Autosomal trisomy is a genetic condition where an individual has an extra copy of a non-sex chromosome, resulting in three copies instead of two. These conditions can affect development and overall health, with severity varying by the specific chromosome involved.
Understanding Chromosomes and Trisomy
Humans have 46 chromosomes organized into 23 pairs within each cell. Of these, 22 pairs are known as autosomes, which carry genetic information for most of the body’s characteristics. The remaining pair consists of sex chromosomes, determining an individual’s biological sex. One chromosome from each pair is inherited from each parent, ensuring the correct genetic complement.
A trisomy is a numerical abnormality in chromosome count, known as aneuploidy, meaning an individual will have 47 chromosomes in their cells rather than the usual 46. Autosomal trisomies involve an extra copy of one of the 22 autosomal chromosomes.
How Autosomal Trisomy Develops
Autosomal trisomy results from an error during cell division called non-disjunction. Normally, during meiosis, the process that creates egg and sperm cells, chromosomes separate evenly, so each gamete receives one copy of each chromosome. Non-disjunction happens when chromosome pairs or sister chromatids fail to separate properly.
This error can occur during either meiosis I or meiosis II in the formation of the egg or sperm. If a gamete with an extra chromosome is fertilized by a normal gamete, the resulting embryo will have three copies of that chromosome. Advanced maternal age is a recognized risk factor for non-disjunction in maternal meiosis.
Major Types of Autosomal Trisomy
Trisomy 21, Trisomy 18, and Trisomy 13 are the most commonly encountered autosomal trisomies in live births. Each condition presents with a unique set of physical characteristics and health considerations.
Trisomy 21
Trisomy 21, also known as Down Syndrome, is the most frequent autosomal trisomy, occurring in approximately 1 in 700 live births. Individuals exhibit mild to moderate intellectual disability and distinct facial features, such as upward-slanted eyes, a flattened nasal bridge, and a single deep crease across the palm. Common health concerns include congenital heart defects, affecting about half of those with Down syndrome, and gastrointestinal abnormalities, vision and hearing problems, and increased risk of respiratory infections.
Trisomy 18
Trisomy 18, or Edwards Syndrome, occurs in about 1 in 5,000 live births. Babies are often born small and may present with a prominent back of the head, small jaw and mouth, unusually shaped ears, and clenched fists with overlapping fingers. Common health issues include severe heart defects, kidney malformations, feeding difficulties, and profound intellectual disability. Survival beyond the first year is uncommon (5-10%).
Trisomy 13
Trisomy 13, also known as Patau Syndrome, is less common, affecting approximately 1 in 10,000 to 16,000 live births. It is characterized by severe intellectual disability and multiple physical abnormalities. Babies often have a small head, small or poorly developed eyes, cleft lip or palate, extra fingers or toes (polydactyly), and weak muscle tone. Infants often have serious heart defects, brain or spinal cord abnormalities, and underdeveloped internal organs. Most infants with Trisomy 13 do not survive beyond their first days or weeks of life; only 5% to 10% live past their first year.
Detection and Diagnosis
Detecting autosomal trisomy often begins with prenatal screening tests to estimate the likelihood of a condition. Non-invasive prenatal testing (NIPT) is a blood test, offered after 10 weeks of pregnancy, that analyzes fragments of fetal DNA in the mother’s bloodstream to assess the risk of Trisomy 21, 18, and 13. Other screening methods include the combined test, performed between 10 and 14 weeks, involving a maternal blood test and an ultrasound to measure nuchal translucency. If performed after 14 weeks, a quad screen blood test can be offered.
If screening tests indicate an increased risk, diagnostic tests confirm the presence of an extra chromosome. Chorionic villus sampling (CVS) can be performed between 10 and 13 weeks of pregnancy by taking a tissue sample from the placenta. Amniocentesis, done between 15 and 20 weeks, involves collecting amniotic fluid. Cells from these samples are then analyzed using karyotyping, a laboratory technique that images chromosomes to identify numerical or structural abnormalities. Trisomy can also be identified after birth through a physical examination, with confirmation by karyotype analysis.