Nondisjunction is a chromosome-sorting mistake. When a cell divides, a pair of chromosomes fails to separate, so the new cells get an uneven share. An egg or sperm can end up with an extra chromosome or be missing one, and a pregnancy that starts from it carries that error in every cell. It is the main reason behind Down syndrome, yet most of its effect goes unseen, because it also ends many pregnancies very early.1PubMed Central. Chromosomal abnormalities in products of conception of first-trimester miscarriages detected by conventional cytogenetic analysis: a review of 1000 cases The error is almost never caused by anything a parent did. The strongest known link is simply how long an egg has been waiting to divide.2PubMed Central. Meiosis and Maternal Aging: Insights from Aneuploid Oocytes and Trisomy Births
How Chromosomes Are Supposed to Separate
Human cells carry 46 chromosomes, the packages of DNA that hold your genes. They come in 23 pairs, with one of each pair from each parent. That means eggs and sperm must carry only 23, one from each pair, so that a fertilized egg returns to 46. They are made by a special division called meiosis, which happens in two rounds. The first round splits the pairs. The second splits each chromosome’s two copied halves.
A first-round failure keeps the two chromosomes of a pair together, and a second-round failure keeps the two identical halves together. A first-round mistake leaves two different chromosomes from that parent, while a second-round mistake leaves two identical copies.3PubMed Central. Integrated functional genomic analyses of Klinefelter and Turner syndromes reveal global network effects of altered X chromosome dosage That difference lets labs work out which round failed. Either way the result is an egg or sperm with 24 or 22 chromosomes, so one new cell has 24 and the other 22. Add the normal 23 from the other parent and you get 47 or 45.
Doctors call having the wrong number of chromosomes aneuploidy, a word you may see on genetic reports.
Most Errors Start in the Egg
In Down syndrome, the extra chromosome came from the mother in 188 of 197 traced cases and from the father in only 9.4PubMed Central. The meiotic stage of nondisjunction in trisomy 21: determination by using DNA polymorphisms Among the egg errors, about 77% happened in the first round and 23% in the second.5PubMed Central. The meiotic stage of nondisjunction in trisomy 21: determination by using DNA polymorphisms The rare sperm errors lean the other way, with about two-thirds happening in the second round.6PubMed Central. Investigation of Factors Associated With Paternal Nondisjunction of Chromosome 21
Eggs are more exposed because of how they are made. A woman’s eggs begin the first round of meiosis before she is born, then stop partway through and wait. The division finishes only when a particular egg is released at ovulation, which can be decades after it started. The second round pauses again and completes only if a sperm fertilizes the egg. Sperm are made continuously in adult men, so each one has gone through division recently, without a long wait in the middle.
The pattern varies by chromosome. For trisomy 18 the pattern flips: second-round egg errors are more common.7PubMed Central. Trisomy 18: studies of the parent and cell division of origin and the effect of aberrant recombination on nondisjunction And in some trisomies the extra copy appears after fertilization, during the embryo’s own cell divisions, in about 5 to 15% of cases.8PubMed. Origin and mechanisms of non-disjunction in human autosomal trisomies The error is a random mechanical slip, and the parent’s own chromosomes are usually normal.
Why a Mother’s Age Raises the Risk
Beyond about age 35, the risk of eggs with the wrong chromosome number rises sharply, bringing more infertility, miscarriage and Down syndrome.9PubMed Central. Meiosis and Maternal Aging: Insights from Aneuploid Oocytes and Trisomy Births Among early embryos created in IVF, about 30% have the wrong number when the woman is under 35, and over 90% when she is over 44.10PubMed Central. Impact of Maternal Age on Oocyte and Embryo Competence Those IVF figures need perspective. They count early embryos tested in a lab, not babies born. Many wrong-numbered embryos never implant or are lost early, often before anyone knows about them. So the chance that an older mother has a baby with a trisomy is far lower than the share of embryos with errors, and most babies born to women over 35 have a normal number of chromosomes.
A key cause in older eggs is early loss of the protein “glue” that holds paired chromosomes together.11PubMed Central. Meiosis and Maternal Aging: Insights from Aneuploid Oocytes and Trisomy Births A ring-shaped protein complex does much of this work, and it is set in place before birth. It has to last for decades, and the egg apparently cannot replace it, so it gradually weakens. When the glue loosens too early, chromosomes can drift apart or separate the wrong way before the division finishes. Weaker cell checkpoints, which normally pause division until chromosomes are lined up, may add to errors.12PubMed. Faulty spindle checkpoint and cohesion protein activities predispose oocytes to premature chromosome separation and aneuploidy
Crossovers matter too. Before separating, paired chromosomes swap pieces, and these swaps help hold them in place.13PubMed. Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II Too few swaps near the center raises first-round errors, while a swap too close to the center raises second-round errors for chromosome 21.14PubMed. Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II
A father’s age shows no link to the extra chromosome in Down syndrome.15PubMed Central. Investigation of Factors Associated With Paternal Nondisjunction of Chromosome 21 Older men do make slightly more sperm with extra sex chromosomes, though.16PubMed. Non-disjunction in human sperm: evidence for an effect of increasing paternal age The plastic chemical BPA caused these errors in mouse eggs at everyday-level doses, but this has not been shown in people.17PubMed. Bisphenol a exposure causes meiotic aneuploidy in the female mouse
Down Syndrome and Other Trisomies
A trisomy means three copies of a chromosome instead of two. Down syndrome, or trisomy 21, comes in three forms: a full extra chromosome 21, a mosaic form, and a translocation form where the extra chromosome 21 is attached to another chromosome.18PubMed Central. Down Syndrome – Genetics and Cardiogenetics. In one series of 117 people, about 84% had the full trisomy, 11% were mosaic and 5% had translocation.19Journal of the Anatomical Society of India. Cytogenetic variations in a series of cases of Down Syndrome So most Down syndrome comes straight from nondisjunction, while the translocation type can run in families.
Full trisomy 21 usually arises from a chance error in one egg or sperm, so the chance of it happening again in a later pregnancy is generally low, though a little above baseline. The translocation form can sometimes be traced to a parent who carries a rearranged chromosome without having the condition. When a child has translocation Down syndrome, doctors commonly offer to check the parents’ chromosomes to find out, and the result guides genetic counseling about the chance of recurrence.
In the mosaic form, only some of the person’s cells carry the extra chromosome 21, because the error happened after fertilization. Features can be milder in some people, but the range is wide, and the proportion of affected cells does not reliably predict how someone will be affected.
Trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) cause serious heart and developmental problems, and many babies do not survive the first year. Chromosomes 13, 18 and 21 carry fewer genes than the others, which is why these are the trisomies that can survive birth. Extra copies of larger chromosomes disrupt development too much, and those pregnancies usually end very early.
Sex Chromosome Conditions
Klinefelter syndrome is a male with an extra X (XXY). It is caused by an error in egg-making in either round, or in the first round of sperm-making.20glowm.com. Klinefelter Syndrome In one group of 14 men, 12 had two different X chromosomes, pointing to first-round errors.21PubMed Central. Integrated functional genomic analyses of Klinefelter and Turner syndromes reveal global network effects of altered X chromosome dosage The condition 47,XYY comes from a sperm error that leaves an extra Y in every cell.22MedlinePlus Genetics. 47,XYY syndrome Triple X (XXX) is a female with an extra X, and it often goes unnoticed.
Turner syndrome is a female with a single X (45,X). It is the only missing whole chromosome a baby commonly survives, and it is often mosaic.
These conditions tend to be milder than an extra chromosome 21. The body switches off extra X chromosomes in most cells, and the Y carries few genes, so the effects are much smaller.
Many people with XXY, XYY or triple X have mild or no obvious signs, so they may never be diagnosed, or they are found by chance during testing for something else. Klinefelter syndrome is often noticed in adolescence or adulthood, through tall stature, delayed or incomplete puberty, or difficulty with fertility. Turner syndrome is more often noticed earlier, through short stature and missing or delayed puberty. Hormone treatment helps in both Klinefelter and Turner syndrome.
Nondisjunction and Miscarriage
About half of first-trimester miscarriages have abnormal chromosomes: 50.1% in a study of 1,000 cases.23PubMed Central. Chromosomal abnormalities in products of conception of first-trimester miscarriages detected by conventional cytogenetic analysis: a review of 1000 cases Trisomies make up about 60% of those errors.24PubMed Central. Chromosomal abnormalities in products of conception of first-trimester miscarriages detected by conventional cytogenetic analysis: a review of 1000 cases Trisomies 16 and 22, which are almost never seen in born babies, are among the most common in miscarriages.25PubMed Central. Incidence and Types of Chromosomal Abnormalities in First Trimester Spontaneous Miscarriages: a Greek Single-Center Prospective Study
So an early miscarriage is often the body ending a pregnancy that could not develop, not the result of exercise, stress, or a single missed precaution. Many such losses happen so early that the person has only a late period or a short positive test.
After one miscarriage caused by a trisomy, the chance that the next pregnancy has the same kind of error is usually only modestly higher than average. Most people who have one loss go on to have a healthy pregnancy later. Testing the tissue can show whether a chromosome error was involved, and because such errors are so common in miscarriage, a single loss rarely calls for extensive investigation. Repeated losses are a reason to ask about parental chromosome testing.
Errors After Fertilization and Mosaicism
Nondisjunction can also happen in the embryo’s own cells. The very first division of a human embryo is error-prone; in one small study a third showed chromosome missorting.26PubMed Central. The first mitotic division of human embryos is highly error prone The first few divisions rely on machinery and supplies left over from the egg, and the embryo’s checking systems seem to work imperfectly. A missorting then can leave different cells with different chromosome counts. About 80% of embryos with mixed chromosome 21 counts started from a normal fertilized egg.27PubMed. Chromosome 21 mosaic human preimplantation embryos predominantly arise from diploid conceptions
Sometimes an embryo with an extra chromosome later loses one copy. If both remaining copies came from one parent, conditions like Prader-Willi syndrome can result.28PubMed. Mosaic maternal uniparental disomy of chromosome 15 in Prader-Willi syndrome: utility of genome-wide SNP array
Mosaic means only some cells carry the error, and the effects depend on which tissues are involved and what share of cells. IVF embryo test reports may label embryos as mosaic. Some mosaic embryos appear to correct themselves over time, as abnormal cells divide poorly or are outgrown, but this is not guaranteed. Others lead to miscarriage, and some lead to a baby with a mosaic condition, depending on the chromosome and how many cells are affected. A genetic counselor is the person to ask about a specific report.
How Chromosome Errors Are Found in Pregnancy
NIPT, a blood test of the mother that looks at fragments of the baby’s DNA, can be done from about 10 weeks. The blood screening test is now offered to pregnant people of any age, though older age raises the chance of a positive result. It is a screening test, not a diagnosis, so chorionic villus sampling or amniocentesis is still needed to confirm a trisomy.29PubMed Central. Non-Invasive Prenatal Testing (NIPT): Reliability, Challenges, and Future Directions NIPT is highly accurate for common trisomies but can misread mosaic results, as it did in one Turner mosaic case.30PubMed Central. Comparison of Efficiencies of Non-invasive Prenatal Testing, Karyotyping, and Chromosomal Micro-Array for Diagnosing Fetal Chromosomal Anomalies in the Second and Third Trimesters
CVS takes a tiny sample of placenta at about 10 to 13 weeks. Amniocentesis takes fluid from around the baby from about 15 weeks. Both give a definite chromosome count, and both carry a small risk of miscarriage.
Researchers are testing ways to strengthen the chromosome glue in aging eggs, so far only in mice and lab cells.31Edinburgh Research Archive. Manipulating sister chromatid cohesion to prevent aneuploidy Future fertility treatment may aim to cut errors in eggs, but none exists yet.32PubMed Central. A versatile cohesion manipulation system probes female reproductive age-related egg aneuploidy