A person’s genetic blueprint is organized into structures passed between generations. This genetic information is contained in DNA. While many genetic variations are a normal part of human diversity, some can result in specific health conditions.
Chromosomes and the Concept of Trisomy
Human cells contain chromosomes, which are made of tightly coiled DNA and carry the genes that direct development and function. Humans have 46 chromosomes arranged in 23 pairs, with one set inherited from each parent.
A trisomy is a condition where an error during cell division results in an extra chromosome in some or all of a person’s cells, making a total of 47. This event happens spontaneously when egg or sperm cells are formed, and the risk increases with the mother’s age. While a trisomy can occur with any chromosome, most are not compatible with life. The most common trisomies seen in live births involve chromosomes 21, 18, and 13.
There are different types of trisomy. The most common is Standard Trisomy, where the extra chromosome is in every cell. In Mosaic Trisomy, only some cells have the extra chromosome, which can lead to milder characteristics of the condition. A rarer form is Translocation Trisomy, where part of the extra chromosome becomes attached to another chromosome.
Down Syndrome (Trisomy 21)
Down syndrome (Trisomy 21) is the most common chromosomal condition, affecting about 1 in 700 babies in the United States. It is characterized by a range of physical traits, including a flattened facial profile, upward-slanting eyes, and a single deep crease across the palm of the hand. Individuals experience lifelong intellectual disability and developmental delays, though the severity varies significantly.
People with Down syndrome have an increased risk for several health issues. About half are born with a congenital heart defect, and other potential problems include:
- Digestive system issues
- Hearing and vision impairments
- Thyroid conditions
- A higher susceptibility to infections
- Leukemia
With appropriate medical care and support, life expectancy has dramatically increased, and many people with Down syndrome live fulfilling lives into their 60s and beyond.
Edwards Syndrome (Trisomy 18)
Edwards syndrome (Trisomy 18) is the second most common trisomy, occurring in approximately 1 in 5,000 live births and is more prevalent in female infants. The condition disrupts development in the womb, leading to health complications and a high rate of miscarriage or stillbirth.
Babies with Edwards syndrome are born with low birth weight and have distinct physical traits, such as a small jaw, clenched fists with overlapping fingers, and feet with rounded bottoms, often called “rocker-bottom feet.” They also face serious health problems including heart and kidney defects, breathing difficulties, and gastrointestinal issues.
The prognosis for Trisomy 18 is poor. About half of babies carried to term are stillborn. Of those born alive, roughly half do not survive beyond the first week, and fewer than 10% live past their first birthday. A small number of individuals, often with a mosaic form of the condition, may live into their 20s or 30s.
Patau Syndrome (Trisomy 13)
Patau syndrome (Trisomy 13) is a rare genetic condition that affects about 1 in every 4,000 to 16,000 live births. The additional genetic material disrupts fetal development, leading to multiple abnormalities and a high rate of pregnancy loss.
Infants born with Patau syndrome have a low birth weight and a wide range of health problems. A common feature is holoprosencephaly, where the brain fails to divide into two hemispheres, which can affect facial development and sometimes lead to a cleft lip or palate. Other characteristics can include:
- Extra fingers or toes (polydactyly)
- Heart defects
- Kidney abnormalities
- Malformations of the ears
The prognosis for Patau syndrome is poor, with many infants dying within the first few days or weeks of life. Only about 5% to 10% of children with this condition survive past their first year. In some cases, particularly with mosaic or partial trisomy, survival can be longer.
Detection of Trisomies 13, 18, and 21
Trisomies 13, 18, and 21 can be identified during pregnancy using prenatal screening and diagnostic tests. Screening tests assess the probability of a condition but do not provide a definitive diagnosis. A common screening is non-invasive prenatal testing (NIPT), which analyzes fetal DNA in the mother’s blood to detect the risk for these trisomies with high accuracy.
If screening indicates an increased risk, a diagnostic test can be performed to confirm the diagnosis. These tests, which include chorionic villus sampling (CVS) and amniocentesis, are invasive and carry a small risk but provide a conclusive answer. CVS takes a tissue sample from the placenta, while amniocentesis collects amniotic fluid.
Cells from these samples are then used for a karyotype analysis. This laboratory technique creates an image of the fetus’s chromosomes, allowing doctors to count them and confirm the presence of an extra chromosome.