Can identical twins be male and female?
Twins of different genders are universally accepted as a sound basis for a clinical determination that boy and girl twins are not identical. However, in extremely rare cases, boy and girl twins can stem from the same fertilized egg, like identical twins do. Twins, who stem from the same fertilized egg, are called monozygotic twins. Washington State Twin Registry stresses that it’s an extremely rare event as only a handful of cases have been reported in the medical literature. Inaccurate copying of the sex chromosomes in male twins, that began as monozygotic twins, have resulted into a few cases of male/female twins pairs. These twins are genetically identical, except for their sex chromosomes. Here are two examples from Washington State Twin Registry on how it could happen.
- There could be a glitch in copying sex chromosomes, resulting in a male zygote (fertilized egg) that starts out with an extra X chromosome (XXY). This is opposed to the usual XX for a girl or XY for a boy. Through a complicated series of events, this zygote could result in the birth of monozygotic twins who are male (XY) and female (XX).
- After a male (XY) zygote is formed, it starts to develop and splits into two embryos (identical male twins). Early in this process, inaccurate copying of the sex chromosomes could result in the loss of the Y chromosome in some cells. The embryo that is formed from the cells missing the Y chromosome would develop into a female (XO). The other embryo would still develop into a male (XY). The twin with only one X chromosome would be female, but her cells would have only one copy of the X chromosome. This condition is called Turner Syndrome. Turner Syndrome is a chromosomal condition that affects development in females.
Read more about these examples and references
What are identical twins?
A woman ovulates and releases one egg. The egg is fertilized by a sperm cell (now called zygote). The zygote splits into two within a few days after fertilization. The two zygotes are developing rapidly into two embryos that have the same chromosomes and genes. However, although they share the same genetic characteristics, identical twins are not necessarily exactly alike. Environmental differences in the womb – and when the twins are growing up – can affect the way they look and behave. Read more about the environmental differences.
Identical twins can share a placenta or have one each. They can be either di-di twins (dichorionic diamniotic), mo-di twins (monochorionic diamniotic) or mo-mo twins (monochorionic monoamniotic). Mo-mo twins are extremely rare. Read more about how identical twins are formed.
What are fraternal twins?
A women ovulates and releases two eggs. The eggs are fertilized by two different sperm cells. Fraternal twins are no more closely related in terms of DNA than regular siblings (if they are conceived by the same mother and father). Fraternal twins will share about 50 percent of their DNA.