Mendel's Laws and nashtOn's Mouse



From the (pinko-liberal) New York Times:
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"Mice Ignore Mendel's Laws In Passing On Their Traits"
by Nicholas Wade
Copyright New York Times Company May 30, 2006

Gregor Mendel might never have figured out his well-known laws of
inheritance if he had had to work with a perplexing strain of mice
developed at the University of Nice in France.

These brown laboratory mice have white feet and white-spotted tails
because of a genetic defect engineered into a gene that controls
pigmentation. But the defective gene behaves in a bizarrely
non-Mendelian way, a group of researchers led by Minoo Rassoulzadegan
report in the current issue of Nature.

When a mouse with one normal and one defective version of the
pigmentation gene is crossed with another such mouse, Mendel's laws
predict there will be three types of offspring, one with two normal
genes, one with two defective genes and one with genes of each kind.

In this case, the defective gene dominates the normal version, so the
mice with one defective copy should have white feet and tails, just
like their parents, but the mice with two normal genes should have
brown feet. (Having two defective genes is lethal, and mice of this
kind died shortly after birth).

The French researchers were very surprised to discover that almost all
the surviving offspring had white extremities, including those with two
normal genes. Crossing these white-footed, genetically normal mice with
other normal mice, they found it took six generations for the trait to
disappear.

Though they have not nailed down the cause of this apparent exception
to Mendel's laws, they believe the defective gene must produce some
special kind of RNA, the chemical cousin of DNA that helps translate
its genetic information into protein.

The defective gene's RNA, they suggest, interferes with the RNA
produced by the normal gene, which may help explain why the defective
gene is dominant.

In addition, the defective RNA may accumulate in sperm and continue to
silence even the two normal genes possessed by the next generation of
mice. Dr. Rassoulzadegan has detected RNA in the sperm of the mice. She
reports that when this RNA is injected into the eggs of normal mice,
the eggs develop into mice with the same white extremities.

Thomas R. Cech, an RNA expert and president of the Howard Hughes
Medical Institute, said it was hard to envisage how the defective
gene's RNA avoided being quickly diluted to ineffectiveness between one
generation of mice and the next. The defective RNA might get amplified
each generation by a special enzyme that makes copies of RNA. But such
an enzyme, though known to exist in roundworms, has not yet been
detected in mice, he said.

Dr. Cech characterized the French mouse experiment as fascinating but
not yet fully explicable. ''The whole things strikes me as a bit like
Swiss cheese -- a lot of solid material holding the story together but
with also a lot of holes,'' he said.

This is not the only known exception to Mendel's rules. Last year
Robert Pruitt, a geneticist at Purdue University, discovered that
mustard plants with two defective copies of a gene called hothead could
nevertheless produce some normal offspring, as if the defective gene
had somehow been repaired.

But the repair would be possible only if the plant possessed an
undamaged copy of the gene from which to restore the correct sequence
of DNA units. Failing to find any DNA version of the correct genetic
information, Dr. Pruitt suggested that the plant might carry a cryptic
backup version of its DNA genome in the form of RNA, perhaps broken up
into so many small pieces that it had escaped detection.

Dr. Pruitt said the French observation did not involve any correction
of a gene, like his finding, but it did suggest that RNA is passed from
parent to offspring, an assumption made by his hypothesis, too.

But in his view neither finding requires a rewriting of Mendel's laws,
which correctly describe the conventional transmission of the
hereditary information carried by DNA. The new results suggest there
may exist ''an alternative pathway that involves the transmission of
hereditary information via RNA,'' and that in special circumstances the
RNA pathway can modify the information in the DNA pathway, Dr. Pruitt
said.

Anomalous findings are often dismissed in science, because researchers
believe the experiment must have come out wrong or that it is not worth
the effort of tracking down the cause. ''It happens all the time in
science,'' Dr. Cech said. ''You come to a fork in the road, and you
know you may waste time if you wander down it, but there's a small
chance you'll discover something totally new that others have
bypassed.'


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Somebody may post a link to this... or visit the NY Times if you like.

(signed) marc

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