"Are they identical?" As the mother of a boy girl set of twins, that question sometimes annoyed me. It seemed to me that it should be obvious to anyone observing them that opposite sex twins are not identical, (the splitting of one egg fertilized by one sperm), but fraternal (two eggs being fertilized during the same ovulation cycle, by two different sperm). The nature of the word identical invokes thoughts of two or more things being exact in appearance.

In the world of twins, conventional wisdom stated that the only possible combination of identical twins was either boy/boy or girl/girl. The idea of boy/girl identical twins was not seriously undertaken, until recently.

Although extremely rare, there are now documented cases of boy/girl identical twins.

A brief science lesson as to how the phenomena could realistically take place would begin with an egg being fertilized by one sperm, then dividing itself into two separate eggs, which is also the exact way identical same-gendered twins begin life. However, in the case of boy/girl identical twins, those fertilized eggs initially both contain male DNA only.

We all learned in high school biology that male DNA have an X-chromosome and a Y-chromosome and it’s the Y-chromosome which makes him male and female DNA have two X-chromosomes. Well, in the case of mixed-gendered identical twins, which always begin male, the Y-chromosome disappears, and science has offered no explanation for it. As a result of losing the Y-chromosome, the male twin becomes female with an XO DNA chromosome pairing. The name for this condition is called Turner's Syndrome.

According to the Turner's Syndrome Society of the United States, the disease is a chromosomal affliction that affects 1 in 2000 births. The most prominent characteristics of Turner's Syndrome is short stature and infertility, with the average woman afflicted with the condition reaching a height of 4'8". Thirty percent of women with Turner's Syndrome also suffer from kidney and heart ailments, according to the group, which is the largest advocacy organization for those associated with the disease.

Most girls born with Turner's Syndrome did not begin as a twin, as most pregnancies where one of the fetuses has the disease ends in miscarriage. The overwhelming majority of boy/girl twins are born fraternal. Scientific studies have shown that there are only 6 known cases of identical boy girl twins worldwide as most go unreported.