Hi Maryann,
The point is that the terms homozygous, heterozygous and hemizygous can only refer to genes, but, of course, red cells do not have a nucleus (it has been extruded during the maturation process), but, in any case, the terms should not be used for antigens.
In this case the closest to being correct is that M+N- red cells have "homozygous expression" and M+N+ red cells have "heterozygous expression". However, particularly in the case of the MNS Blood Group System, this terminology is not completely correct. There are many low prevalence antigens associated with both the glycophorin A and the glycophorin B molecules, and the many hybrids therein, that a red cell that groups as M+N- may not be as a result of MM homozygosity at a genetic level, but may have heterozygous expression, because there is one "M" antigen (for want of a better way of putting it), and one low prevalence antigen expressed on glycophorin A (or a hybrid), so that, in terms of expression, the M antigen has the strength of an M+N+ red cell, rather than an M+N- red cell.
I hope that helps.
Malcolm