Chromosomes are the structures that hold our DNA. Our DNA contains the instructions, called genes, that tell our bodies how to develop and function. Humans have 23 pairs of chromosomes, so 46 in total. One of each pair is inherited from our mother and the other from our father. Click here to view an illustration of a chromosome and DNA.
Chromosomes vary in size. Each chromosome has a constriction, called the centromere, at one point along its length dividing it into a shorter arm, called the p arm, and a longer arm, called the q arm. Chromosome tests allow us to take a closer look at the chromosomes and identify changes, such as missing or added pieces. To read more about chromosomes, click here.
Last updated: 3/16/2011
What is a chromosome inversion?
A chromosome inversion occurs when a piece of a chromosome breaks off and then reinserts itself up-side down. The inversion may involve just one "arm" of the chromosome or could extend across the centromere and involve material on both arms. When the inversion involves just one arm, it is called a paracentric inversion. If the inversion involves both arms, it is called a pericentric inversion.
Last updated: 3/16/2011
What are the effects of a chromosome 9 inversion?
Pericentric inversions (or inversions involving the centromere) of chromosome 9 are the most common type of inversion seen in humans. The two most common inversions are inv(9)(p11q12) and inv(9)(p11q13). These inversions usually cause no signs or symptoms. Also, they do not tend to increase a person's risk for pregnancy loss or any other reproductive problem.
One study reviewed the results of 652 chromosome studies on mid-trimester amniocentesis (16–18 weeks) during a 10-year period from January 1985 to February 1995. Of 652 patients, pericentric inversion of chromosome 9 was found in 27 cases. The indications for amniocentesis among these 27 patients included advanced maternal age, abnormal ultrasound findings, abnormal maternal serum screening tests, and family history of a child with a disability. Immediate evaluation of these infants after birth was normal in 24 and abnormal in 3; one had a low Apgar score at 1 and 5 min, one had bilateral capillary hemangioma over the eyelids, and one had a heart murmur and an irregular heart rhythm. The researchers concluded that inversion of chromosome 9 is associated with a normal outcome in the majority of cases.
However, it is important to discuss an inversion chromosome 9 found through amniocentesis with a genetics professional. This type of health care professional can help you understand the size and location of the inversion and the effect, if any, this finding may have on the fetus.
Last updated: 3/28/2011
How can I find a genetics professional in my area?
Ait-Allah AS, Ming PL, Salem HT, & Reece EA. The Clinical Importance of Pericentric Inversion of Chromosome 9 in Prenatal Diagnosis. Journal of Maternal-Fetal Investigation. 1997; 7(3):126-128. http://www.springerlink.com/content/6c1lvkqpng218v64/. Accessed 3/16/2011.
Gardner and Sutherland. Chromosome Abnormalities and Genetic Counseling, 3rd Edition. NY, NY: Oxford University Press; 2004;