Genetics Terms Starting With X
Genetics Glossary: X
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X Inactivation
/ eks in-ak-tih-VAY-shun / · English: X + Latin: inactivare (to make inactive)
X Inactivation is the epigenetic silencing of one X chromosome in each somatic cell of female mammals, equalizing X-linked gene dosage between XX females and XY males.
Inactivation is initiated early in embryonic development by the XIST non-coding RNA, which coats the inactive X chromosome and recruits repressive chromatin modifications including histone methylation and DNA methylation. The choice of which X to inactivate is random in each cell but is stably maintained through subsequent mitotic divisions, producing a mosaic of cells expressing maternal or paternal X-linked alleles. Skewed X-inactivation, in which one X is preferentially silenced in most cells, can cause female carriers of X-linked recessive conditions to show symptoms.
Calico cats are almost always female because their orange and black fur patches result from random X-inactivation of different color genes on each X chromosome. Male calico cats are extremely rare and usually carry an extra X chromosome, giving them an XXY genotype.
X inactivation does not silence all genes on the inactive X. About 15 to 25 percent of X-linked genes escape inactivation and are expressed from both X chromosomes in females.
In women who carry a hemophilia A mutation, skewed X-inactivation that preferentially silences the normal X chromosome in blood cells can produce mild bleeding symptoms even in heterozygous carriers. Studies have found that roughly 10 to 15 percent of female carriers show measurable factor VIII activity below the normal range of 50 to 150 percent, correlating with the degree of skewing toward the mutant X in hematopoietic cells.
X Linked
/ EKS-LINKT / · Letter X (named for its unknown nature when discovered); linked
X Linked is the inheritance category for traits or diseases caused by genes carried on the X chromosome, affecting males and females differently because males have only one X chromosome.
Because males carry only a single X chromosome, a recessive mutation on that X has no second copy to mask its effect, so males express X-linked recessive conditions far more often than females. Females carry two X chromosomes, so a working allele on the second X typically compensates for a faulty copy on the first, making heterozygous females carriers who usually show no symptoms. X-linked dominant conditions follow a different pattern, affecting females who carry just one mutant copy, though the same mutation is often more severe or lethal in males.
Duchenne muscular dystrophy, caused by mutations in the DMD gene on the X chromosome, affects approximately 1 in 3,500 male births worldwide and is one of the most common X-linked recessive diseases.
Queen Victoria carried a mutation causing hemophilia on her X chromosome and passed it to several of her descendants, including Alexei, the son of the last Russian Tsar, affecting royal families across Europe through her female carrier daughters.
X-linked conditions only affect males. Females can be affected by X-linked recessive conditions when they inherit a faulty allele from both parents, and X-linked dominant conditions affect females who carry just one faulty copy.
Red-green color blindness affects about 8 percent of males but only 0.5 percent of females because males need only one faulty OPN1MW or OPN1LW gene copy on their single X chromosome to be color blind. A female requires faulty alleles on both of her X chromosomes to express the same recessive phenotype, which occurs far less frequently given the allele frequencies in the population.
X-Linked Inheritance
/ eks linkt in-HAIR-ih-tents / · English: X-linked + inheritance
X-Linked Inheritance is the pattern of heredity exhibited by genes located on the X chromosome, resulting in different phenotypic expression between males and females because of their different sex chromosome constitutions.
X-linked recessive traits are expressed in males who carry a single copy of the recessive allele, while females require two copies and usually remain unaffected carriers if heterozygous. Dominant X-linked traits are expressed in both sexes but may be more severe in males. Pedigree analysis of X-linked traits characteristically shows no father-to-son transmission, because fathers pass their Y chromosome rather than their X to sons.
Thomas Hunt Morgan demonstrated X-linked inheritance in Drosophila melanogaster in 1910 when he showed that white eye color was transmitted exclusively through females to sons, providing the first direct evidence that a specific gene resides on a specific chromosome.
X-linked inheritance does not mean sex-limited expression. X-linked conditions affect both sexes but with different frequencies and severities because males have only one X while females have two.
Red-green color blindness affects about 8 percent of males but only 0.5 percent of females in populations of European descent, a disparity that reflects X-linked recessive inheritance. Because males carry a single X chromosome, one recessive allele at OPN1MW or OPN1LW is sufficient to produce the phenotype, while females must inherit the allele on both X chromosomes.
Autosomal Recessive Inheritance →