"Even though corn looks different from other plants, it's not that
far removed from its predecessor, teosinte. Experiments reported in the
1970's by geneticist George Beadle suggested that changes in as few as
five of the plant's thousands of genes are responsible for the major
differences between teosinte and maize. Dr. Doebley has confirmed the
results.
"One of the genes, abbreviated tga, makes it easy to eat the kernels.
In teosinte, the tga gene directs the plant to form a casing around teach
kernel. This fruit case makes the kernel hard to get to. But in modern
corn, the tga gene has a different form, one that stops the fruit case
from completely enclosing the kernel. Without the genetic change, corn
kernels would be really tough to sink your teeth into, Dr. Doebley said.
"And corn would be tough to harvest, too if it weren't for a gene
called tb1. In teosinte, the gene causes the plant to have many stalks,
which means there are a lot of ears on each plant.
"'Although that may sound good for farming, it really isn't.' Dr.
Doebley said. To understand why, he said, think of the sunflowers that
have one big flower at the top and those that have several smaller
flowers.
"Say you'd like to get a thousand sunflower seeds,' he said. 'Would
you rather pick one head and get a thousand seeds, or 10 heads with a
hundred seeds?'
And even though the 10 flowers are all on the same plant, they are
likely to go to seed at different times, so a farmer would have to go back
to the plant more than once.
"Dr. Doebley and his co-workers have isolated the tb1 gene and
compared the teosinte version with the corn version. Preliminary
experiments suggest that the gene's job is to prevent side branches from
growing too big. In teosinte, the long branches grow. But in corn, it
turns out, the gene is more active causing fewer, shorter branches. The
result is that corn plants, depending on the variety, have only two to
five ears. Teosinte can have a dozen.
"The corn version of the third gene causes the ear to have more
kernels, and influences branch length and the arrangement of the branches
on the main stalk. The fourth gene also affects the branching pattern,
and the fifth seems to modify many different traits."~Sue Goetinck,
"Geneticists working to Learn How Ancient Plants Became Grains that Feed
the World," Dallas Morning News, Monday, June 17, 1996, p 6D, 7D.
glenn
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