Why don't identical twins always have the same mental or physical conditions?

Question

I recently read that an ADHD study was conducted on sets of both identical and fraternal twins. In fifty to ninety percent of all identical twin sets, both twins had ADHD, whereas that was the case in only 33 percent of all fraternal sets. There is obviously a genetic basis for ADHD, but why is it that in only fifty to ninety percent of identical twin sets, both twins had ADHD? Why not one hundred percent? After all, identical twins are genetically identical, right? Not only that, but I would imagine that in most cases, these twins are exposed to the same environmental factors. Why is it that not all identical twins have the same mental or physical conditions in common?

Answer 1

Because not all ADHD cases are due to genetics. Read this article and you get your answer. Very interesting!

http://www.nimh.nih.gov/health/publicati...

Answer 2

Identical twins have identical genes. But, DNA is a very stagnant molecule. Environment depends a lot on a person's environment. Say one twin decides they like Sesame Street and the other prefers Teletubbies. The one who watches Sesame street will be much more aware of letters and numbers than the one who prefers Teletubbies.

Or, say one prefers to watch TV while the other isn't really interested and prefers to play with Toys. There have been studies to indicate that television at a young age can trigger ADD or ADHD, because it doesn't allow for certain lobes to be stimulated at a young age, so they shut down. So, this would be how one twin would have ADHD and the other wouldn't.

See, even though they have the same genes, things like personality and mental capacity are not hard wired.... it all depends on environment.

Answer 3

1. Not all diseases such as ADHD are exclusively genetic. There is likely a strong environmental component as well. Some diseases, such as Huntington's, seem to be almost exclusively genetic, some such as rheumatoid arthritis seem to be primarily environmental. Most diseases such as various forms of cancer, schizophrenia, and phenylketonuria seem to have both genetic and environmental factors. This is known as gene penetrance: http://en.wikipedia.org/wiki/Penetrance

2. Identical twins are identical at conception. Following conception differences will occur: somatic cell mutations such as single base-pair changes up to changes involving whole chromosomes, VDJ recombination in the immune system, and differences in X inactivation of the X chromosome (females only,except in some rare cases) are some of the reasons that identical twins differ.

Answer 4

I guess because of the genes.

Answer 5

Virtually nothing is absolutely 100% determined by genes.  Small differences in their pre-natal womb conditions and small differences in how they were raised are important.  Also, nobody is exposed to EXACTLY the same environmental conditions; think about the friends they made, having separate rooms/beds, etc. account for the cases where identical twins did not turn out the same.

The womb conditions are probably the best bet though (i.e. embryo and fetal development).

"Nurture > nature"

Answer 6

well i am a twin and we don't have the same mentalor physical conditions cuz,we r already identical we don't have to do the same things so people will know which 1 is which.