Twin Studies

From: Michael Ragland (ragland37_at_webtv.net)
Date: 08/23/04 

Date: Mon, 23 Aug 2004 20:01:27 +0000 (UTC)

Twin Studies
Scientific studies performed on pairs of twins.
These studies can answer many questions in medical genetics. They are
especially useful for deciding whether a certain condition is genetic,
environmental, or both. To understand how twin studies work, you must
know two things:
There are two types of twins: (a) Identical twins, who have 100% of
their genes in common, and (b) non-identical twins, who have 50% of
their genes in common (on average).
If they are raised together, twins share a similar environment.

Here's an example of the information that twin studies can provide. In
1995, researchers in New Jersey studied 84 pairs of twins, in which at
least one twin had asthma. For each pair, they recorded the type of twin
(identical or non-identical) and whether one or both twins had asthma:

Type of twin pair
Number of twin pairs
Twin pairs in which both twins had asthma ("concordance rate")

Number of identical twins pair 39
concordance rate 59%
(23 out of 39)

Number of Non-identical twins 55
concordance rate 24%
(13 out of 55)

If asthma were governed only by genes, then every time one identical
twin had asthma, the other should have it, too. (Because identical twins
have the same genes.) In other words, the "concordance rate" should be
100%. The table shows, however, that instead of a 100% concordance,
there is only a 59% concordance between identical twins. This is good
evidence that asthma is not purely a genetic disease.

Ragland:
No, its not a purely genetic disease but the fact the study showed the
concordance rate is more than twice as high in identical twins (59%) as
in non-identical twins (24%) is [as the article states] good evidence
that asthma is not purely an environmental disease. The article notes,
"This type of analysis works for almost all conditions, not just
asthma."

Despite the interaction between genes and environment are twin studies a
good barometer for determining which conditions are more often
influenced by genes in interaction with the environment? Looking at the
bibliography the bias would seem to be yes.

______________________________________

If asthma were purely an environmental condition, then genes should make
no difference at all. The concordance rate would, therefore, be the same
for identical twins as for non-identical twins. The table shows,
however, that the concordance rate is more than twice as high in
identical twins (59%) as in non-identical twins (24%). This is good
evidence that asthma is not purely an environmental disease.
This type of analysis works for almost all conditions, not just asthma.

Bibliography
Laitinen T, Rasanen M, Kaprio J, Koskenvuo M, Laitinen LA. Importance of
genetic factors in adolescent asthma: a population-based twin-family
study. Am J Respir Crit Care Med 1998 Apr;157(4 Pt 1):1073-8. 
Sarafino EP, Goldfedder J. Genetic factors in the presence, severity,
and triggers of asthma. Arch Dis Child. 1995 Aug;73(2):112-6. 
 
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