X-chromosome inactivation in the context of "Mary F. Lyon"

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⭐ Core Definition: X-chromosome inactivation

X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome (see dosage compensation).

The choice of which X chromosome will be inactivated in a particular embryonic cell is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism (its cell line). The result is that the choice of inactivated X chromosome in all the cells of the organism is a random distribution, often with about half the cells having the paternal X chromosome inactivated and half with an inactivated maternal X chromosome; but commonly, X-inactivation is unevenly distributed across the cell lines within one organism (skewed X-inactivation).

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X-chromosome inactivation in the context of DNA methylation

DNA methylation is a biological process by which methyl groups are added to the DNA molecule. Methylation can change the activity of a DNA segment without changing the sequence. When located in a gene promoter, DNA methylation typically acts to repress gene transcription. In mammals, DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis.

As of 2016, two nucleobases have been found on which natural, enzymatic DNA methylation takes place: adenine and cytosine. The modified bases are N-methyladenine, 5-methylcytosine and N-methylcytosine.

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X-chromosome inactivation in the context of X-linked recessive

X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily hemizygous for the gene mutation because they have one X and one Y chromosome) and in females who are homozygous for the gene mutation (see zygosity). Females with one copy of the mutated gene are carriers.

X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. Females have two X chromosomes while males have one X and one Y chromosome. Expression of X-linked conditions in female carriers can vary greatly due to random X-chromosome inactivation (Lyonization) within each cell. Differences in X-chromosome inactivation (known as skewed X-inactivation) occurs due to some cells expressing one X allele and some the other. Decades of research has led to the consensus that conditions be considered X-linked and that the "dominant" and "recessive" designations be retired.

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LINKED FROM (PARENTS)
← DNA methylation ← X-linked recessive
LINKS TO (CHILDREN)
Tortoiseshell cat → Calico cat → Alleles → Chromosome → Mitosis → Fluorescence in situ hybridization → DAPI → Histone → Confocal microscopy → Xist → Mary F. Lyon → X chromosome → Theria → Mammal → Heterochromatin → Gene product → Dosage compensation → Eutheria → Skewed X-inactivation →