Final answer:
In mammals, each inactivated X chromosome forms a darkly staining body known as a Barr body, which is a result of X inactivation, compensating for the double genetic dose of the X chromosome in females.
Step-by-step explanation:
In mammals, an inactivated X chromosome condenses into a dormant structure that can be observed as a darkly staining body within the nucleus of a cell. This structure is known as a Barr body, named after its discoverer, Murray Barr. During embryonic development in female mammals, the process of X inactivation occurs to compensate for the females possessing two X chromosomes. It ensures that females have the same effective dose of X chromosome genetic information as males, who only have one X chromosome.
One X chromosome in each embryonic cell is randomly chosen to be inactivated and condenses into a Barr body. Once an X chromosome is inactivated in a particular cell, all the descendant cells from that progenitor will maintain the same inactive X chromosome. In certain cats, known as "tortoiseshell" cats, this phenomenon can be visibly observed as patches of different coat colors, which correspond to the inactivated X chromosome carrying the gene for that color.