March is National Trisomy Awareness Month. What is a Trisomy?
Most people are born with 46 chromosomes which are divided into 23 pairs. During sperm or egg production, these 23 pairs are separated so each sperm or egg only has one copy of each chromosome. On occasion, a pair of chromosomes do not separate, resulting in both copies of a chromosome being incorporated into one sperm or egg cell. When a sperm or egg with an extra chromosome unites with a partner’s egg or sperm, the resulting baby will have three copies of one chromosome. This third copy of a chromosome is a “trisomy.”
Most chromosomes contain too much genetic information for the body to function with an extra dose of genes. Pregnancies with an extra copy of these gene-dense chromosomes will result in a miscarriage. However, there are four chromosomes which can result in a viable trisomy: 21, 18, 13, and the sex chromosome X. Most people have heard of or know an individual with Trisomy 21, which is more commonly referred to as Down Syndrome. But what do you know about other trisomies?
Trisomy 18 (previously known as Edwards Syndrome)
About 1 in 5,000 babies in the US is born with an extra copy of chromosome 18. These babies are born small, with a small head and jaw, heart defects, and other organ defects. While symptoms vary from individual to individual, most babies with this condition experience numerous physical problems and die from complications shortly after birth. Babies who do survive typically experience severe intellectual and developmental disability. About 10% will live past their first birthday. On rare occasions, individuals with Trisomy 18 may be “mosaic” which means they do not have the extra copy of chromosome 18 in all of their cells. These individuals can live to adulthood, but will likely have severe intellectual disability and will require full-time care.
Trisomy 13 (previously known as Patau Syndrome)
About 1 in 16,000 babies in the US is born with an extra copy of chromosome 13. Babies with Trisomy 13 have many physical problems including brain or spinal cord abnormalities, heart defects, poorly developed eyes or even one centrally located eye, cleft lip with or without cleft palate, extra fingers or toes, and weak muscle tone. Most babies with Trisomy 13 die in the first days or weeks of life. Babies who do survive typically experience severe intellectual and developmental disability. About 5-10% may live to their first birthday.
Trisomy X (also known as Triple X Syndrome or 47,XXX Syndrome)
About 1 in 1000 girls are born with an extra copy of the X chromosome. While the X chromosome is a large chromosome containing many genes, girls inheriting an extra copy of the X chromosome often show very mild symptoms if any at all. This is due to the process of X inactivation. In all women, one copy of the X chromosome is inactivated or “turned off” at random, leaving only one copy to function within the cell. For women with three copies of the X chromosome, two copies will be turned off at random. However, there are a few genes on the X chromosome which are not turned off on the inactivated copy. For women with Trisomy X, these extra copies of inactivated genes may result in taller than average height, learning disabilities particularly in language skills, delayed motor skills, weak muscle tone, and behavioral or emotional difficulties. Symptoms of Trisomy X are so mild that women with this condition often go undiagnosed.
So in the third month, let’s think about the many individuals living with their third chromosome! If you would like to learn more about trisomies, please visit the Support Organization for Trisomy 18, 13 and related disorders (SOFT) or the National Institutes of Health (NIH) Genetic and Rare Disease (GARD) pages for Trisomy 18, Trisomy 13, and Trisomy X.
Diane Biederman is a genetic counseling graduate student at the University of South Carolina Genetic Counseling Program.
Find a genetic counselor at Palmetto Health USC Genetic Counseling or feel free to contact us at 803-545-5775 or genetics@uscmed.sc.edu.
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