UPDATE 3: A chromosome can be a hybrid chromosome. For example, you can have a chromosome that is part X and part Y. In this case either the X or the Y dominates. If the X dominates you have a female (a zX chromosome [an XzX chromosomal combination but still CALLED “an XY chromosomal combination”]). If the Y dominates you have a male (a zY chromosome [an XzY chromosomal combination but still CALLED “an XX chromosomal combination”]).