Types of X-Chromosome Problems
About half of the individuals with Turner syndrome
have monosomy X, which means each cell in the individual's body has only one copy of the X chromosome, instead of the usual two sex chromosomes (see X Chromosome Monosomy).
Some women with Turner syndrome have a chromosomal change in only some of their cells, which is known as X chromosome mosaicism (see Mosaic Turner Syndrome).
Turner syndrome research scientists have not yet determined which genes on the X chromosome cause most signs and symptoms of Turner syndrome
. They have, however, identified one gene called SHOX (short for "short stature homeobox") that is important for bone development and growth. The SHOX gene is responsible for making a protein (called the SHOX protein) that it is especially important for the growth and maturation of bones in the arms and legs. One copy of the SHOX gene is located on each of the sex chromosomes (the X and Y chromosomes) in an area called the pseudoautosomal region. Although many genes are unique to the X or Y chromosome, genes in the pseudoautosomal region are present on both chromosomes. As a result, men and women each have two functional copies of the SHOX gene.
Turner syndrome occurs when one sex chromosome is missing or altered in some or all of a female's cells. As a result, most women with Turner syndrome have only one copy of the SHOX gene instead of the usual two copies, which decreases the amount of SHOX protein by half. Researchers believe that this deficiency is at least partly responsible for short stature and skeletal abnormalities (such as unusual rotation of the wrist and elbow joints) in women with this condition.