What determines whether the maternal or paternal allele is expressed? I am taking cell biology and have this question: During the process of gene expression, it is possible to express either the maternal allele or the paternal allele. When and how is the determination made to express one allele over the other?

Except in the case of X inactivation in females, and genes on the Y chromosome in males, generally speaking _both_ alleles are expressed in cells.

A good example is the ABO blood type marker system. If the mother is type AA, she will pass on an _A_ allele, which codes for the A antigen. If the father is type BB, he will pass on a _B_ allele, coding for the B antigen. Therefore, any children will be type AB, as the maternal chromosome 9 will express the A form, and the paternal chromosome 9 will express B.