Confused with JavaScript prototype

They are two simiar code below:
1.
function User(name) {
this.name = name;
}
User.prototype = { 'sex': 'man' };
var user = new User('Zhang');
alert(User.prototype['sex']);   // man
alert(user.constructor.prototype['sex']);   // undefined
alert(user.constructor.prototype.constructor === Object);   // true My problem:
I'm wondering why the first piece code  「user.co nstructor.prototype［'sex'］」return「undefined」、I think it would be「man」.
Can somebodl tell me?
--
Thanks for your comment Sky.
In the first implement、User.prototype.com.nstructor is modified to Object、not User itself.The refore user.co nstructor is equal to User.prototype、they ar all Object.Wecan solitype protors protrappect.