Opposite sex in identical twins: Does it really happen?

From conventional wisdom, we know that in humans twinning results from one of two possible events: 

1. The mother's ovaries release two oocytes (eggs) and two sperm from the father fertilize each the eggs = dizygotic or fraternal twins.
   
   
2. The ovaries release one oocyte and it is fertilized by one sperm; however, the zygote (the fertilized egg) splits into two separate embryos = monozygotic or identical twins.

Since fraternal twins come from completely unique gametes (sperm and egg), fraternal twins can be of opposite sex or the same sex. For example, if a sperm carrying a Y chromosome fertilized one oocyte and another sperm carrying an X chromosome fertilized the other (remember: the oocytes from the mother all carry one X chromosome) the result would be one boy (XY) and one girl (XX) fetus. If sex chromosomes from each sperm were both X chromosomes, both fetuses would be female. Similarly, if both spermatozoa carried Y chromosomes, the resulting fetuses would both be male.

Since identical twins come from the same set of gametes, they are "always" the same sex. Identical twins are either both male or both female fetuses because their chromosomes are identical to each other. In the classical sense and under normal conditions, it is not possible to have monozygotic twins of opposite sexes.

Biology; however, does not always follow the same rules that biologists and doctors do. For every rule or well-accepted theory, biology allows an anomaly to occur to keep things interesting. A short string of case reports in medical journals have described the extremely rare occurrence of opposite sex (sex discordant) monozygotic twins as early as 1961. The anomaly of opposite sex monozygotic twins occurs due to chromosomal defects and/or gene mutations in sex-determining genes.

One of the first reports of this phenomenon came from Turpin et al. in 1961 where the authors described a set of monozygotic twins where the male displayed a normal XY karyotype (chromosome analysis by microscopy), while the female displayed an abnormal XO karyotype--only had one sex chromosome. It was not until much later that an explanation for this occurrence was clear, yet it was apparent that one embryo lost its copy of the Y chromosome at some point in development and thus developed as female (with Turner's syndrome). Nonetheless, other researchers and doctors continued reporting these rare cases following the Turpin et al. report. 

In 1966, Edward et al. reported a new karyotype associated with this phenomenon, where the female exhibited a 45X/46XY mosaicism (chromosome content was not the same in each of her cells), while the male was phenotypically normal with a 45X karyotype (missing one sex chromosome). Nine years later, Karp et al. published on monozygotic twins where the male had a normal 46XY karyotype, yet the female had a mosaic 46XY/45X karyotype which the authors speculated was caused by structural abnormalities in the Y chromosome. The next year, Schmidt et al. also described a case of monozygotic twins with a normal 46XY male and an abnormal 46XY/45X mosaic female. As in many of the other studies, Schmidt et al. were able to confirm that the twins were monozygotic by serum protein tests--comparing blood group antigens and serum proteins of the twins to that of the parents. The authors speculated that the monozygotic twins ended up with opposite sex resulting from the original 46XY zygote splitting into two identical embryos, yet one of the embryos lost the Y chromosome from most of its cells after suffering an error in mitosis (cell replication process). The embryo that lost the Y chromosome in most of its cells still retained the Y in some of its cells, which is what led to the 46XY/45X mosaic karyotype. Since the Y chromosome was not present in enough of the necessary tissues, this likely led to the mosaic embryo becoming female.

The next reporting of this phenomenon was not seen until 24 years later when Wachtel et al. from the University of Tennessee delivered a case report on 46XY monozygotic twins with opposite sex. This case was interesting because both twins had an identical karyotype and both possessed a normal SRY gene (testis-determining gene), yet were of opposite sex. The authors offered up a possible explanation for the discordant sex in the monozygotic twins:

• The sex-determining genes downstream (needed after) of SRY may have experienced a mutation in the female.
• The promoter (gene sequence needed to express SRY) for SRY may have been mutated in the female.
• The possibility of cryptic mosaicism (mosaic chromosomes that weren't detected). Perhaps the twins' real karyotype was 46XY/45X mosaic and the 45X cells were predominantly in the gonads for the female fetus.

Conclusion: In practically all cases of twins, dizygotic (fraternal) twins can be either the same sex or opposite sexes, while monozygotic twins MUST be the same sex, by definition. Discordant sex in monozygotic twins is extremely rare (the only reported cases in medical journals have been included here) and only happens when sex chromosomes are lost or imbalanced (mosaic) in one of the two embryos, or if there is a mutation in a sex-determining gene.

Image credit:

https://wespeakscience.com/identical-twins/

Referenced Articles:

1. TURPIN, R., LEJEUNE, J., LAFOURCADE, J., CHIGOT, P. L. & SALMON, C. [Presumption of monozygotism in spite of a sexual dimorphism: XY male subject and haploid X neuter subject]. C. R. Hebd. Seances Acad. Sci. 252, 2945–6 (1961). 
2. Edwards, J. H., Dent, T. & Kahn, J. Monozygotic twins of different sex. J. Med. Genet. 3, 117–23 (1966).
3. Karp, L., Bryant, J. I., Tagatz, G., Giblett, E. & Fialkow, P. J. The occurrence of gonadal dysgenesis in association with monozygotic twinning. J. Med. Genet. 12, 70–8 (1975).
4. Schmidt, R., Sobel, E. H., Nitowsky, H. M., Dar, H. & Allen, F. H. Monozygotic twins discordant for sex. J. Med. Genet. 13, 64–68 (1976). PMID: 944767
5. Wachtel, S. S., Somkuti, S. G. & Schinfeld, J. S. Monozygotic twins of opposite sex. Cytogenet. Cell Genet. 91, 293–5 (2000).
6. Somkuti, S. G. et al. 46,XY monozygotic twins with discordant sex phenotype. Fertil. Steril. 74, 1254–1256 (2000).