gistfile1.js

var Book = Backbone.Model.extend();
var book = new Book();
book instanceof Book // true
book instanceof Backbone.Model // true

var Relation = function () {};
    Relation.extend = Backbone.Model.extend

var RelatedBook = Relation.extend(Book.prototype);
var relatedBook = new RelatedBook();

relatedBook instanceof RelatedBook // true
relatedBook instanceof Relation // true
relatedBook instanceof Book // true
relatedBook instanceof Backbone.Model // false??