Use of twins to study complex diseases

[Pooka1]

And excerpt from this article...(emphasis added)

http://www.biostat.sdu.dk/courses/f1...g/hall2003.pdf

Use of twins to study complex disease

Comparisons of monozygotic and dizygotic twins are
frequently used to estimate the environmental and
heritable factors that contribute to complex diseases. This
comparison assumes that twin pregnancies, births, and
early childhood are representative of singleton pregnancies,
births, and early childhood. Growth of twins in utero is not
typical of singletons. The effect of intrauterine growth on
later adult diseases could also be atypical. All twins can be
expected to have many kinds of in-utero differences, such
as placental flow in monozygotic twins and the amount of
microchimerism in dizygotic twins.

Findings of twin studies need to be viewed with some
reservation. Twins may not be representative of singletons
in relation to some complex diseases. Furthermore,
increasing use of artificial reproductive technologies,
presence of high concentrations of gonadotropins,
intentionally broken or abnormal zona pellucidas, and
physiological abnormalities related to culture media might
have effects on childhood or adult disorders not yet
recognised or not even present in spontaneous twinning.
Long-term studies and outcomes must record whether
twinning was spontaneous, the type of artificial
reproductive technology used, complications of twin
pregnancies, placental flow, changes in maternal
physiology, type of chorion, and microchimerism during
pregnancy to establish whether twin pregnancies can be
representative of singleton pregnancies.

The in-utero adjusted growth rate of both monozygotic
and dizygotic twins could also alter other pathways that
protect against certain diseases and predispose to others.
Differences in maternal metabolism during twin
pregnancies versus singleton pregnancies—with lower
packed-cell volumes, more gestational diabetes, greater
cardiac output, etc—could also alter the metabolism of
the embryo and fetus.

These observations have interesting ramifications for
the Barker hypothesis,93 which suggests that diminished
intrauterine growth predisposes to early onset of
hypertension, cardiovascular disease, and diabetes in later
life.However, the smaller twin at birth in a twin pair does
not seem to have the same type of risk for these diseases as
is seen in singletons.94–96 An increase in breast cancer97,98
and testicular cancer94,99,100 has been noted in twins.
Further, risk of autism has been reported to be increased
in both monozygotic and dizygotic twins.101,102

So it seems possible all these twins studies are not going to helpful in the case of complex disease.

[Pooka1]

Non-identical Monozygotic twins

http://8e.devbio.com/article.php?id=111

Most monozygotic twins are remarkably similar. However, there are some equally remarkable exceptions that show that monozygotic twins do not have to be "identical."

It is usually assumed that monozygotic twins always share the same genome and that their hereditary endowments are the same. Indeed, twins have long been used in studies that attempt to separate "nature" from "nurture". Galton (1875) proposed that the comparison of monozygotic twins to dizygotic twins would show which characteristics are genetic ("nature") and which are conditioned by the environment ("nurture"). However, recent data have suggested that each of a pair of one-egg twins can develop very differently from the other, and these findings of "discordant monozygotic twins" have raised concerns about the assumptions underlying those studies that presume each of a pair of identical twins will develop the same way. In fact, Hall (1996) hypothesizes that the mechanism underlying the separation of blastomeres (which leads to identical twinning) may be genetic changes that distinguish the cells from each other. In this scenario, "identical" twinning is caused by the non-identity of the early blastomeres.

There are several cases in which identical twins can have discordant phenotypes. The major routes involve aneuploidy, X-chromosome inactivation differences, imprinting, and circulatory differences.

[rohrer01]

Good find! This is what I was trying to say in many other threads, but put more succinctly! I don't think it dicredits all of the studies, however, because their DNA is most similar to any other's on the planet.

[Pooka1]

this needs re-posting