He or She in Unknown Gender
For years, if the gender of an individual referred to in a sentence is unknown,
 “he” would be used as the generic pronoun.

“We don’t know who started the fire,” a police officer might say, 
“but he will be held responsible.”
It is understood, by both the police officer and any listeners, 
that “he” could refer to either a woman or a man.

However, as culture changes, so does the language along with it, 
and many believe that the exclusive use of “he” 
for a person of unknown gender is sexist. 
There are a few options in this situation.

An archaic way of dealing with the issue is to use “one,” 
as in “One never knows what one can expect.”
Using this pronoun is often clunky 
and results in some strange-sounding sentences.

“He or she” can be used in moderation, but it cannot be used too many times at once: 
“he or she knows that if he or she needs to talk, he or she can visit his or her professor.”
Some use “they,” but this word cannot be used with a singular antecedent—
it is only used with plurals.
Sometimes rewriting a sentence may help, but unfortunately you will at some point 
be forced to make a choice between sexist, clunky, or technically incorrect!


Read more at 

http://grammar.yourdictionary.com/style-and-usage/grammar-rules-for-he-she-usage.html#cH2Vqc3vUkUIidSv.99