Diseases Linked to the Sex Chromosome X

In the past century we have come so far in genetic sciences as to be able to determine exactly what gene or genes cause birth defects or diseases. While many of these diseases are linked to autosomal chromosomes - the non-sex linked chromosomes, 22 of the 23 chromosomes - there are some which are linked to the sex chromosomes, also known as the X and Y chromosomes. Such diseases as Cystic Fibrosis, Huntington Disease, and Sickle Cell Anemia, are all notorious autosomal diseases. However, diseases like Hemophilia and Diabetes Insipidus are linked to the sex chromosomes.

Before we get any further, it's important to discuss the sex chromosomes in more detail. The sex cells, as mentioned earlier are the 23^rd pair of chromosomes, and each person is born with two sex chromosomes. Males and females are distinguished by their XX or XY pairing. In females, the pairing is XX, meaning both chromosomes carry the "female gene." In males, the pairing is XY, meaning one is a female cell and the other a male cell. As the male matures each of these sex chromosomes will break apart and some sperm cells will carry X chromosomes, and some Y. Thus, when two X sex cells combine, a girl is formed. When an X and Y bind, a boy is formed.

Additionally, our genetic make-up is determined on a dominant - recessive pattern. There are also cases of incomplete dominance, in which combined inheritance is seen. These reasons usually cause autosomal diseases. However, diseases linked to sex chromosomes are usually the result of an extra or missing chromosome. Furthermore, a principle called X-linked inheritance is at work making males more likely than females to be affected by problems with incomplete dominance. Remember that females have two X chromosomes, so, more often than not the second X can kick in and play the dominant role. In males, their sex linked chromosomes don't match, and thus they are more susceptible to diseases due to a defective chromosome.

The first sex chromosome linked disease, and one of the most well known, is hemophilia. This is a condition in which an individual's blood doesn't clot in a normal manner, and as such puts the individual's life at risk even with a small injury. Internal bleeding (damage to organs) and tissue damage are common reasons why hemophilia is life-threatening. The disease occurs in about 1 out of every 5000 males, give or take. Although females can be carriers of this disorder, they are much less to be affected by it. Currently there is no cure for hemophilia. Instead, the disorder is treated by regular blood transfusions, and learning to live a lifestyle less littered with risk, and wearing equipment to prevent common injuries.

Another sex chromosome linked disease is Duchenne muscular dystrophy. This is a disorder in which one's ability to walk is severely impaired. The individual is usually able to walk, although abnormally until the age of 7 (at the earliest) at which point the muscles are too degenerated to continue to walk. Furthermore, this muscle degeneration affects organs as well, and death usually results. This is passed down from a recessive chromosome and affects around 1 in 4,000 male births. Again, this can affect females, but is most common in males as they genetically more susceptible.

Lastly, Diabetes Insipidus may sounds like a familiar condition, but it is actually a disease involving the hormone vasopressin. The disease suppresses production, thus leading to excessive thirst, and despite this, excessive urination. As a result dehydration is of serious concern. The only treatment available is hormone replacement therapy.

Abnormalities of sex chromosomes, such as inheriting an extra or missing one are another form of sex linked illness. Some such diseases include Triple X syndrome in girls, which is the inheritance of an extra X chromosome, Turner syndrome in girls caused by a missing X, or Klinefelter syndrome in boys caused by the presence of an extra X chromosome. While these syndromes impact the life of the carrier, they are generally milder in symptoms. For example, Triple X syndrome causes intellectual difficulties with vocabulary and reading.

It is important to note that most people with sex chromosome linked illnesses go on to lead fairly normal lives, despite regular medical treatment. This is in contrast to autosomal diseases, in which the individual is at great risk for early death, or severe complications during life. There are treatments for many, and Cystic fibrosis treatments have made life significantly easier for sufferers, however, for the most part, individuals with these disorders perish with a certain timeframe (depending on which disease) after symptoms arise. Perhaps this is an argument for reproductive technologies such as genetic counseling? But I guess that is another discussion for another time.

References:

Chiras, D. (2005). Human Biology, 5th Ed. Sudbury, MA: Jones and Bartlett Publishers.

Bainbridge, D. (2004). The X in Sex: How the X Chromosome Controls Our Lives. MASS: Harvard University Press.