Turner Syndrome in Discordant Dizygotic Twins: Genetic Origins and Twin Relations
In October 2025, I received an email message from a woman (T1) who explained that she was a dizygotic (DZ) twin whose twin sister (T2) was affected with Turner syndrome (TS). The chromosomal constitution of affected females is 45, X0, but there are variations. For example, T2’s genetic makeup includes one full X chromosome in some cells and one partial X chromosome in other cells, a condition known as mosaicism. TS results from non-disjunction (failure to separate) of chromosomes during meiosis — it is a random process that can lead to the loss of either an X chromosome from an egg or an X or a Y chromosome from a sperm (Knopik, et al., Reference Knopik, Neiderhiser, DeFries and Plomin2017). Structural abnormalities of the X chromosome during cell division can also result in TS (Sharma & Kikker, Reference Sharma and Kikker2025).
TS was first discovered by Dr Henry Turner, in 1938. The condition is also known as Ullrich-Turner, Bonnevie-Ullrich Turner syndrome, and gonadal dysgenesis (Turner Syndrome Foundation, 2024). As the second most common genetic disorder, TS has been estimated to affect 1/2000−1/2500 females (Sharma & Kitter, Reference Sharma and Kikker2025); however, it may occur more frequently because the associated symptoms and severity vary widely, such that cases of TS may be misdiagnosed or remain undetected. The symptoms variously include short stature, ear infections, osteoporosis, diabetes, infertility, problems involving the heart, kidneys, and assisted reproductive treatments (if patients try to become pregnant). TS is also linked to a range of cognitive, emotional, and social problems. Some visual-spatial skills are impaired, specifically those involving perception and drawing (Temple & Carney, Reference Temple and Carney1995). The verbal skills of TS females do not appear to be affected, so their lower average intelligence test scores mostly reflect their spatial deficits.
When I wrote my first book, Entwined Lives (Segal, Reference Segal1999), I discovered a study showing that when 45/XO girls inherited their father’s X chromosome they were happier, had more friends, and were more aware of others’ feelings, relative to 45/XO girls who inherited their mother’s X chromosome (Skuse et al., Reference Skuse, James, Bishop, Coppin, Dalton, Aamodt-Leeper, Bacarese-Hamilton, Creswell, McGurk and Jacobs1997). This finding suggested genomic imprinting in which a gene (or genes) are expressed differently, depending on which parent transmitted them to their offspring. I wondered if subsequent studies had been conducted along this line. I since discovered a TS study showing that a maternally derived X chromosome was associated with lower total and lower density lipoprotein cholesterol and higher body mass index than if the X chromosome was paternally derived. In contrast, ocular difficulties and higher academic achievement were observed in conjunction with a paternally derived X chromosome (Sagi et al., Reference Sagi, Zuckerman-Levin, Gawlik, Ghizzoni, Buyukgebiz, Rakover, Bistritzer, Admoni, Vottero, Baruch, Fares, Malecka-Tendera and Hochberg2007). Unfortunately, behavioral traits like happiness were not investigated. Interestingly, a report comparing the sperm of a Turner syndrome father to the sperm of volunteers detected an increased tendency toward meiotic nondisjunction in the sperm of the Turner syndrome father (Martinez-Pasarell et al., Reference Martinez-Pasarell, Templado, Vicens-Calvet, Egozcue and Nogues1999).
T1, who is now 43 years of age, revealed that her twin sister was diagnosed with TS just 10 years earlier. This is both surprising and not surprising — it is surprising because of the wealth of genetic knowledge and technology that is currently available; however, it is not surprising because TS’s symptoms and severity occur in a number of different conditions; furthermore, T2 had a partial form of the disorder.
In speaking with T1, I learned why TS’s diagnosis was delayed: When the twins were born, the nurse informed their mother that she had beautiful monozygotic (MZ) twin girls. The mother asked whether the nurse had really looked at them, saying that they appeared completely different. The doctor subsequently identified a single placenta, but decided the twins were DZ; it was later confirmed that two separate placentae had fused. Over the years, the twins’ mother encouraged her physicians to explain her second born twin’s developmental difficulties and physical handicaps, but the physicians became defensive. Finally, 10 years ago the family consulted a new doctor because T2 was experiencing abnormal menstrual periods — he immediately asked if she had been tested for TS. Note that XO females typically do not menstruate, but T2’s mosaicism is a likely explanation for why she did have periods.
T1 was eager to share her life experiences as a typical individual raised alongside a twin sister with various developmental delays. It is important to tell her story because there is limited research on situations like hers. T1 was also curious to learn how commonly chromosomal anomalies occur among twins and if she could possibly transmit the condition to her children. I will tell her story first.
T1’s Story
T1 and T2 were born on June 23, 1982, and delivered naturally 11 minutes apart without complications. Their mother was 26 and their father was 30 years of age at the time. The multiple pregnancy was not detected until the 6th month when two heartbeats were identified. T1 was born first and weighed 6 pounds, 11 ounces; T2 was born second and weighed 4 pounds, 5 ounces. The twins have a brother who is older by 5 years.
T1 is a science teacher with a master’s degree in curriculum, instruction, and assessment, as well as education credits. She is married and has two children. T2 has been part of special education programs her entire life, leading to a certificate of high school completion. She received state assistance enabling her to enroll in a work program, but she is unable to maintain employment for long periods and continues to have speech and language delays that may be unrelated to 45/XO. However, T2’s memory for important dates, phone numbers and people’s names is excellent, and she is very sociable. 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 currently differ by 3 inches in height (T1 is taller) and differ by approximately 40 pounds in weight (T2 is heavier). They are shown as infants, young children, and adults in Figures 1, 2 and 3.
Figure 1. T1 (right) and T2 (left) as infants. Photo courtesy: T1.
Figure 2. T1 (right) and T2 (left) as young children. Photo courtesy: T1.
Figure 3. T1 (left) and T2 (right) celebrating their fortieth birthday together. Photo courtesy: T1.
T1 was very protective of T2 as they were growing up — other children knew not to tease T2 when T1 was nearby. T1 explained, ‘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. I was this little blonde girl, but they quickly knew not to ever say anything to her [my twin] — she was always so kind that it was hard to hurt her, so people learned quickly.’ T1 admitted that her parents focused considerable attention on her twin sister, leaving T1 on her own much of the time. As a result, she became very independent, almost to a fault. She credits her choice of a teaching profession to the care she had to extend to her twin. However, there were difficulties along the way.
As a teenager, before T1 truly grasped her sister’s situation, she objected that she was expected to perform certain household chores, such as doing the laundry, whereas her twin was not. T1 was also reluctant and embarrassed to bring friends and boyfriends to her home, because she did not want them to be disrespectful to her sister, and she did not want to hear comments such as ‘there are two of you’.
I wondered if T1 felt resentful about forfeiting opportunities and parental attention because of her sister. She denied this, then discussed the guilt she felt from being the unaffected twin. Her guilt stemmed from knowing she could do so many things that T2 could not do, like drive a car, dance at a high school prom, and have children. In fact, T1 delayed getting her driver’s license for a year because T2 could not get one. As we spoke about these things, T1 began to cry.
T1 and T2 now live in different parts of their state, but they have never gone for long periods of time without visits. They usually celebrate their birthdays together, something that their mother encourages. T1 recalls that she and her sister had played with dolls together, saying that the ‘hardest part‘ of T2’s situation is that T2 had always wanted to be a mother and cannot be. However, T2 is content with being an aunt to T1’s two children and they fully accept her.
As a teacher and as a twin with a disabled sister, T1 is highly sensitive to situations similar to hers. She referenced a young twin pair in her school that is discordant for Down syndrome or trisomy 21. She is interested in knowing more about how the unaffected twin manages the situation, and how the twins’ parents divide their time between their two children. Hopefully, currently increased twinning rates and greater attention to multiple birth children will encourage research that can address these questions.
Turner Syndrome: Frequency in Twins
I was interested in knowing how often TS occurs in DZ twins and the proportion of sets that are discordant for the condition. However, several literature searches that included the terms ‘dizygotic’, ‘XO’ and ‘Turner syndrome’ retrieved mostly references on concordant and discordant MZ twins. In a strange twist, as I was researching this article, I emptied an old book bag and discovered a January 2025 note from a woman with a question about her opposite-sex twins who were discordant for 45/XO; of course, her daughter was the affected twin. I attempted to contact her, but I was unsuccessful.
In answer to T1’s questions, I was unable to find statistics on DZ twins and TS; however, it is the case that TS is rare among MZ twins. As indicated above, its frequency is 1/2000−-1/2500 female births, making it the most common chromosomal anomaly in females. It is worth noting that TS occurs less often among twins than nontwins — since 1938 and as of 2024, only 30 cases have been reported worldwide. In addition, there have been only six reports of MZ nonmosaic twins with TS (Rehman et al., Reference Rehman, Shariff, Arif and Memon2024).
Many studies have found a lower frequency of congenital anomalies among nontwin than twin pregnancies, in which chromosomal anomalies account for 25%. In fact, in a comprehensive analysis, the presence of trisomy 21 proved lower in twins than in singletons, especially among MZ pairs (Konishi et al., Reference Konishi, Samura, Muromoto, Okamoto, Takahashi, Kasai, Ichikawa, Yamada, Kato, Sato, Hamada, Nakanami, Machi, Ichizuka, Sunami, Tanaka, Yonetani, Kamei, Nagamatsu and Sago2022). Hopefully, better data on DZ twins and TS will be forthcoming. Finally, it is believed that because TS results from random nondisjunction, this tendency is not transmitted from parent to child; however, recall the report by Martinez-Pasarell et al. (Reference Martinez-Pasarell, Templado, Vicens-Calvet, Egozcue and Nogues1999) cited above.
Twin Research Reviews
Prevention of Premature Twin Birth
Researchers from Vietnam and Australia compared the effects of vaginal progesterone and pessary for preventing premature birth in women with twin pregnancies whose cervical length was under 38 mm. (A pessary is a device inserted into the vagina for support of pelvic organs; Cleveland Clinic, 2025). Participants included 300 women pregnant with twins, organized into the two groups of 150 each (women receiving either cervical pessary or 400 mg progesterone). The pregnant women were recruited for study from the My Duc Hospital, in Ho Chi Minh City, Vietnam, between March 2016 and June 2017 (Dang et al., Reference Dang, Nguyen, Pham, He, Vu, Phan, Le, Le, Vuong and Mol2019). At less than 34 weeks of gestation, the frequency of preterm birth was the same for the two groups. However, some additional analyses suggested that women whose cervical length was 28 mm or less might benefit more from cervical pessary.
Twin Gestation with Hydatidiform Mole
Full and partial hydatidiform moles are atypical gestational features. Hydatidiform moles are rare masses or growths that can form inside the uterus at the start of a pregnancy (MedlinePlus, 2024). Some types are paternal in origin, while others include both paternal and maternal contributions. Xing et al. (Reference Xing, Adams, Zou, Morsberger, Scanga, Gao, Barker, Vang and Ronnett2022) reported a study of 21 cases identified through records from the Johns Hopkins Hospital, in Baltimore, Maryland. Genotyping revealed the parental chromosomal contributions to each twin/multiple gestation, as well as the zygosity for example, it was found that most pregnancies with complete hydatidiform mole or CHM (71% or 5/7) most likely derived from one oocyte fertilized by two sperm. This research promises to offer valuable insights into twinning processes, although much remains to be explored.
Update on Feingold Syndrome Twins
In an earlier issue of Twin Research and Human Genetics I profiled a pair of 18-month-old MZ male twins with Feingold syndrome (Segal, Reference Segal2025). As I explained, both Type 1 and Type 2 ‘involve characteristic features of the hands (shortening of the second and fifth fingers; inward curvature of the fifth finger), head (small head; small jaw), and feet (fusion of the second and third or fourth and fifth toes). Type 1 usually includes blockage of the digestive system, a feature that is not present in Type 2.
I was interested in the progress of this pair and in October 2025 I received an updated report from their mother: ‘They’re doing well! They are 21 months old now. Lots more baby talk babbling, no true speech yet. But they do seem to understand a lot and follow commands and try to communicate more. We had a developmental evaluation, and they [the assessment team] seemed pretty confident they [the twins] don’t have autism, just a delay that they will hopefully catch up on soon.’ It is reassuring to know that therapies for rare conditions are being developed and are improving the lives of affected twins and nontwins.
Qualitative MZ Twin Difference Studies
Marzecki et al. (Reference Marzecki, Widanaralalage, Bhandoh, McAdams, Ahmadzadeh and Zavos2025) remind us that most environmental factors implicated in individual differences are not shared between family members. They propose that an alternative approach to quantitative analyses is the qualitative study of MZ co-twin differences. Specifically, they suggest that MZ co-twins be interviewed in depth and detail to identify idiosyncratic events associated with differences between them. A review of seven qualitative twin difference studies, covering a range of ages and phenotypes, was performed. Personality and sexual orientation were highlighted as uniquely linked to differences between co-MZ twins. It was concluded that nonshared environmental events are partly idiosyncratic and conducive to qualitative study in partnership with quantitative methods.
Media
Identical Twins Turn 100 Years of Age
Identical male twins celebrated their 100th birthday together on September 1, 2025 (Supermission, 2025). The brothers shared a century that included wars, various hardships, and a dramatically changing world — when they were teenagers in 1938 there was no internet or AI. The two are shown standing side by side with a cake that reads ‘100 Happy Birthday’. The twins still look remarkably alike, with matching eyeglasses, partially bald heads, and half smiles. They also appear to be matched in height and weight. These similarities are evidence of twin research showing genetic influences on physical development (Koenis et al., Reference Koenis, Brouwer, van Baal, van Soelen, Peper, van Leeuwen, Delemarre-van de Waal, Boomsma and Hulshoff Pol2013).
Twins in Famous Families
Most famous people are not twins, but some have twins in their families. Former United States President Thomas Jefferson often commented on the importance of family. Jefferson was raised in Virginia in a family that included 10 children, among them fraternal twin siblings, Anne and Randolph The late singer Michael Jackson had five brothers and three sisters in his family. His brother Marlon, with whom he sang, was a twin whose brother Brandon died shortly after birth (Adcock, Reference Adcock2025). We do not know the twin type of twin pair they were — given the musical talents of the Jackson family members, we can wonder if, and how, Brandon’s presence might have affected his siblings’ performances had he lived.
Celebration of Yorùbá Twins of Nigeria
In 2002, the London Nigerian art collector, Kayode Adegbola, received a pair of Yorùbá twin statuettes, known as ibeji (Adedayo, Reference Adedayo2025). This gift triggered the Ibeji Project to revive the Yorùbá’s unique history of twinning. Twins are honored among the Yorùbá people of Nigeria, capturing their special spiritual, cultural, and artistic qualities; some statues are carved after living twins. In October 2024, Adegbola organized a display of ibeji in conjunction with London’s Tafeta Gallery. A picture of this captivating collection is provided in the media coverage of this collection.
In an earlier issue of Twin Research and Human Genetics, I described a display of ibeji exhibited at the University of California’s Fowler Museum, in Los Angeles (Segal, Reference Segal2014). The show, titled ‘Double Fortune, Double Trouble: Art for Twins Among the Yorùbá’, included over 250 carved wooden statues or memorial figures. Each one, carved by a Nigerian artist, reflected different regional styles and materials. My article also includes detailed information and traditions linked to these statues, and a snapshot of this impressive collection.
Identical Artistic Partners
Thumbing through a recent issue of The New Yorker magazine, I discovered the Jiménez twins, Erick and Elliot (Zatarain, Reference Zatarain2025). The twins were born in Miami, Florida soon after their parents relocated to the United States from Cuba. (A photograph of the twins strongly suggests they are identical.) Both twins are photographers who read a 1954 book titled, El Monte, by Cuban author Lydia Cabrera — one twin claims they were in their twenties at the time, although his brother is less certain. Regardless, when they first encountered the book, both twins were uninvolved in American culture, but that changed — at age 36, they reread the book as Americans. They went on to use El Monte to create a photograph series that is on display — their first solo exhibition — at Miami’s Pérez Art Museum. Their artistic work is described as reflecting multiple dualities; I wonder if this quality reflects their multiple birth status. The exhibit closes on March 22, 2026.
Rare Conjoined Twins Separated
A rare pair of conjoined twins was surgically separated at Seattle’s Children’s Hospital in 2025 (Sacks, Reference Sacks2025). The twin girls, Ally and El, were joined at the pelvis; there are a variety of classifications for twins connected in this way, depending upon their positioning and shared features. The operation lasted for 18 hours and was performed when the twins were 15 months old. Prior to the procedure, the medical team familiarized themselves with the twins’ anatomy using magnetic resonance imaging (MRI), fluoroscopy, and endoscopy. The twins are still under the care of hospital staff but are faring well physically. They are receiving treatment and therapy from rehabilitation medical professionals, occupational therapists, and language specialists. The are expected to be released from the hospital in May 2026.
