Mom just sent me this year’s newsletter from the Mid-Atlantic Twin Registry. I was telling Laura and Steph Lamb that I didn’t know many of the facts about twins in the newsletter’s “Did you know?” section. Did you know these? I didn’t even know that there was such a thing as “Half-Identical Twins”.

Identical Twins

• Only 1/3rd of all twins are identical.
• Contrary to popular belief, no genetic proof exists that identical twins “run in the family.” No hereditary influence for twinning has yet been identified.
• May have one shared placenta, two separate placentas, tor two placentas fused into one.
• They share 100% of their genetic markers.
• Are always the same sex.
• Have the same blood type.
• Not caused by fertility treatments or maternal age.
• Can result in conjoined twins or mirror image twins. Mirror image twins are created when the fertilized egg splits quite late (around days 9-12); any later and twins can be conjoined.

Fraternal Twins

• Can be hereditary on mother’s side. The tendency to hyperovulate, or release more than one egg in a cycle, is a genetic trait that can be passed from mother to daughter.
• May have two separate placentas or two placentas fused into one.
• Share about 50% of their genetic markers, or the same as signleton siblings.
• May be same sex or male/female.
• May or may not have the same blood type.
• Can be attributed to fertility treatments or advanced maternal age.

Half-Identical Twins aka “Polar-Body Twins”

• It is not known what percentage of twins is half-identical.
• May have two separate placentas or two placentas fused into one.
• Share about 75% of their genetic markers, mor than fraternals, but less than identicals.
• May be same sex or male/female.
• May or may not have the same blood type.
• These twins occur when the mother’s egg splits before fertilization and then each half is then fertilized by a sperm. This results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins.

If you want to read the newsletter yourself, a PDF is available here.

Now that I’ve posted it, I can stop carrying around this newsletter to show you next time I see you.

Stacey! This is so cool there is one and only one reaction I could have.. THE HAPPY DANCE! That’s right, doing it right now and starting to get a little bit dizzy. I don’t know what I would do without you and now without this blog! I have been deep in thought about how to solve this logistical issue of us being far apart, my thoughts below:

- kidnap you in the middle of the night (your nightmares are bad enough as is)
- shrink you and put you in my pocket (still working on the technology looks like a ‘08 idea)
- Call Scotty and have him “beam” you here (He isn’t available)

Back in the days of the overstuffed backpack, Stacey, you made 1/3 nerd the cool it is today. I only have known one other person that was as original and authentic as you, our brother, Art. While I tried on trends and personalities you guys didn’t need to, you already knew who you were without question. I remember Mom wondering who “the twins” really were; I surely was not like the boy who organized his hats by region and sport on his wall or the little girl who read Gone with the Wind in third grade and then returned it to her alphabetized library. As I searched and experimented my Sun and Moon were constants–leading me by example–by simply being yourselves.

I see that now. I am now in my glory as the yoga, gardening, historical-biography-reading nerd that is me. Thank you for being my constant, my guide, my best friend.

Nerds are cool and always have been.