Final answer:
The phenomenon where only one allele from either the maternal or paternal side is expressed, while the other is inactive, is known as X inactivation. It is a process of dosage compensation observed early in the development of female mammals.
Step-by-step explanation:
Phenomenon of Inactive Alleles
The phenomenon where only one allele is active during genomic imprinting, even though offspring receive two alleles (one from each parent), is known as X inactivation. This is particularly evident in female mammals where one of the two X chromosomes condenses into a structure known as a Barr body. Early in development, each cell randomly inactivates one of its X chromosomes, and all descendant cells from that original cell inherit the same inactivation. This ensures dosage compensation between males (who have one X chromosome) and females (who have two X chromosomes). An example of this process is seen in tortoiseshell cats, where color variation in the coat illustrates which X chromosome is inactive in each region of cells.