X-linked disorders affecting only boys.

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 X-linked disorders,  so severe that they kill male fetuses and seriously affect only the surviving females, are rather rare.  The typical X-linked hereditary disease clinically affects boys and affects girls much more mildly  -to such a point that the disorder goes completely unnoticed.   There are many such disorders.   One of the saddest of these is Lesch-Nyhan syndrome.  A single recessive mutant gene causes a disorder of purine metabolism resulting in a severe mental deficiency and self-mutilation in the male infant.  The child chews his own lips and tears away at his own body tissues (eyes, fingers, legs) -probably as a reaction to the extreme pain experienced in those tissues. Life expectancy is only a few years,  due to kidney failure.   Hunter syndrome is another X-linked metabolic disorder which affects only boys. There is a deficit of a single enzyme necessary for the catalysis (breakdown) of complex carbohydrates causing accumulation of polysaccharides. These boys are also mentally deficient and have major deformities of the body (the general appearance has been termed gargoylism), cardiopathy, deafness, and low life expectancy (10-22 years).   Fragile-X syndrome,  of the hereditary type, is not directly caused by the mutant gene located on the X chromosome,  but by the effect of this gene on the integrity of the X chromosome.  It is relatively common (1/1700 births) making it the major known cause of hereditary mental deficiency.  The break-off of one long arm often results in a syndrome comprising many of the symptoms of autism.   The infant fails to socialize,  does not acquire language,  is often mentally deficient,  and manifests the usual bizarre behaviors of autism including neophobia (intolerance of anything new or surprising or upsetting),  self-stimulation (finger fanning, rocking, head banging),  strange movement disorders such as toe walking,  and so forth.   In addition,  the fragile-X variant of autism presents with a few markers that help the clinician pose a provisional diagnosis:   macroorchidism (a large genital apparatus) and a long face with large ears.  More subtle markers include hyperextensible finger joints,  arched palate, pectus excavatum (flat chest), flat feet, prolapse of the mitral valve (a cardiac defect), and dental malocclusion (misalignment of the teeth).    I have stated elsewhere in this book that the male sex is more at risk for mental deficiency,  and I have proposed that this seems explainable by X-linked brain disorders.    Indeed,  in addition to the syndromes I have just mentioned,  other X-linked brain disorders causing mental deficiency include the syndromes of Allan,  Atkin, Davis, FitzSimmons, Garéis, Golabi, Holmes, Juberg, Rénier, Lujan, Renpenning, Schimke, Seemanova,  Vasquez...   and there are many more.   The reader can look these up in any good medical dictionary.