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Reader Question: Can Identical Twins Be Different Genders?

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Updated June 04, 2014

Q: Barbara says: I know the difference in twins as far as fraternal & identical. My question is can identical twins be different sexes? I say no but my husband says they can be. Who is right? Thanks for your help.

A: Actually, you are both somewhat right. 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 a few 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, so you can tell your husband that you are 99% correct in your assumption that identical twins are always the same sex.

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

More resources:
Turner Syndrome Society
Turner Syndrome explained by About.com's Guide to Rare Diseases
Understanding Zygosity
Unique and Unusual Types of Twins
Tatum and Aiden
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