Mosaic trisomy 14


What is mosaic trisomy 14?

In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Each pair is numbered 1 through 22 except for the sex chromosomes which are denoted by "X" and "Y." Women have two Xs while men have one X and one Y. In complete trisomy 14, ALL of the cells of the body have an extra chromosome 14, which means that there are 47 chromosomes in all the cells. A fetus with an extra chromosome 14 in all its cells will usually miscarry and not make it to term.

In mosaic trisomy 14, some of the cells of the body have an extra chromosome 14, which means that there are 47 chromosomes in those cells, and some of the body's cells have the normal number of 46 chromosomes. The cells with the extra chromosome 14s are called "trisomic" as they have the three copies of chromosome 14. The word "mosaic" means that only some cells have the extra chromosome, while other cells are normal (they do not have any additional or missing chromosome material).

An extra chromosome 14 causes a range of symptoms including intellectual disability, specific facial features, heart problems, being smaller than peers, and slower development. Babies with mosaic trisomy 14 may also be small before they are born and have "failure to thrive", which means they do not weigh enough for their age. Everyone with mosaic trisomy 14 can have a different set of symptoms because they may have a different number of trisomic cells, which may be present in different parts of the body.

When mosaic trisomy 14 is found before a baby is born, through chorionic villus sampling (CVS) or amniocentesis, it can be very difficult to predict what health problems will occur. If the CVS test identified mosaic trisomy 14, then there is a chance that all the trisomy 14 cells are only in the placenta and not in the baby (confined placenta mosaicism). In this case, an amniocentesis may be suggested to look at the baby's cells directly. If the cells with extra chromosome 14 are in the baby and the placenta, usually the baby's health is suggested based up on the percent of cells that have the extra chromosome 14. The baby's health issues will vary and may be determined by factors such as if the test showed the extra chromosome 14 by chorionic villus sample or amniocentesis or both, if the extra chromosome is from mom or dad, and if there are health problems or birth defects seen on ultrasound. Medical geneticists and genetic counselors can be very helpful in discussing the possible outcomes based on this information.

Most often infants with many cells that have the extra chromosome 14 disorder are very sick and many die early in life. However, individuals with less cells with the extra chromosome 14 have less health issues and may live longer. Rarely, individuals with a very small amount of extra chromosome 14 cells found on prenatal testing may have very few to no medical issues and live a normal life span.

SOURCE: Emory University - Department of Human Genetics in collaboration with ThinkGenetic • • DATE UPDATED: 2016-06-13


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