"Based on what is, to our knowledge, the largest twin
study of age-related macular degeneration to date ... we
quantified substantial genetic influences," study author
Johanna Seddon of Harvard Medical School wrote in the most
recent issue of the Archives of Ophthalmology.
In the study of 840 people with a twin, identical twins
with macular degeneration were twice as likely than fraternal
twins with the disease to suffer the same degree of blindness.
Among identical twins with the disease, 55 percent had
the same severity of the disease, compared to 25 percent
of the fraternal twins, researchers said. Environmental
factors, such as whether the victim smoked or was obese,
contributed to the severity in 19 percent to 37 percent
of cases.
Identical twins are monozygotic, coming from the same
egg that splits after fertilization and sharing the same
genes. Fraternal twins are dizygotic, meaning two eggs were
fertilized simultaneously.
Last week, scientists reported the discovery of a variation
in a single gene that could be responsible for half of all
cases of age-related macular degeneration, which afflicts
between 10 million and 15 million people in the United States.
The gene is involved in a component of the immune system
that regulates inflammation, they reported in the journal
Science.
Macular degeneration involves the accumulation of waste
deposits in the macula, the center of the retina that is
packed with cones which help in seeing color, detecting
motion and making out fine detail. It erodes the center
of the field of vision.
