Properties of already existing instances depend on how i affect the prototype properties

Question

Could someone explain me why if I affect an object to a prototype, properties are not added to already existing instances of this object, while if I affect individually each property of the prototype, properties are added to all already defined instances. Example1 and example2 below illustrate my question (https://jsfiddle.net/tgyakpe0/5/). Result of the example is:

• example 1 My father is English I am English

• example 2 My father is English I am undefined

• example 3 My father is English and lawyer I am English and lawyer

• example 4 turnaround My father is English and lawyer I am English and lawyer

In example 4 I made a simple turnaround for this behaviour, but I wonder if it's a safe practice. Thank you.

/* example 1 */
        function person1(first, last, age, eyecolor) {
            this.firstName = first;
            this.lastName = last;
            this.age = age;
            this.eyeColor = eyecolor;
        }
        var me1=new person1("lolo","lala",35,"blue");
        person1.prototype.nationality="English";
        var myFather1 = new person1("John", "Doe", 50, "blue");
        document.getElementById("example1").innerHTML ="* example 1 <br>My father is " + myFather1.nationality+" <br>I am "+me1.nationality;
        /* example 2 */
        function person2(first, last, age, eyecolor) {
            this.firstName = first;
            this.lastName = last;
            this.age = age;
            this.eyeColor = eyecolor;
        }
        var me2=new person2("lolo","lala",35,"blue");
        person2.prototype={nationality:"English"};
        var myFather2 = new person2("John", "Doe", 50, "blue");
        document.getElementById("example2").innerHTML ="* example 2 <br>My father is " + myFather2.nationality+" <br>I am "+me2.nationality;
        /* example 3 */
        function person3(first, last, age, eyecolor) {
            this.firstName = first;
            this.lastName = last;
            this.age = age;
            this.eyeColor = eyecolor;
        }
        var me3=new person3("lolo","lala",35,"blue");
        person3.prototype.job="lawyer";
        person3.prototype.nationality="English";

        var myFather3 = new person3("John", "Doe", 50, "blue");
        document.getElementById("example3").innerHTML ="* example 3 <br>My father is " +myFather3.nationality+" and "+myFather3.job+" <br>I am "+me3.nationality+" and "+me3.job;
        /* example 4 */
        function proto(f,prot){
          for (k in prot) {
          f.prototype[k]=prot[k];
          }
        }
        function person4(first, last, age, eyecolor) {
            this.firstName = first;
            this.lastName = last;
            this.age = age;
            this.eyeColor = eyecolor;
        }
        var me4=new person4("lolo","lala",35,"blue");
        prototo={job:"lawyer",nationality:"English"};
        proto(person4,prototo);
        var myFather4 = new person4("John", "Doe", 50, "blue");
        document.getElementById("example4").innerHTML ="* example 4 <br>My father is " +myFather4.nationality+" and "+myFather4.job+" <br>I am "+me4.nationality+" and "+me4.job;