2007-08-27 09:45:09 · 7 answers · asked by Anonymous in Science & Mathematics ➔ Biology
Identical Twins have the exact same DNA, Siamese or not. This is because the same female egg split in two after fertilization and both babies developed.
When you test the DNA of a twin each one will have the same. The simple fact that an error in reproduction caused the two separate fetus to join makes no difference. But, this only true in the case of identical twins.
In the case of Siamese twins the bodys may have formed differently, sharing organs or one body missing an organ, but that was caused by the error in the fetus; not by the original DNA split at fertilization.
If the twins are brother and sister then the X component and all the genes attached to it are different. But that would be true for any sibling. The DNA is closer and the female part is probably identical in faternal twins (mixed sexes) but the male component will always be different. In this case if a male and female pair of Siamese twins are born they will NOT have identical DNA. Most Saiamese twins do though since most of them are the same sex and the error was made because the bodies are so similar that it is hard to differinate them.
2007-08-27 09:57:03 · answer #1 · answered by Dan S 7 · 0⤊ 0⤋
It depends on the defect that caused the twins to be conjoined. An embryo develops with a strong emphasis on symmetry. If the embryo partially splits early enough, you could result with a fairly symmetrical pair of conjoined twins (i.e. two twins sharing the same torso or set of legs). The various physiological effects of being conjoined could cause differences in development between the two, so they might not look identical at birth or as they age, but the basic body plan would be symmetrical.
Two identical or fraternal twins could also fuse together at a later point in development. This would be the case if the twins were conjoined in an off-center location, such as the head or part of the torso. In this case they could have different DNA if they were fraternal twins. Since they fused at such an early age, their immune systems would be "used" to each other, so there would be no chance of one twin rejecting the other, despite the difference in genetics.
The second case is related to chimerism, where, very early in development, one embryo (just a ball of cells at this stage) absorbs the other. They then act as one embryo, and develop into an otherwise normal person with two sets of DNA. The locations of the different sets of DNA depend on where the absorbed cells end up. The result could be a person with his "brother's" kidneys, liver, heart, etc. Portions of the skin or hair may be visibly different (depending on natural variations between complexion and hair color in fraternal twins), but otherwise, the only way one would know that he (or she) was a chimera would be through genetic analysis.
2007-08-28 09:04:49 · answer #2 · answered by andymanec 7 · 0⤊ 0⤋
Yes, always. They come from the same fertilized egg. At some point the developing embryo splits, but instead of coming all the way apart, in which case you'd just have identical twins, it doesn't come all the way apart, so they develop into two people who are stuck together.
If you want to be really technical about it, maybe if one of them gets a really bad sunburn and smokes a lot, he'll manage to change his DNA a little, but it was definitely the same to start out with.
2007-08-27 09:57:55 · answer #3 · answered by Anonymous · 0⤊ 0⤋
Some do, and others do not. Siamese twins are the result of two separate fetuses that fuse together. Of course two fetuses in the womb are twins, but some twins are identical and other twins are fraternal. So if fraternal twin fetuses fuse and become siamese, they will not have the same dna. It will be as different as normal brothers or sisters. If two identical twin fetuses fuse, then these siamese twins will have the same DNA.
2007-08-27 10:39:07 · answer #4 · answered by Chris 3 · 0⤊ 0⤋
Yes, it is one cell that started to split in half to form identical twins, then did not complete the action. Fraternal twins have DNA as different as any two siblings.
2007-08-27 10:27:05 · answer #5 · answered by Sandy Sandals 7 · 0⤊ 0⤋
Yes, they do. Except for random small defects caused by mutation or damage (such as radiation), identical twins' DNA is just that: identical.
However, their appearances may not be exactly the same. This is because their growth prenatal and postnatal may be affected by different factors.
2007-08-27 09:59:08 · answer #6 · answered by TheHonorableReese 6 · 0⤊ 0⤋
I think so.
2007-08-27 09:52:40 · answer #7 · answered by Blackbird 5 · 0⤊ 0⤋