I receive many emails from twins and their families with important stories to tell. In October 2025, I received a message from a fraternal female twin to say that she had a twin sister with Turner syndrome. (For reasons of confidentiality, I will refer to the unaffected twin as Twin 1 and the affected sister as Twin 2.)

What is Turner syndrome?

Dr. Henry Turner first discovered Turner syndrome in 1938 (Turner Syndrome Foundation, 2024). This condition occurs only in females and is identified by a chromosomal constitution of 45, XO. Typical females have two X chromosomes as indicated by a chromosomal constitution of 46, XX. In the case of Turner syndrome, it seems that one X chromosome is lost very early in the reproductive process.

If, for example, an egg loses one X chromosome and is fertilized by a healthy sperm, the result is XO. It is also possible that a sperm might lose an X chromosome or a Y chromosome—if that sperm fertilizes a healthy egg, then the result would be the same. However, due to the mysteries of reproduction and cell division, a female diagnosed with Turner syndrome might have some cells with the typical XX makeup and some cells with XO, a situation known as mosaicism.

It is recognized as the second most common genetic disorder, estimated to affect between 1/2,000 and 1/2,500 females. In fact, the condition may occur more frequently because the symptoms and severity vary widely, such that some cases of Turner syndrome may be misdiagnosed or even undetected (Sharma & Kitter, 2025). The symptoms vary and can include short stature, ear infections, osteoporosis, diabetes, infertility, and problems with the heart and kidneys. Turner syndrome has also been associated with certain cognitive, emotional, and social problems. Some visual-spatial skills, such as those involving perception and construction, are impaired (Temple & Carney, 1995). In contrast, the verbal skills of these females do not appear to be affected, so their lower average intelligence test scores mostly reflect their spatial problems.

Interestingly, there is an older study showing that when Turner’s syndrome girls inherited their father’s X chromosome, they were happier, had more friends, and were more aware of others’ feelings, relative to affected girls who inherited their mother’s X chromosome (Skuse et al., 1997). When genetic expression in a child depends on which parent transmitted the genes, it is called genomic imprinting. A more recent study showed that if the X chromosome comes from the mother, it is associated with lower total and lower-density lipoprotein cholesterol and higher body mass index than if the X chromosome comes from the father. In contrast, ocular difficulties and higher academic achievement were observed if the X chromosome came from the father (Sagi et al., 2007).

The experience of two twins

Twin 1, who is now 43 years old, explained that her twin sister was diagnosed with Turner syndrome at the age of 33! This is surprising and not surprising. It is surprising because there is so much genetic technology available to physicians and scientists. It is not surprising because the condition’s symptoms and severity characterize many medical disorders. Moreover, Twin 2 had a partial or mosaic form of the disorder, probably making it harder to identify.

According to Twin 1 (whom I interviewed), their mother tried for a long time to obtain accurate information about her affected daughter’s developmental difficulties and physical handicaps, but the doctors grew defensive. Finally, 10 years ago, the family consulted a new doctor because Twin 2 was experiencing abnormal menstrual periods—he immediately asked if she had been tested for Turner syndrome. It should be noted that XO females typically do not menstruate, but Twin 2’s mosaicism probably accounts for why she did have periods.

Twin 1’s story may help other twins cope with having an affected twin sibling. The twins were delivered naturally, 11 minutes apart and without complications. Their mother was 26, and their father was 30 years of age at the time. Twin 1 was born first and weighed 6 pounds, 11 ounces; Twin 2 was born second and weighed 4 pounds, 5 ounces. Twins were not detected until the sixth month of pregnancy.

Twin 1 is married and has two children. She is a science teacher with a graduate degree in curriculum, instruction, and assessment, as well as several education credits. Twin 2 has been enrolled in special education programs her entire life, leading to a certificate of high school completion. She received state assistance that allowed her to participate in a work program, but she cannot maintain employment for long periods and has speech and language delays that may be unrelated to Turner syndrome. However, her memory for significant dates, phone numbers, and people’s names is excellent. She is married and has had a hysterectomy due to her difficult periods, and because she and her spouse would be unable to raise a child. The twins are shown as infants in the figure that accompanies this article; Twin 1 is on the right.

Understanding Twins Essential Reads

Twin 1 was very protective of Twin 2 as they were growing up. Twin 1 recalled, “I was a shy kid, that’s the crazy part. If anybody was ever making fun of her, this beast came out of me, and I would threaten the kids.”

Twin 1 admitted that her parents gave considerable attention to her twin sister, leaving Twin 1 on her own much of the time. As a result, she believes she became very independent. She also believes that her choice of a teaching profession reflects the care she had to give to her twin—she also acknowledged some difficulties growing up.

As a teenager, before Twin 1 fully understood her sister’s medical status, she objected that she had to perform certain household chores, whereas her twin did not. Twin 1 was also reluctant and embarrassed to invite friends and boyfriends to her home, because she did not want them to disrespect her sister, and she did not want to hear comments such as “there are two of you.” At the same time, Twin 1 confessed to guilty feelings from being the unaffected twin. Her guilt stems from knowing she could do many things that her sister could not do, like drive a car and dance at a high school prom.

Twin 1 and Twin 2 live in different parts of their state, but they see each other fairly often, especially for birthday celebrations. Twin 1 recalls that she and her sister played together with their dolls when they were young children, so a difficult part of Twin 2’s situation is that she cannot be the mother she always yearned to be. However, Twin 2 is happy being an aunt to her sister’s two children.

Unfortunately, there is relatively little research done on how having a disabled twin sibling affects the behavior and well-being of a typical twin. Given the increase in twinning rates in Western nations since 1980, from 1/60 births to about 1/31 in 2022, greater scientific and clinical attention will need to be paid to this aspect of twinship. (Note that the twinning rate has remained stable since 2022; Osterman et al., 2024).

A longer version of this article is forthcoming in the journal Twin Research and Human Genetics.* *I am grateful to Twin 1 for sharing her story and for providing photographs.

References

Osterman, M. J., Hamilton, B. E., Martin, J. A., Driscoll, A. K., & Valenzuela, C. P. (2024). Births: Final Data for 2022. National Vital Statistics Reports: From the Centers for Disease Control and Prevention, National Center for Health Statistics, National Vital Statistics System, 73(2), 1-56.

Sagi, L., Zuckerman-Levin, N., Gawlik, A., Ghizzoni, L., Buyukgebiz, A., Rakover, Y., Bistritzer, T., Admoni, O., Vottero, A., Baruch, O., Fares, F., Malecka-Tendera, E., & Hochberg, Z. (2007). Clinical significance of the parental origin of the X chromosome in Turner syndrome. The Journal of clinical endocrinology and metabolism, 92(3), 846–852. https://doi.org/10.1210/jc.2006-0158

Sharma, L., & Kikker, N.S. (2025). Turner Syndrome. National Library of Medicine: National Center for Biotechnology Information, https://www.ncbi.nlm.nih.gov/books/NBK554621/

Skuse, D. H., James, R. S., Bishop, D. V., Coppin, B., Dalton, P., Aamodt-Leeper, G., ... & Jacobs, P. A. (1997). Evidence from Turner’s syndrome of an imprinted X-linked locus affecting cognitive function. Nature, 387, 705-708.

Temple, C. M., & Carney, R. A. (1995). Patterns of spatial functioning in Turner’s syndrome. Cortex, 31(1), 109-118.