X-linked

(adjective)

Associated with the X chromosome.

Related Terms

Examples of X-linked in the following topics:

  • Sex-Linked Traits

    • When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked.
    • Eye color in Drosophila was one of the first X-linked traits to be identified, and Thomas Hunt Morgan mapped this trait to the X chromosome in 1910.
    • Males are said to be hemizygous, because they have only one allele for any X-linked characteristic.
    • When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected.
    • Eye color in Drosophila is an example of a X-linked trait

  • Chromosomal Theory of Inheritance

    • At that time, he already knew that X and Y have to do with gender.
    • He was able to conclude that the gene for eye color was on the X chromosome.
    • This trait was thus determined to be X-linked and was the first X-linked trait to be identified.
    • Males are said to be hemizygous, in that they have only one allele for any X-linked characteristic.
    • In Drosophila, the gene for eye color is located on the X chromosome.

  • Genetic Linkage and Distances

    • Linked genes can become unlinked during recombination; the probability of genes separating depends on their distance from each other.
    • Morgan identified a 1:1 ratio between a segregating trait and the X chromosome, suggesting that the random segregation of chromosomes was the physical basis of Mendel's model.
    • This also demonstrated that linked genes disrupt Mendel's predicted outcomes.
    • The fact that each chromosome can carry many linked genes explains how individuals can have many more traits than they have chromosomes.
    • During meiosis, linked genes somehow became unlinked.

  • Human Sex-Linked Disorders

  • History of DNA Research

    • In 1919, Phoebus Levene identified the components of DNA (the four bases, the 5-carbon sugar, and the phosphate chain) and showed that the they were linked in the order phosphate-sugar-base.
    • He called each of these units a nucleotide and suggested the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups, which are the 'backbone' of the molecule.
    • In 1937, William Astbury produced the first X-ray diffraction patterns from DNA.
    • At King's College Maurice Wilkins and Rosalind Franklin examined X-ray diffraction patterns of DNA fibers.
    • In 1948, Linus Pauling discovered that many proteins included helical shapes from X-ray patterns and from attempts to physically model the structures.

  • Neurodevelopmental Disorders: Autism and ADHD

    • ., Fragile X syndrome) also have intellectual disability.
    • Except for some well-characterized, clearly-genetic forms of autism (e.g., Fragile X and Rett Syndrome), the causes of ASD are largely unknown.
    • There has been some unsubstantiated controversy linking vaccinations and autism.
    • In the 1990s, a research paper linked autism to a common vaccine given to children.
    • There are several candidate genes that may contribute to the disorder, but no definitive links have been discovered.

  • X-Inactivation

    • The presence of extra X chromosomes in a cell is compensated for by X-inactivation in which all but one X chromosome are silenced.
    • Females possess two X chromosomes, while males have only one; therefore, if both X chromosomes remained active in the female, they would produce twice as much product from the genes on the X chromosomes as males.
    • So how does X-inactivation help alleviate the effects of extra X chromosomes?
    • An individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each of her cells.
    • Individuals with three X chromosomes, called triplo-X, are phenotypically female, but express developmental delays and reduced fertility.

  • Physical Maps and Integration with Genetic Maps

    • Radiation hybrid mapping uses radiation, such as x-rays, to break the DNA into fragments.
    • An EST is a short STS that is identified with cDNA libraries, while SSLPs are obtained from known genetic markers and provide a link between genetic maps and physical maps.

  • Epistasis

    • A cross between heterozygotes for both genes (AaCc x AaCc) would generate offspring with a phenotypic ratio of 9 agouti:3 solid color:4 albino .
    • That is, every possible genotype other than aabb results in triangular seeds; a cross between heterozygotes for both genes (AaBb x AaBb) would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.
    • Note that we are assuming the interacting genes are not linked; they are still assorting independently into gametes.

  • Types of Receptors

    • There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.
    • G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein.
    • Cell signaling using G-protein-linked receptors occurs as a cyclic series of events.
    • Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme.
    • An example of this type of enzyme-linked receptor is the tyrosine kinase receptor.