American Medical Association Family Medical Guide - American Medical Association [678]
If your child has Ehlers-Danlos syndrome, he or she needs to avoid contact sports and other physically stressful activities that could result in injuries and increase the risk of chronic pain later in life. Also, make sure your child knows not to show off the unusual positions he or she can put his or her joints into, because doing so can damage the joints. Make sure that other family members and your child’s friends and teachers have knowledge about Ehlers-Danlos syndrome that will enable them to help your child if necessary.
X-Linked Disorders
X-linked disorders, also called sex-linked disorders, are genetic disorders that are transmitted by a defective gene on an X chromosome. The X and Y chromosomes determine sex. Females have two X chromosomes; males have one X chromosome and one Y chromosome. All egg cells contain an X chromosome. If a sperm that fertilizes an egg carries an X chromosome, the resulting embryo will have two X chromosomes and will be female. If a sperm carries a Y chromosome, the embryo will have an X and a Y chromosome and will be male.
Most X-linked traits and disorders are recessive and are passed from mothers to sons. Women are rarely affected by X-linked disorders because their other X chromosome has a healthy copy of the gene, which overrides the effects of the disease gene. However, females can be carriers of an X-linked disorder, and they have a 50 percent chance of transmitting the X chromosome with the disease gene to a son, who will have the disease. Boys who receive an X-linked gene from their mother will be affected because they have only one X chromosome (and a Y chromosome from their father).
The daughters of female carriers of X-linked disease genes also have a 50 percent chance of inheriting their mother’s X-linked disease gene; they, however, will be unaffected carriers. An X-linked recessive disease gene can be transmitted through many generations of a family by healthy carrier mothers before it becomes apparent when, for the first time, a boy in the family is born with the disease.
X-linked inheritance
Disorders that are caused by defective genes on the X sex chromosome generally are transmitted from mothers to sons, who receive one X chromosome (only from their mother). A woman who has the defective gene on the X chromosome is usually unaffected. She has a 50 percent chance of passing the gene on to a son, who will be affected, and a 50 percent chance of passing the gene on to a daughter, who will be an unaffected carrier like her mother.
Fragile X Syndrome
Fragile X syndrome is caused by a defective gene on the X sex chromosome that narrows the affected part of the chromosome and makes it look fragile. Fragile X syndrome is the most common inherited form of mental retardation. Males are more likely to have the disorder and to have more severe symptoms than females. Because females can have one defective X chromosome and one healthy X chromosome, they can be carriers of fragile X and either have no symptoms or have a mild form of the disease. They have a 50 percent chance of transmitting their defective X chromosome to a son, who will be affected. If you have a family history of fragile X syndrome or if you have a child with fragile X syndrome, you can benefit from talking to a genetic counselor (see page 952), who can evaluate your risk of transmitting the disorder to a future child.
Symptoms
Physical features that are characteristic of fragile X syndrome may be evident at birth, but the first noticeable sign is usually developmental delay that is apparent by age 2 or 3. Most boys have some degree of mental impairment, ranging from below-average intelligence to severe mental retardation. The severity of the mental impairment may become more apparent with age. About 20 percent of boys have symptoms similar to those of autism (see page