robert@stevie.cs.unlv.edu (Robert Cray) (01/23/89)
I was reading a genetics book a while ago and came across "...almost all MZ twins are same sex...". Thinking about it, I can't figure out how you could have MZ twins of different sex, anyone know? Was the author just hedging? --robert
hes@ecsvax.uncecs.edu (Henry Schaffer) (01/24/89)
From, TEXTBOOK OF HUMAN GENETICS by Max Levitan, 1987, p. 354 In a section on Zygosity Diagnosis, talking about concordance/discordance in MZ/DZ twins. "... chromosome abnormalities or mutation may develop in one MZ twin and not in the other, giving rise to phenotypic differences. In a few instances, it has even turned out that twins of unlike sex were really monozygotic, as one exhibited XO Turner's syndrome and the other was a normal male, or both were XY by only one showed testicular feminization." --henry schaffer n c state univ
kanov@bimacs.BITNET (Mechael Kanovsky) (01/30/89)
In article <1046@jimi.cs.unlv.edu> robert@jimi.cs.unlv.edu (Robert Cray) writes: >I was reading a genetics book a while ago and came across "...almost all >MZ twins are same sex...". Thinking about it, I can't figure out how you >could have MZ twins of different sex, anyone know? Was the author just >hedging? > > --robert Probably he is, but I still can think of a long shot of a case where MZ (monozygot) twins will be of different sexes. If in one of the twins we have a mutation that will occur on the Y gene and for that reason there will be a defect in the membrane protien that recognizes testosterone what you will get is an XY female. Actualy there are a few other genetic defects where XX will have male characteristics and XY where they will be female. But I can't think of a case where the MZ twins will be of different sexes and they will both be normal. -- Mechael Kanovsky : BITNET kanov@bimacs.bitnet Math & CS Dept. : UUCP uunet!mcvax!humus!bimacs!kanov Bar-Ilan University : ARPA kanov%bimacs.bitnet@cunyvm.cuny.edu Ramat-Gan Israel : CSNET kanov%bimacs.bitnet%cunyvm.cuny.edu@csnet-relay ! You can't propel yourself forward ! ! by patting yourself on the back ! #! rnews 1
ayermish@asylum.SF.CA.US (Aimee Yermish) (02/01/89)
Another very long shot on how you could get monozygotic twins to be of different sexes: If they both started out female, but right early on (I mean *really* early on, although I don't know enough about developmental bio to say more specifically) one of them had a breakage (or maybe a very lucky translocation) in one X chromosome cutting it short such that it was for most intents and purposes a Y chromosome. Of course, the chances that the break would happen in just the right place are vanishingly small, leaving the poor kid with some gene dosage screwups. I *said* it was a very long shot... --Aimee -- Aimee Yermish ayermish@asylum.sf.ca.us Program in Cancer Biology ayermish@portia.stanford.edu Stanford University 415-594-9268
chiaravi@silver.bacs.indiana.edu (Lucius Chiaraviglio) (02/01/89)
In article <1091@asylum.SF.CA.US> ayermish@asylum.UUCP (Aimee Yermish) writes: >Another very long shot on how you could get monozygotic twins to be of >different sexes: If they both started out female, but right early on >(I mean *really* early on, although I don't know enough about >developmental bio to say more specifically) one of them had a breakage >(or maybe a very lucky translocation) in one X chromosome cutting it >short such that it was for most intents and purposes a Y chromosome. Won't work. If you break an X chromosome in an XX individual you still get a female, no matter where you break it. This is because merely cutting an X chromosome short does not make it a Y chromosome -- a Y chromosome has different contents than an X chromosome. If you mess up one X chromsome badly enough you will cause abnormality (known as Turner's Syndrome, whose most extreme form consists of the individual having only 1 X chromosome), but the individual will still be female. -- | Lucius Chiaraviglio | ARPA: chiaravi@silver.bacs.indiana.edu BITNET: chiaravi@IUBACS.BITNET (IUBACS hoses From: fields; INCLUDE RET ADDR) ARPA-gatewayed BITNET: chiaravi%IUBACS.BITNET@vm.cc.purdue.edu Alt ARPA-gatewayed BITNET: chiaravi%IUBACS.BITNET@cunyvm.cuny.edu
ayermish@asylum.SF.CA.US (Aimee Yermish) (02/02/89)
I stand corrected. Thank you, Lucius. --Aimee -- Aimee Yermish ayermish@asylum.sf.ca.us Program in Cancer Biology ayermish@portia.stanford.edu Stanford University 415-594-9268
mkkuhner@codon3.berkeley.edu (Mary K. Kuhner;335 Mulford) (02/03/89)
In article <1046@jimi.cs.unlv.edu> robert@jimi.cs.unlv.edu (Robert Cray) writes: >I was reading a genetics book a while ago and came across "...almost all >MZ twins are same sex...". Thinking about it, I can't figure out how you >could have MZ twins of different sex, anyone know? Was the author just >hedging? > --robert The apparent sex of a human being depends on a lot of things besides the sex chromosomes present, so this statement does make some sense. I know of at least one case in which an infant boy, one of a pair of MZ twins, had his genitals so severely injured that the decision was made to surgically "correct" him to female, and he was raised as such. Hormone treatment of the mother and some other environmental stimuli could also result in chromosomal sex not matching physiological sex, though having this happen to just one of a pair of twins seems farfetched. Mutation to the testis-determining-factor gene could also cause mismatched twins, but I don't know of an example. This would be excessively rare. I think the author was just being grotesquely overcautious..... Mary Kuhner