Nondisjunction is an error during cell division where chromosomes fail to separate properly, leaving some cells with too many chromosomes and others with too few. This is the primary cause of conditions like Down syndrome and Turner syndrome, and it’s the reason chromosomal abnormalities become more common as maternal age increases. Humans normally have 46 chromosomes in every cell, and nondisjunction disrupts that number in ways that can range from undetectable to life-altering.
How Nondisjunction Works
During normal cell division, chromosomes line up in the middle of the cell and then get pulled apart so each new cell gets the right number. Nondisjunction happens when that pulling-apart step fails. Two chromosomes that should go to opposite sides of the cell instead travel to the same side. One resulting cell ends up with an extra chromosome (47 total in humans), while the other is missing one (45 total).
This error can happen during three different types of cell division, and the consequences differ for each.
During Egg and Sperm Formation
The most consequential form of nondisjunction occurs when the body makes eggs or sperm, a process called meiosis. Meiosis has two rounds of division, and the error can strike during either one.
If it happens in the first round, paired chromosomes fail to separate. This produces four reproductive cells that are all abnormal: two with an extra chromosome and two missing one. If it happens in the second round, the paired copies (sister chromatids) fail to split apart. Because the first round went normally, only two of the four resulting cells are affected. The other two are completely normal. This distinction matters because errors in the first round affect a larger proportion of eggs or sperm, making them more likely to produce a chromosomal abnormality in a future pregnancy.
After Fertilization
Nondisjunction can also happen during ordinary cell division (mitosis) after an embryo has already formed. When this occurs, some cells in the body carry the normal 46 chromosomes while others have 45 or 47. This patchwork of genetically different cells is called mosaicism. A person with mosaic Turner syndrome, for example, has some cells with one X chromosome and other cells with the typical two. The severity of symptoms in mosaic conditions depends on how many cells are affected and where in the body those cells end up.
Why Chromosomes Fail to Separate
Cells have a built-in safety system, often called the spindle checkpoint, designed to prevent exactly this kind of error. Before chromosomes are pulled apart, the checkpoint verifies that every chromosome is properly attached to the molecular cables (spindle fibers) that will tow them to opposite ends of the cell. If a chromosome isn’t attached correctly, the checkpoint delays division until the problem is fixed.
Nondisjunction happens when this system fails or when the physical structures holding chromosomes together malfunction. Chromosomes are held in pairs by a protein complex called cohesin, which acts like a molecular clasp. At the right moment, an enzyme cuts cohesin so the chromosomes can separate. If cohesin breaks down too early, chromosomes drift apart prematurely and land on the wrong side. If it holds too tightly, they can’t separate at all and both get dragged to one side.
Why Maternal Age Is the Biggest Risk Factor
The link between a mother’s age and the risk of chromosomal errors in her eggs is one of the strongest and most well-documented patterns in human genetics. In women aged 25 to 30, roughly 30% of embryos carry an abnormal chromosome count. By ages 36 to 40, that figure rises to about 59%. In women over 40, it reaches nearly 69%. Women over 40 are more than four times as likely to produce embryos with chromosomal errors compared to women 35 and under.
The reason comes down to cohesin and the unique biology of egg cells. A woman’s eggs begin developing before she is born, during fetal development. Cohesin is loaded onto the chromosomes during this early stage and is never replenished. Those same cohesin proteins must hold chromosomes together for decades, from before birth until the egg is finally released during ovulation. Over 30 or 40 years, cohesin gradually degrades. Studies in both human and mouse eggs show that chromosome-associated cohesin is severely reduced in older eggs. Without enough cohesin, chromosomes are far more likely to separate incorrectly.
This degradation accelerates through several mechanisms: a protective protein that shields cohesin is lost prematurely, the enzyme that cuts cohesin becomes overly sensitive, and accumulated oxidative damage weakens the remaining protein. The spindle checkpoint in eggs also appears to be less strict than in other cell types, allowing errors to slip through that would be caught elsewhere.
Conditions Caused by Nondisjunction
When an egg or sperm with the wrong chromosome count successfully fertilizes or is fertilized, the resulting embryo has an abnormal number of chromosomes in every cell. Having three copies of a chromosome instead of two is called trisomy. Having only one copy is called monosomy. Most of these abnormalities are incompatible with life. Chromosomal errors account for 50 to 60% of all spontaneous miscarriages, the vast majority occurring in the first trimester.
A small number of trisomies and monosomies can result in a live birth:
- Down syndrome (trisomy 21): Three copies of chromosome 21, the most common survivable trisomy. People with Down syndrome have 47 chromosomes per cell. It causes intellectual disability, characteristic facial features, and increased risk of heart defects, though outcomes vary widely.
- Edwards syndrome (trisomy 18): Three copies of chromosome 18. This causes severe developmental problems, and most affected pregnancies end in miscarriage. Infants born with full trisomy 18 typically have very short life expectancies.
- Patau syndrome (trisomy 13): Three copies of chromosome 13. Like Edwards syndrome, this causes severe abnormalities and most cases do not survive to birth.
- Turner syndrome (monosomy X): A single X chromosome and no second sex chromosome, resulting in 45 total chromosomes. This affects females and causes short stature, delayed puberty, and sometimes heart or kidney differences. Many people with Turner syndrome live full adult lives.
- Klinefelter syndrome (XXY): An extra X chromosome in males, giving 47 chromosomes total. This often causes tall stature, reduced fertility, and sometimes mild learning differences. Many men with Klinefelter syndrome are never diagnosed because symptoms can be subtle.
The reason only certain trisomies are survivable has to do with the size and gene content of the affected chromosome. Chromosome 21 is the smallest human chromosome with relatively few genes, so the disruption from an extra copy is less severe. Extra copies of larger, gene-dense chromosomes cause such widespread developmental problems that the embryo cannot survive.
How Nondisjunction Is Detected
Chromosomal abnormalities from nondisjunction can be identified through several prenatal and postnatal tests. Standard karyotyping, where a lab technician photographs and arranges a cell’s chromosomes by size, remains a core method for spotting extra or missing chromosomes as well as mosaic patterns. A technique called FISH (fluorescent in situ hybridization) uses glowing molecular probes that bind to specific chromosomes, allowing rapid detection of common trisomies like 13, 18, and 21 without waiting for a full karyotype.
For more detailed analysis, array-based testing (aCGH) scans the entire genome for smaller duplications or deletions that karyotyping might miss. In prenatal settings, these tests are performed on cells collected through amniocentesis or chorionic villus sampling. Non-invasive prenatal screening, which analyzes fragments of fetal DNA circulating in the mother’s blood, can flag likely trisomies as early as 10 weeks of pregnancy, though it’s a screening tool rather than a definitive diagnosis.
After birth, the same laboratory techniques confirm suspected chromosomal conditions. Karyotyping is typically the first step, with FISH and array testing added when more detail is needed or when mosaicism is suspected.