Reader Question: Can Identical Twins Be Different Genders?
Tuesday June 17, 2008
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
• Submit a question.
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
• Submit a question.
I have 3 years old fraternal twins. Everyone gets excited at teh mention of twins so they still ask is it a boy and a girl. Well it is. The next question 80% of the time is “Are they identical?” Most of the time I politely say no. I guess not very meany people paid attention to their Biology or Anatomy classes. Any way, what really gets me is that I have had nurses ask me that very question. Ok you are a nurse, they didn’t teach you anything about identical twins in any of your science classes? To those people I can’t help but explain that identical twins can’t be opposite sexes. (execpt in the rare cases you explained)
I am a freternal twin. Whenever people ask about my other, they always assume its a girl. Its quite annoying, then I usally hae to explain to the differences between identical and fraternal twins. *Sigh*, only if people knew more.