X-linked recessive inheritance

X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males, who are necessarily homozygous,  for the gene mutation because they have one X and one Y chromosome. and in females who are homozygous for the gene mutation.



Females with one copy of the mutated gene are carriers.



X-linked inheritance means that the gene causing the trait or the disorder is located on the X chromosome. 



Female carriers who have only one copy of the mutation do not usually express the phenotype, although differences in X-chromosome inactivation can lead to varying degrees of clinical expression in carrier females, since some cells will express one X allele and some will express the other. 



Inheritance of X-linked recessive traits follows a pattern of three points: affected fathers cannot pass x-linked recessive traits to their sons because fathers give Y chromosomes to their sons; x-linked recessive traits are more commonly expressed in males than females; x-linked recessive traits tend to skip generations, meaning an affected grandfather will not have an affected son, but could have an affected grandson through his daughter.



All daughters of an affected man will obtain his mutated X, and will then be either carriers or affected themselves depending on the mother. 



The resulting sons will either have a 50% chance of being affected if the mother is a carrier, or 100% chance of mother is affected.



Common X-linked recessive disorders are:



Red–green color blindness (Between seven and ten percent of men and 0.49% to 1% of women are affected by red-green color blindness).



Hemophilia A, a blood clotting disorder caused by a mutation of the Factor VIII gene. leading to a deficiency of Factor VIII. 



Hemophilia B, also known as Christmas disease, a blood clotting disorder caused by a mutation of the Factor IX gene and leading to a deficiency of Factor IX. 



Hemophilia B is rarer than hemophilia A. 



Duchenne muscular dystrophy: associated with mutations in the dystrophin gene, and characterized by rapid progression of muscle degeneration, eventually leading to loss of skeletal muscle control, respiratory failure, and death.



Becker’s muscular dystrophy: a milder form of Duchenne, which causes slowly progressive muscle weakness of the legs and pelvis.



X-linked ichthyosis: an inheredited deficiency of the steroid sulfatase (STS) enzyme. 



X-linked agammaglobulinemia (XLA), affects the body’s ability to fight infection not being able to generate mature B cells.



Glucose-6-phosphate dehydrogenase deficiency, which causes nonimmune hemolytic anemia in response to a number of causes, most commonly infection or exposure to certain medications, chemicals, or foods. 



Adrenoleukodystrophy; leads to progressive brain damage, failure of the adrenal glands and eventually death.



Alport syndrome; glomerulonephritis, endstage kidney disease, and hearing loss. 



Androgen insensitivity syndrome; variable degrees of undervirilization and/or infertility in XY persons of either sex.



Barth syndrome; metabolism distortion, delayed motor skills, stamina deficiency, hypotonia, chronic fatigue, delayed growth, cardiomyopathy, and compromised immune system.



Blue cone monochromacy; low vision acuity, color blindness, photophobia, infantile nystagmus.



Centronuclear myopathy; cell nuclei are abnormally located in skeletal muscle cells. 



Charcot–Marie–Tooth disease characterized by loss of muscle tissue and touch sensation, predominantly in the feet and legs but also in the hands and arms in the advanced stages of disease.



Coffin–Lowry syndrome; severe mental retardation sometimes associated with abnormalities of growth, cardiac abnormalities, kyphoscoliosis as well as auditory and visual abnormalities.



Fabry disease; A lysosomal storage disease causing anhidrosis, fatigue, angiokeratomas, burning extremity pain and ocular involvement.



Hunter syndrome; potentially causing hearing loss, thickening of the heart valves leading to a decline in cardiac function, obstructive airway disease, sleep apnea, and enlargement of the liver and spleen.



Hypohidrotic ectodermal dysplasia, presenting with hypohidrosis, hypotrichosis, hypodontia


Kabuki syndrome (the KDM6A variant); multiple congenital anomalies and mental retardation.



Spinal and bulbar muscular atrophy; muscle cramps and progressive weakness



Lesch–Nyhan syndrome; neurologic dysfunction, cognitive and behavioral disturbances including self-mutilation, and uric acid overproduction.



Lowe syndrome; hydrophthalmia, cataracts, intellectual disabilities, aminoaciduria, reduced renal ammonia production and vitamin D-resistant rickets.



Menkes disease; sparse and coarse hair, growth failure, and deterioration of the nervous system



Nasodigitoacoustic syndrome; misshaped nose, brachydactyly of the distal phalanges, sensorineural deafness



Nonsyndromic deafness; hearing loss



Norrie disease; cataracts, leukocoria along with other developmental issues in the eye



Occipital horn syndrome; deformations in the skeleton



Ocular albinism



Ornithine transcarbamylase deficiency; developmental delay and mental retardation, liver damage, skin lesions, and brittle hair may be seen



Siderius X-linked mental retardation syndrome, with cleft lip and palate and facial dysmorphism, caused by mutations in the histone demethylase PHF8



Simpson–Golabi–Behmel syndrome; coarse faces with protruding jaw and tongue, widened nasal bridge, and upturned nasal tip



Spinal muscular atrophy caused by UBE1 gene mutation; weakness due to loss of the motor neurons of the spinal cord and brainstem



Wiskott–Aldrich syndrome; eczema, thrombocytopenia, immune deficiency, and bloody diarrhea.



X-linked severe combined immunodeficiency (SCID); infections, usually causing death in the first years of life.



X-linked sideroblastic anemia; skin paleness, fatigue, dizziness and enlarged spleen and liver.



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