Another side point not mentioned here otherwise: genomic imprinting.

As a general rule, genes are expressed whether they came from the mother or father. Understanding just that general rule will give one a good grasp of the principles. But bill often has exceptions. There are a few genes which are "imprinted" in sperm or eggs and are not equally expressed during development. The imprinting tends to favor paternal gene copy expression and typically involves epigenetic modification of the chromosome structure. The sequence of the nucleotides in the genes doesn't change, but the DNA itself may be chemically modified, e.g. with methyl groups added to cytosines. Those modifications recruit proteins which tend to suppress gene expression and maintain the methylation patterns over time.