Missing an entire chromosome is almost always fatal before birth. Humans normally carry 46 chromosomes arranged in 23 pairs, and losing one from most of those pairs causes a pregnancy to end in miscarriage, typically in the first trimester. The one well-known exception is the X chromosome: females born with a single X instead of the usual two (a condition called Turner syndrome) can survive and live full lives, though they face specific health challenges. Losing part of a chromosome, rather than the whole thing, is more survivable but still causes a range of developmental and medical conditions.
Why a Missing Chromosome Is Usually Fatal
Your body relies on having two working copies of most genes. When an entire chromosome is gone, hundreds or thousands of genes are cut to a single copy, and for many of those genes, one copy simply cannot produce enough protein to keep cells functioning normally. This concept is called haploinsufficiency. Some proteins operate near a functional threshold, meaning even a modest drop in quantity pushes the system below what’s needed. Others are building blocks of larger molecular machines that require precise ratios of components to assemble correctly. Halving the supply of one component can reduce the output of the whole machine by far more than 50%.
This is why missing any of the 22 numbered chromosome pairs (called autosomes) is incompatible with life. About 60% of first-trimester miscarriages involve a chromosomal abnormality, and whole-chromosome losses and gains account for roughly 73% of those abnormalities. Most embryos missing an autosome stop developing within the first few weeks, often before a person even knows they’re pregnant.
The Exception: Turner Syndrome
Turner syndrome is the only full monosomy that allows survival to birth. It occurs when a female has just one X chromosome instead of two, giving her 45 chromosomes total. The condition affects roughly 32 out of every 100,000 female live births. Even so, the vast majority of embryos with this chromosome pattern do not survive: only about 3% of 45,X pregnancies make it to term.
The two hallmark features of Turner syndrome are short stature and ovarian insufficiency. Girls with the condition typically do not experience the growth spurts other children go through during puberty, and their adult height is significantly shorter than expected for their family. The ovaries either don’t develop or stop functioning early, which means puberty may not begin on its own, menstrual periods may end early, and conceiving without fertility treatment is unlikely for most.
Beyond height and reproductive development, Turner syndrome affects multiple organ systems. Heart defects are common and detected in many cases before or shortly after birth. Kidney and urinary tract abnormalities show up in 60 to 80% of those with the condition, with horseshoe kidney (where the two kidneys are fused at the base) occurring in 20 to 45% of patients, far more often than in the general population. Duplication of the urinary tract and other structural kidney differences are also frequent. Because of these risks, doctors routinely screen the heart and kidneys of anyone diagnosed with Turner syndrome.
Physical features that may be visible at birth include a wide or webbed neck, a broad chest with widely spaced nipples, swollen hands and feet, low-set ears, and upward-turned fingernails. Not every person with Turner syndrome has all of these traits, and the severity varies widely.
How Turner Syndrome Is Managed
Growth hormone therapy is the primary treatment for short stature in Turner syndrome. It’s approved by regulatory agencies worldwide and has been shown to meaningfully increase adult height when started in childhood. The treatment involves daily injections given under the skin, typically continued for several years.
Estrogen replacement is used to trigger puberty when the ovaries don’t produce enough hormones on their own. The timing varies by individual, but the goal is to initiate breast development, bone strengthening, and other changes of puberty at an age close to what peers experience. Ongoing hormone therapy often continues into adulthood to protect bone density and cardiovascular health. With appropriate medical care and monitoring, most people with Turner syndrome lead healthy, independent lives.
Missing Part of a Chromosome
You don’t have to lose an entire chromosome for problems to arise. Sometimes a segment breaks off and is lost, a situation called partial monosomy or a chromosomal deletion. The effects depend entirely on which genes sit in the deleted region.
Cri du chat syndrome results from losing a piece of the short arm of chromosome 5. Infants with this condition have a distinctive high-pitched cry that sounds like a cat, along with intellectual disability, a small head, and characteristic facial features. Jacobsen syndrome comes from a deletion on the long arm of chromosome 11 and causes growth delays, bleeding disorders, and distinctive facial features. The 1p36 deletion syndrome, involving the tip of chromosome 1, leads to intellectual disability, seizures, and heart defects. Each of these conditions has its own spectrum of severity, partly depending on exactly how much genetic material is missing.
How a Chromosome Goes Missing
The most common cause is an error during the formation of eggs or sperm, a process called nondisjunction. Normally, when a cell divides to produce reproductive cells, chromosome pairs separate so each egg or sperm gets exactly one copy of each chromosome. In nondisjunction, the pair fails to split apart. One resulting cell ends up with an extra copy, and the other ends up with none. If the cell missing a chromosome is the one that goes on to be fertilized (or does the fertilizing), the embryo will have only 45 chromosomes.
This error can happen at two different stages. If it occurs during the first round of cell division, all four resulting reproductive cells will be abnormal. If it happens during the second round, two of the four cells will be normal and two will not. The risk of nondisjunction increases with parental age, particularly maternal age, though it can happen at any age.
When Only Some Cells Are Affected
Sometimes the chromosome loss doesn’t happen during egg or sperm formation but instead occurs after fertilization, during one of the early cell divisions of the embryo. When this happens, some cells in the body have the normal 46 chromosomes while others have only 45. This is called mosaicism.
People with mosaic Turner syndrome, for example, tend to have milder symptoms than those whose every cell is missing an X chromosome. The severity generally depends on what proportion of cells are affected and which tissues those cells ended up in. However, mosaic monosomy still carries real risks. Research on embryos with mosaic chromosome losses shows increased miscarriage rates compared to embryos with normal chromosome counts, regardless of whether the level of mosaicism is high or low.
How a Missing Chromosome Is Detected
During pregnancy, a missing chromosome can sometimes be spotted on ultrasound. Turner syndrome, for instance, may show up as large fluid collections on the back of the fetal neck, heart abnormalities, or unusual kidney structures. Definitive diagnosis requires genetic testing.
The traditional test is a karyotype, which photographs and arranges all 46 chromosomes from a cell sample to check for missing or extra ones. It’s reliable for detecting whole missing chromosomes but can only spot deletions larger than about 5 to 10 million DNA base pairs. Microarray analysis is a newer approach that catches the same large-scale problems plus much smaller deletions, down to segments of about 500,000 base pairs. In studies comparing the two, microarray analysis detected about 42% more genetic abnormalities than standard karyotyping. It’s also more likely to produce a usable result from difficult samples, succeeding in about 87% of cases where karyotyping might fail.
After birth, genetic testing is typically prompted by physical features, growth patterns, or developmental delays that suggest a chromosomal condition. A simple blood draw is usually all that’s needed to run either test.