What is the estimated number of people with an extra X chromosome?
07.06.2025 04:50
Some nondisjunction data, especially in older literature, suffers bias from the way the studies are done. There is seldom any reason to test newborns for these syndromes, so we really don’t know how common these syndromes are. Early studies often got their data from institutionalized individuals, as in psychiatric hospitals and prisons. If the reasons for institutionalization were in any way shaped by the presence of XXX or XXY chromosome sets, the data would not represent the population at large. Suppose, for example, the personality issues described in the two quotes above—low self-esteem, impulsive behaviors—influenced social isolation and behavioral problems that led to psychiatric or criminal institutionalization. In short, we should be suspicious of any such data unless the researchers have specified what measures they took to avoid sampling bias.
In males, such individuals have two X’s and one Y, or in other words, XXY instead of the normal XY male condition. This is called Klinefelter syndrome and reportedly occurs in about 1 out of every 500–1000 males. Such males tend to be unusually tall with long limbs; to be developmentally delayed and show testicular atrophy and sterility; and to have female-like breasts (gynecomastia) and body hair distribution. Klinefelter individuals also reportedly can have various personality and behavioral challenges:
If source 3 is correct, it would seem to me that we couldn’t accurately call the triplo-X condition a disease, as there are no symptoms and it would go unnoticed in the vast majority of such individuals. Only persons with some reason to get genetic testing would ever know they had it—just like some people can have two spleens or three kidneys and never know it unless it’s revealed as an incidental finding of a CT scan, surgery, or something like that.
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The other shortcoming of the NORD data is that they don’t say whether they mean the prevalence of these conditions globally or US only. It’s entirely possible that the data could vary between different countries. Both conditions result from abnormalities in meiosis called nondisjunction of the sex chromosomes—failure of a person’s sex chromosomes to separate normally in anaphase and go to different sperm or egg cells so each sex cell gets only one —see my chart at the end of this essay. Nondisjunction occurs because of the failure of any one of three enzymes that normally causes chromosomes and chromatids to separate at anaphase, and/or failure of a spindle assembly checkpoint (SAC) that normally prepares mitotic and meiotic chromosomes for separation.
Therefore, when any information source like NORD reports such data, they should ideally specify whether they mean global data or data for the U.S. or some other country only, and they (or more likely their primary sources) should describe the study population and show avoidance of sampling bias. All too many sources, though, are vague on these and other issues affecting the reported prevalence of various disorders.
From my text, Anatomy & Physiology—The Unity of Form and Function, 10th ed. (McGraw Hill Education)
Klinefelter syndrome may increase the risk of: Anxiety and depression. Social, emotional and behavioral problems, such as low self-esteem, emotional immaturity and impulsiveness. Infertility and problems with sexual function.
The frequency data I’ve given are from the U.S.-based National Organization for Rare Diseases (NORD) (references 1–2) and are flawed in two ways. One is that they say “males” for XXY but “live births” for XXX—two different standards of comparison. Does “males” mean only live-born males or include the stillborn? For the female data, does “live births” mean all live births or only live-born females? Is it 1 in 1,000 females or 1 in 1,000 people? For coming from a single source, this is carelessly inconsistent reporting and we can’t make a direct comparison.
My search for better data from sources other than NORD hasn’t borne fruit so far, so this is about the best I can do for now to directly answer the QPG question. But it’s been, for me, an interesting exploration and thought exercise to start my day. Now I can log off, make some fresh coffee, and have breakfast.
Either a male or a female can have an extra X chromosome.
In females, such individuals have three X chromosomes, or XXX instead of the normal XX. This is called triplo-X syndrome and occurs in every 900 to 1,000 live births. That source says that XXX doesn’t usually cause any abnormalities in female appearance or function; at a cellular level, a cytologist can see that each cell inactivates two of the three X chromosomes; these show in cells as condensed Barr bodies and only one of the X chromosomes remains active.
Barr bodies (inactivated X chromosomes) of a triplo-X female and a normal XX female.
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But source 3 may be wrong. A UK source says of XXX females:
Another is that these nondisjunction syndromes rise with the age of the mother, and countries differ significantly in life expectancy and ages at which females give birth. A country with older new mothers, on average, might be expected to have higher incidences of these and other nondisjunction disorders (such as Down syndrome). In the Hong Kong, the average mother has her first child at 32.8 years old; in Bangladesh, 25.7. There might be even greater differences in the ages at which women have their last child. How much influence does this have on the incidence of congenital genetic disorders, including (speculatively) XXX and XXY?
Low self-esteem seems to be the most common feature, and shyness is also common in triple -X females. Receptive and expressive language disorders are common. These language disorders may be responsible for social problems, as is challenging behaviour, although this behaviour is less common.
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I suspect few readers want to know any more detail about the enzymes or the SAC than that, so I won’t go into further depth here. They do raise a possibility in my mind, however, that the prevalence of XXY and XXX syndromes could vary geographically. One reason is that varying levels of environmental contaminants might (I’m only speculating) subject persons to mutations in the genes for those enzymes or SAC proteins, as might differences in diet, nutritional status, and food quality. For environmental reasons, the incidence of nondisjunction could hypothetically be different in a large urban population of Mumbai, Accra, or Shanghai than it is in Oslo, Helsinki, or Stockholm.
Below is my chart comparing normal disjunction (separation) of the X chromosomes on the left, and two conditions on the right, including XXX, resulting from nondisjunction.
This is a Quora Prompt Generator question, not being asked by a person, and it’s flawed. But it’s something that interests me, and considering other human-to-human questions recently, some actual human beings in Quoraland may be interested in the information. So here goes.