makes you a man?

Well!
Here is where i left off, and i was wondering what people think…

I was arguing that what makes a person of a gender is if they have all, or parcially at least, the “Sexual Organs” of that gender.
Since we are in the age of cosmetic surgery, the fake versions of those “parts” also count for me!

My sister was arguing that if you are born of a gender than you are of that gender, even if you no longer have the corresponding parts; if you make the change to another gender, than you are of that gender and not of the previous one…

The big thing was! What happens if a person doesn’t have those parts!
(I’m sorry about the incorrect naming, but english is my second language)

If a person doesn’t have male or female parts, than; according to my sister the gender they had when they where born prevails…
I said that it didn’t! That person was genderless!