Hi all

A while ago I mentioned a website with a scientific explanation as to why "identical" twins aren't really identical. Here is the link (you have to go through Twini Basics/How Multiples are Formed) to get to this article:

http://mypage.direct.ca/c/csamson/multiples.html

I've copied and pasted it for anyone who is interested. It's a bit complicated but very interesting:

"Scientifically speaking, "identical" twins are twins who arised from a single zygote, and are called monozygotic. A zygote is the cell that results from the fertilization of an egg (oocyte) and a sperm.

A zygote has a single nucleus which contains a single set of genes (the nuclear DNA, two strands, one from each parent). The cytoplasm of the cell contains the mitochondria, which have genes of their own, which are inherited from the mother only. When DNA testing is done, it is the nucleus that is tested and typed, so identical twins will show up as identical based on nuclear DNA testing. It is possible to find differences in identical twins if the mitochondrial DNA is tested.

When a cell splits into two new cells to form monozygotic twins, the mitochondrial DNA may not divide equally between the two cells. The mitochondrial DNA may also mutate and change over time, so twins that come from a single zygote can have slightly different mitochondrial genes and traits determined by these genes.

Dr. Charles Boklage, a geneticist, states that: "It seems that the mitochondrial genes in the egg cell often have two or more different versions (it's called heteroplasmy) which can be sorted into cells of the embryo (and potentially into co-twins) in different proportions. So even twins from a single zygote can differ considerably in mitochondrial genes and in traits determined by those genes. There can be as much difference in mitochondrial genes between monozygotic co-twins as between dizygotic twins. "

This can explain differences in identical twins outward appearance as well as personality differences. This is also why it is near impossible for geneticists to test for polar body (half-identical) twinning.

In addition, the expression of genes in monozygotic twins can be different. For example, a minor infection can trigger the genes that code the immune system to respond in different ways, resulting in different antibody protein molecules to be found in each twin. Mutations and rearrangement of pieces of DNA can slightly change as the twins grow, causing differences.

Female monozygotic twins can also differ because of X inactivation. Females get two X chromosomes, one from the father and one from the mother. You only need one to be active, and early in the embryo formation, a random shut down of half of each X chromosome occurs. It is random and permanent, and it's possible that one twin may shut down one version of the same chromosome, while the other shuts down the other. This is why some monozygotic female twins will have one twin affected with a disease that is X-linked (such as Muscular Dystrophy) while the other is not.

Geneticists also cannot prove monozygosity through DNA testing. DNA testing will exclude a set from being dizygotic, but never concretely prove that set is monozygotic. In DNA testing, geneticists test various genetic markers, and if they don't find differences, then they will state the twins to be monozygotic. Differences however, do not prove that a set cannot be monozygotic. Many dizygotic twins have cells of their co-twin in their bodies due to sharing of the bloodstream in utero, or placentas fusing.

[Note: Most of this information was provided by geneticist Dr. Charles Boklage, the University of Western Australia's Life and Physical Sciences Department, the adulttwins Yahoo group, Nancy Segal's book "Entwined Lives" and a few posters on the fanfic_med Yahoo group.]"



Maree

Eleanor 1/8/03
Harriet 1/8/03

This message was edited by nakigirl on Tuesday, 19 July 2005 @ 9:48 AM

Thanks Maree

Very interesting!!!!!!!

I could understand most of it LOL

Thanks again
Wems

WOW! Thanks for sharing that info with us.

Belinda

That was interesting. Thanks for that. Out of curiosity, Maree, is your interest in things scientific due to having twins, or are you a scientist/medical/health professional yourself?
Cheers
Mimzie

Hi Mimzieb - No, I'm not a doctor or scientist - my interest is just from having twins. Although I've watched 11 series of ER so maybe that qualifies me as a doctor now?!

Maree

Eleanor 1/8/03
Harriet 1/8/03