Final answer:
The Lyon Hypothesis explains the random inactivation of one of the X chromosomes in female mammals, known as X inactivation, which is a form of dosage compensation to balance gene expression from the X chromosome.
Step-by-step explanation:
The Lyon Hypothesis is used to describe the process of random inactivation of the X chromosome in female mammals, which serves as a mechanism of dosage compensation to balance the dose of X chromosome gene expression. This explanation corresponds to option 1 in the question posed. In mammals, one of the two X chromosomes in females condenses into a dormant structure known as a Barr body, ensuring that females do not express a double genetic dose of the X chromosome.
The inactivation is random, meaning that in each cell, it can be either the maternally or paternally derived X chromosome that inactivates, and this pattern of inactivation is perpetuated in all cells derived from that original cell. This phenomenon explains how females, like tortoiseshell cats which are heterozygous for an X-linked coat color gene, can express different coat colors in different regions of their body.