| X | Although the letter X is used as a symbol in various ways (such as X-rays, the x-axis of a graph, etc.), today the x usually refers to the x chromosome. (12 Dec 1998) |
|---|---|
| X body | An obsolete term for Langerhans' granule. (05 Mar 2000) |
| X chromosome | <genetics> A sex chromosome. In mammals paired in females (XX), in amphibia paired in males. (11 May 1997) |
| X disease | One of several viral diseases of obscure aetiology. (05 Mar 2000) |
| X disease of cattle | bovine hyperkeratosis |
| x factor | <biochemistry> A precursor necessary to synthesise cytochromes. (09 Oct 1997) |
| X inactivation | <cell biology> The inactivation of one or other of each pair of X chromosomes to form the Barr body in female mammalian somatic cells. Thus tissues whose original zygote carried heterozygous X borne genes should have individual cells expressing one or other but not both of the X borne gene products. The inactivation is thought to occur early in development and leads to mosaicism of expression of such genes in the body. See: Lyon hypothesis. (18 Nov 1997) |
| X linked disease | <disease, genetics> A genetic or inherited disease whose controlling gene or at least part of the relevant genome is carried on an X chromosome, for example haemophilia. most known conditions are recessive and thus since males have only one X chromosome they will express any such recessive character. Few dominants are known and the homozygous states are very rare so that female expression of such diseases is uncommon. (18 Nov 1997) |
| x wave | The wave in the atrial or venous pulse curves produced when ventricular ejection moves the floors of the atria toward the ventricular apices. (05 Mar 2000) |
| X zone | A transient adrenocortical zone present in some rodents at birth, most notably in mice, situated between the zona reticularis and the adrenal medulla; it degenerates in males with the secretion at puberty and in females during their first pregnancy; it slowly enlarges in unmated females after puberty and does not degenerate until middle age; the X zone appears to secrete no hormone. Synonym: androgenic zone. Misnomer for the foetal adrenal cortex of primates. Synonym: foetal reticularis. (05 Mar 2000) |
| X-esotropia | Decreasing convergence from the primary position in both upward and downward gaze. (05 Mar 2000) |
| X-exotropia | Increasing divergence from primary position in both upward and downward gaze. (05 Mar 2000) |
| x-linked | On the X chromosome. Linked in genetics does not mean merely associated. An x-linked gene travels with the x chromosome and therefore is part of the x chromosome. (12 Dec 1998) |
| X-linked gene | A gene located on an X chromosome. (05 Mar 2000) |
| X-linked hypogammaglobulinaemia | A congenital, X-linked recessive, primary immunodeficiency characterised by decreased numbers (or absence) of circulating B-lymphocytes with corresponding decrease in immunoglobulins of the five classes; associated with marked susceptibility to infection by pyogenic bacteria (notably, pneumococci and Haemophilus influenzae) beginning after loss of maternal antibodies. X-linked hypogammaglobulinaemia with growth hormone deficiency, hypogammaglobulinaemia combined with a reduced number of B-cells; characterised by short stature, delayed puberty, and recurrent infections. (05 Mar 2000) |