What are monosomies?
The term monosomy is used to describe the absence of one member of a pair of chromosomes. Therefore, there is a total of 45 chromosomes in each cell of the body, rather than 46. For example, if a baby is born with only one X sex chromosome, rather than the usual pair (either two X's or one X and one Y sex chromosome), the baby would be said to have "monosomy X." Monosomy X is also known as Turner syndrome.
To answer some questions you may have about a child with Turner syndrome, visit the Frequently Asked Questions page in the Genetics Resources.
What is Turner syndrome?
Turner syndrome is a genetic disorder seen in girls that causes them to be shorter than others and to not mature sexually as they grow into adulthood. The severity of these problems varies among affected individuals. Other health problems may also be present involving the heart or renal system (i.e., kidneys). Many problems affecting girls with Turner syndrome can be managed or corrected with appropriate medical treatment. Turner syndrome occurs in one in 2,500 females born.
The name "Turner syndrome" comes from the physician, Dr. Henry Turner, who first described the collection of findings in 1938. It was not until 1959 that the cause of Turner syndrome (having only a single X chromosome) was identified.
What causes Turner syndrome?
Normally in reproduction, the egg cell of the mother and the sperm cell of the father start out with the usual number of 46 chromosomes. The egg and sperm cells undergo cell division where the 46 chromosomes are divided in half and the egg and the sperm cells end up with 23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes, the baby ends up with a complete set of 46 chromosomes, half from the father and half from the mother.
Sometimes, an error occurs when an egg or sperm cell is forming, causing it to have a missing sex chromosome. When this cell fails to contribute a sex chromosome to the embryo, so that there is only one X sex chromosome, Turner syndrome results. Having a single copy of a particular chromosome, rather than the usual pair, is called "monosomy." Turner syndrome is also known as "monosomy X." The missing sex chromosome error can occur in either the mother's egg cell or the father's sperm cell; however, it is usually an error that occurred when the father's sperm cell was forming. There is nothing known that a father could have done or not done which would have caused or prevented the sperm from having a missing sex chromosome. (The chance for Turner syndrome to occur is, therefore, not associated with the increasing age of the mother). The features of Turner syndrome result from having a missing X chromosome in each of the body's cells.
About 50 percent of Turner syndrome results from full monosomy X. One third have two X chromosomes with part of one X missing. Others have a mosaic pattern (two or more chromosome patterns in the cells). A small percent of Turner syndrome results from having the normal number of chromosomes (46 total), but missing a portion of the X chromosome. When only part of an X chromosome is missing (called a deletion), not all of it, girls with Turner syndrome usually have milder features of the syndrome. The features of Turner syndrome that are present depend upon which part of the X chromosome they are missing.
How is Turner syndrome diagnosed?
When a girl is born with features suggestive of Turner syndrome, a small blood sample is usually taken and the chromosomes are analyzed to determine the absence of a sex chromosome. Sometimes, girls with Turner syndrome do not exhibit any problems as infants or children, and it is only when they fail to go through puberty that a physician begins to suspect that Turner syndrome may be present. Again, not all girls with Turner syndrome exhibit all features described here. There is great variability. Some girls with Turner syndrome are diagnosed during the newborn period, while others are diagnosed during childhood and in their late teens.
Chromosomal abnormalities such as Turner syndrome can often be diagnosed before birth by analyzing cells in the amniotic fluid or from the placenta. Fetal ultrasound during pregnancy can also give information about the possibility of Turner syndrome, but ultrasound is not 100 percent accurate, and many babies with Turner syndrome may look the same on ultrasound as those without Turner syndrome. A chromosome analysis, whether performed on a blood sample, cells from the amniotic fluid, or placenta, is over 99.9 percent accurate.