namf
Forum Elite
Topics: 80
Posts: 312
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Hi guys,
Anyone know what criteria in the single/double placenta, chorion, amnion class would be used to definitively distinguish monozygotic vs. dyzygotic twins? For example:
Mono=1 placenta, 1 chorion, 2 amnions
Dy=2 placenta, 2 chorions, 2 amnions
Looking into Langman's Embryo, I couldn't really find definitive differences. Or maybe I'm just missing the obvious. But it seemed with all the exceptions that were illustrated in that book, that there was no defining difference. Please advise.
Any questions which cleverly call upon this knowledge--in grand USMLE fashion--would also be very useful.
Thank you!
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| namf
Forum Elite
Topics: 80
Posts: 312
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Help! <:help:> <:help:>
Thanks! 
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| PsychDr2B
Forum Elite
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Also in grand USMLE fashion, please check out First Aid. In the anatomy-embryology section (2004 edition is on page 75), is an entry for Twinning. There is an illustration and a brief explanation that you already summed up.
Also some good definitions from http://www.dictionarybarn.com/DIZYGOTIC-TWINS.php and http://www.dictionarybarn.com/MONOZYGOTIC-TWINS.p...:
Monozygotic Twins: Identical twins, twins which are the result of a single zygote (fertilized egg) splitting into two cell masses and becoming two individuals. The twins are genetically identical and are always of the same sex (both males or both females).
Dizygotic twins are siblings who have shared a common uterine environment. They are due to fertilization of two different ova by different sperm. Dizygotic twins are also called fraternal twins.
As for an example question, try this:
Are Jack and his sister Jill, both born on the same day, more likely to be Monozygotic or Dizogotic Twins?
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| namf
Forum Elite
Topics: 80
Posts: 312
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Hi PsychDr2B,
Thanks for your reply!
Is the answer dizygotic?
I need to read your links--I did stare and think about that picture in FA, but it never got to the heart of my question. My problem stems from 2 test questions we had I just blew off on consecutive regular-term exams (yeah, they meanly recycled a q!). The qs both presented a given situation--1 placenta, 1 chorion, 2 amnions--whatever, and then wanted you to make a definitive statement like "This MUST be monozygotes" for the answer. I just feel like I would get a question like that thrown at me wrong--so I am trying to fill that gap.
Any ideas anyone? Thanks as always!!
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| neodoc
Forum Newbie
Topics: 1
Posts: 4
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Dizygotic (fraternal) twins - 2 fertilized oocytes;
different genotypes/phenotypes; same or different sex
comprise 2/3 of all twins
ALWAYS diamniotic and dichorionic; may have separate or fused placenta
Monozygotic (identical) - fertilization of 1 oocyte;
same genotype/phenotype
comprise 1/3 of all twins
65% of arise from splitting of the inner cell mass at the end of week 1 (therefore are diamniotic and monochorionic and a single fused placenta)
35% arise by splitting of the zygote at the 2 cell to morula stage at the beginning of week 1 ( are diamniotic and dichorionic and two placenta which may be fused)
Hope this helps
___________________
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