Answer to Question #68276 in Cell Biology for Del Eaton
This is because female mammals have an additional system for differential allele expression - ChrX inactivation. In a tissue, one ChrX is randomly inactivated by heterochromatisation. Therefore, most of the gene on one ChrX are off. That is why, some cells in a tissue will express the maternal allele, while others the parental allele.
Dear Del Eaton, if you have additional questions you can place new question or submit it as assignment.
What about alleles on genes other than those on ChrX? Do the small differences between paternal and maternal alleles that code for the same protein require the cell mechanism to select which of the two alleles the mRNA comes from?