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Glossary of terms used on this site

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Term Definition
X-Chromosome inactivation

X-chromosome inactivation refers to the phenomenon in a female by which either the paternal or maternal X chromosome is inactivated randomly in each early embryonic cell, with subsequent inactivation of that same X chromosome in all cells descended from that cell.


X-linked diseases are single gene disorders that reflect the presence of defective genes on the X chromosome. This chromosome is present as two copies in females but only as one copy in males.

X-linked dominant

The inheritance pattern describing a dominant trait or condition caused by a mutation in a gene on the X chromosome. The condition is expressed in heterozygous females as well as males, who have only one X chromosome. Affected males tend to have more significant disease than affected females. Disorders inherited in this manner are relatively rare.

X-linked recessive

Used to describe a characteristic or condition in which a mutation in a gene on the X chromosome causes the condition in males.

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