What is tetraploidy?
Genetic information is organized in the cells of the body on structures called chromosomes. The chromosomes contain important genetic information needed for the body to grow and develop normally. Most cells in the body typically have 46 chromosomes which are organized into 23 pairs. The first 22 pairs of chromosomes, which are numbered 1-22, are the same in males and females and are called autosomes. The 23rd pair of chromosomes is called the sex chromosomes and are what determine gender (XX for female and XY for male). An individual receives one chromosome from each pair from each parent at the time of conception.
Tetraploidy is a condition in which cells contain four complete sets of chromosomes. Instead of having 46 chromosomes organized into 23 pairs of 2, tetraploid cells have 92 chromosomes organized into 23 sets of 4.
The imbalance of genetic information caused by tetraploidy in the cells typically causes significant developmental problems even from very early in pregnancy. In most cases, pregnancies involving babies with tetraploidy in all cells (complete tetraploidy) will end in first trimester miscarriages. Liveborn babies with complete tetraploidy are extremely rare and usually die within the first days or months of life. As of early 2016 there have only been a few cases reported in the medical literature of live born infants with tetraploidy. The longest reported surviving infant lived to be 26 months old and experienced severe physical and developmental problems.
In some cases, tetraploidy is only present in some, but not all, of the cells in a fetus or liveborn baby. This situation is referred to as mosaicism, and means that there is more than one cell line present in the body. With mosaicism for tetraploidy, some cells in the body have the typical 46 chromosomes while others have 92. When mosaic tetraploidy is identified by prenatal testing before birth, it can be very difficult to predict what health and developmental problems will occur in the baby. If the diagnosis is made by chorionic villus sampling, which is a test that involves a needle biopsy of a small piece of the placenta, then there is a chance that all the mosaic tetraploidy cells are only in the placenta and not in the baby (known as confined placenta mosaicism). When tetraploidy is diagnosed by CVS, an amniocentesis (which is a procedure that involves withdrawing some amniotic fluid from the sac surrounding the baby and doing genetic testing on fetal skin cells that are present in the fluid) may be suggested to test cells from the baby, in order to compare the results from CVS (placental cells) and amniocentesis (fetal cells). Some predictions about the baby's health and development related to the tetraploidy may be made based on the percentage of cells that are tetraploid versus having a typical chromosome content (known as diploid), as well as considering the number of health problems or birth defects identified by ultrasound. Medical geneticists and genetic counselors can provide a family helpful information regarding the possible outcomes for the baby based on this information.
Some individuals are born with the standard 46 chromosomes in every cell of their body (a normal chromosome count, or diploid) but can develop tetraploidy in cancerous or precancerous cells. In this situation, the person does not have tetraploidy throughout his or her body and is not expected to have other health or developmental problems. It is only the precancerous or cancer cells which have the changes in chromosome number. Cancer cells often contain many different types of genetic and chromosome changes that would not be compatible with normal growth and development if present at the time of conception of a baby.
SOURCE: Emory University - Department of Human Genetics in collaboration with ThinkGenetic • https://www.thinkgenetic.com/diseases/tetraploidy-3563/overview/4626 • DATE UPDATED: 2016-06-24
Guc-Scekic, M., Milasin, J., Stevanovic, M., Stojanov, L. and Djordjevic, M. (2002), Tetraploidy in a 26-month-old girl (cytogenetic and molecular studies). Clinical Genetics, 61: 62-65. doi: 10.1034/j.1399-0004.2002.610112.x
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