An Overview of Sex-Linked Disorders

Sex-linked disorders are any diseases, disorders, or abnormal conditions that are caused by one or more defective genes on the X chromosome. The X chromosome is one of the two sex chromosomes, the other being the Y chromosome. In humans, males have XY chromosomes and females have XX chromosome. Sex-linked disorders may also include an aberration in the number of X or Y chromosomes.

X-linked Dominant Disorders

In these disorders the disease is observed in both, males and females. All the daughters of an affected male are affected, while the sons of an affected male remain unaffected; that is there is no male-to-male transmission. Hypophosphatemic rickets and Classic Rett syndrome are two famous examples of X-linked dominant disorders.

Hypophosphatemic rickets: A vitamin-D resistant rickets characterized by a low serum phosphate concentration. The clinical features include dental abscess, tooth decay, leg deformity, and abnormal growth rate.

Classic Rett syndrome: A progressive neurological disorder in girls. The subject is normal for the first 18 months of age, followed by developmental stagnation, which is followed by rapid regression in motor and language skills. The hallmark of the disease is repetitive and stereotypic hand movements.

X-linked Recessive Disorders

These disorders are almost exclusively observed in males, the occurrence in females is extremely rare. Since males have only one X chromosome, that is, males are hemizygous for X-linked genes, they express the disorder. All daughters of an affected male are carriers. X-linked recessive disorders include hemophilia, color blindness, Duchenne muscular dystrophy (DMD), and fragile-X syndrome.

Hemophilia: This is a disorder of the blood-clotting system. In this disorder the blood fails to clot. Clinical symptoms include bruises, joint swelling and pain due to internal bleeding, blood in stool or urine, prolonged bleeding from injuries, cuts, or after surgeries, unexplained nosebleeds, and prolonged bleeding after immunizations.

Color blindness: A disorder that is characterized by an inability to differentiate between certain colors. Specifically the patient suffers from red-green color blindness.

Duchenne muscular dystrophy: It is a disorder of the musculoskeletal system. Symptoms appear in early childhood, and include delays in sitting and standing, muscle weakness and wasting, waddling gait, progressive difficulty in standing and walking which eventually leads to the sufferer being wheelchair bound by the age of 12 years. The hallmark of the disease is Gower maneuver, which is when patient rolls to the prone position, kneels, and pushes up to a standing position while supporting against knees and thighs. Death occurs by 30 years of age due to cardiac or respiratory failure.

Fragile X syndrome: The most common cause of genetic learning disability amongst boys. Disabilities in this disorder may range from mild to severe. Physical presentation of the disorder may include large ears, long face, soft skin and large testicles in post-pubertal males. Connective tissue complications may include ear infections, high arched palate, flat feet, and hyperflexible joints. Behavioral findings may include autism, social anxiety, hand biting or flapping, aggressive behavior, attention deficit disorders, and poor eye contact. Females who have this genetic disorder may present with milder signs and symptoms of the disease.