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Pamela Prindle Fierro

Can a boy and a girl be identical twins?

By June 19, 2012

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It's one of the most common questions about twins: are they identical or fraternal? Well-meaning strangers -- and sometimes friends -- ask the question without really understanding what it means. The terms identical -- and the alternative, fraternal -- don't just describe whether twins look similar. It actually refers to their zygosity, and how they form.

Those that understand zygosity would never question whether a set of boy/girl were identical. Yet, parents of such twins encounter this question on a regular basis. Can boy/girl twins be identical?

Generally, the answer is no. Identical (monozygotic) twins are always same gender because they form from a single zygote that contains either male (XY) or female (XX) sex chromosomes. However, there have been -- very rare -- reported cases of a genetic mutation in male twins where one twin loses an Y chromosome and develops as a female. The female twin would be afflicted with Turner Syndrome, characterized by short stature and lack of ovarian development. This situation is extremely rare, in 99% of twins, identical twins will always be the same sex.

Of course, another explanation for gender differences in identical twins is an identical twin who undergoes a sex change operation.

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September 25, 2013 at 11:32 am
(1) James says:

Well excuse us normal people from not knowing the difference. “Those that understand zygosity would never question whether a set of boy/girl were identical.” I guess we are not all doctors….

November 7, 2013 at 5:53 pm
(2) Beth says:

Wow why so defensive? She literally only gave the reason why some people get asked the question. She didn’t say “Oh those poor dumb laypeople don’t know what they’re doing.”

And you don’t need to be a doctor to learn about zygosity. You learn about it high school biology. So. Calm yourself.

March 21, 2014 at 6:55 pm
(3) kishan says:

Thank you for the clarification.

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