Study to look at large differences in refractive error in identical twins

Studies amongst true identical twins with large population numbers (250
twins) show that many of the twins have large refractive error
differences.

Studying differences in identical twins is therefore a fantastic
opportunity to look at why these twins may have differences that are:

1. Created by certain specific environmental/behavioural conditions or

2. Are random or chaotic and have no known statistically correlating
single item or items.

To my knowledge no such study has ever been done before.

Meanwhile a well known existing Twin myopia study has said "we are
already planning to see whether the MZ discordant twins have had a
different childhood environment in some way"

Andrew

 

Dear Andrew,
Let us suppose that one twin had a refractive status of +1/4 and +1/2
diopter, while
the other twin had a refractive status of -1/2 and -3/4 diopter.

Question: Is that a "large" difference in "refractive error" or a
small difference?

Best,

Otis

 

Your maximum value of differentness is 1.25D

Generally less than 3 is low myopia
3 to 6 is moderate
6 and over is high

The study results were as follows after combining astigmatism to
simplify analysis.

http://www.iovs.org/cgi/content/full/42/6/1232/F2

Exact match for every twin would produce a straight line for the MZ and
DZ groups.

I am not a maths person so i printed the graphs off and figured it out
with a ruler.

You can see that for normal sighted (twin2)twins there are two minus 6
(twin1) twins and many minus 3 twins.

For several -5 region (twin2) twins there are several +1 region twins

So the biggest differences are moderate to high and quite clearly
commonly present in MZ twins

Andrew

 

snip

When I look at the graph, I see classic straight line data. No biological
measurement has exact measures, but it is obvious from the graph that the MZ
twins have much more similar refractive error than the DZ twins.

Thanks for the link to the actual study, here is the conclusion; about 90%
of the refractive error can be explained by inherited factors, the rest
attributed to environment:


"RESULTS. For the continuous spectrum of myopia/hyperopia, a model
specifying additive genetic and unique environmental factors showed the best
fit to the data, yielding a heritability of 84% to 86% (95% confidence
interval [CI], 81%-89%). If myopia and hyperopia ( -0.5 D and 0.5 D,
respectively) were treated as binary traits, the heritability was 90% (95%
CI, 81%-95%) for myopia and 89% (95% CI, 81%-94%) for hyperopia. For total
and corneal astigmatism, modeling showed dominant genetic effects are
important; dominant genetic effects accounted for 47% to 49% of the variance
of total astigmatism (95% CI, 37%-55%) and 42% to 61% of corneal astigmatism
variance (95% CI, 8%-71%), with additive genetic factors accounting for 1%
to 4% and 4% to 18%, respectively (95% CIs, 0%-13% and 0%-60%,
respectively).

CONCLUSIONS. Genetic effects are of major importance in myopia/hyperopia;
astigmatism appears to be dominantly inherited."