Monosomy

Monosomy is a rare chromosome anomaly. Human cells normally contain 23 pairs of chromosomes, with a total of 46 chromosomes in each cell. Monosomy refers to the loss of one chromosome in cells. Any such change of chromosomes shall cause problems pertaining to growth, development and function of the body's systems. Monosomy is a genetic defect caused by an incomplete set of chromosomes. The changes in chromosomes occur during the formation of reproductive cells in early fetal development.

Monosomy can be identified during prenatal testing, especially in women who are at high risk. Prenatal testing such as an amniocentesis can reveal monosomy. As the test results could be very complicated, it is important to receive genetic counseling before undertaking this test. While a negative result indicates that no abnormalities were detected, a positive result suggests that a problem may be present. Since false positives and negatives can also happen, follow up additional testing is also recommended.

Aneuploidy is the term used to refer to chromosomal defects, a gain or loss of chromosomes from the normal 46. In monosomy, which is a kind of anueploid, there is the loss of one chromosome in cells. Another common form of aneuploidy is trisomy where people have three copies of a particular chromosome 21 in each cell instead of the two copies. One common example of the condition caused by trisomy is Down Syndrome.

Turner syndrome is a known example of the condition caused by monosomy. In this syndrome, women typically have only one X chromosome instead of the usual two. Significantly, Turner syndrome is the only full monosomy that is found in human beings. In other full monosomy, the individual will not survive development.

Cri du chat syndrome and 1p36 Deletion Syndrome are instances of partial monosomy caused by deletion of the short p arm of chromosome 5 and chromosome 1 respectively.

Chi du chat syndrome is characterized by a number of symptoms and in particular a malformed larynx which causes the voice to sound strangely high pitched. Chromosome 1p36 deletion syndrome is considered one of the commonest chromosome deletion syndromes. It is characterized by features such as developmental delay, feeding difficulties, low muscle tone, distinctive facial features, hearing loss, heart problems, seizures, vision defects and a large fontanelle that is slow to close. The incidence of monosomy 1p36 has been estimated to be 1 in 5000 to 1 in 10000 live born children. Interestingly, more females than males have been reported.