Ohno's law

Ohno's law is the principle that mammalian X chromosomes retain a broadly conserved set of genes across species, despite extensive change in their autosomes.

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Mammalian sex chromosomes: The X and Y chromosomes that determine sex in most mammals and carry genes with distinct evolutionary histories. Ohno's law describes a conserved feature of the mammalian X chromosome.
Susumu Ohno: A Japanese-American geneticist known for influential work on sex chromosomes, gene duplication, and genome evolution. He formulated the principle and developed its evolutionary interpretation.
Human X chromosome: One of the two human sex chromosomes, carrying genes involved in sex-linked traits and many other functions. It is a well-studied instance of the broadly conserved mammalian X.
X chromosome gene order: The linear arrangement of genes along an X chromosome. Gene order can change even when much of the chromosome's gene set remains conserved.
Muller's ratchet: The accumulation of harmful mutations in a nonrecombining population through repeated loss of the least-mutated class. It explains degeneration of nonrecombining chromosomes, rather than conserved X-linked gene content.
X chromosome inactivation: The transcriptional silencing of one X chromosome in most cells of female mammals. It helps balance X-linked gene dosage between XX females and XY males.
Evolution by Gene Duplication: A 1970 book by Susumu Ohno arguing that gene duplication supplies material for evolutionary innovation. Ohno presented his views on sex-chromosome evolution in this influential work.
Mouse X chromosome: The X chromosome of mice, a model system for mammalian genetics and development. Its gene content can be directly compared with the human X.
Evolutionary gene loss: The disappearance of functional genes from a lineage through mutation, deletion, or other evolutionary processes. Gene loss in particular lineages qualifies any claim of universal X-linked conservation.
Sex chromosome-autosome homology: Similarity between sex-chromosome regions and corresponding autosomal regions, often reflecting shared ancestry. It concerns chromosome ancestry and correspondence, not conservation of genes across species.