Two Copies of the Gene a Exist in a Diploid

Multiple Choice

Two copies of the gene A exist in a diploid mammal. However, only one copy is expressed in every somatic cell. Different somatic cells in the body of each organism have inactivated one or the other allele in a seemingly random fashion, but when they divide, the daughter cells inherit the choice of the inactive allele faithfully. This is an example of …

A) X-inactivation
B) Genomic imprinting
C) Loss of heterozygosity
D) Monoallelic gene expression
E) Alternative gene splicing