Student Article High School Science Journal Author: Ksenia Dyachok

Inheritance: no DNA required!

August 05, 2007

Some plants can fix abnormal genes in their own chromosomes if previous generations carried the normal version, according to a new study by Robert Pruitt, a molecular geneticist from Purdue Department of Botany and Plant Pathology. These findings seem to violate genetic-inheritance laws that have been accepted for more than a century.

The laws of inheritance were derived by Gregor Mendel, a 19th century Austrian monk, who was conducting plant hybridity experiments. Between 1856 and 1863, he cultivated and tested some 28,000 pea plants. His experiments brought forth two generalizations which later became known as Mendel's Laws of Heredity or Mendelian inheritance. In particular, one of the direct consequences of Mendel's laws state that if each parent has two copies of the mutant gene, all their offspring will have the same two copies of the mutated gene.  

Arabidopsis and the ‘Hothead’ gene

While studying Arabidopsis thaliana, a mustard plant commonly used in genetics experiments, Robert Pruitt and his colleagues found that the mutated version of the ‘Hothead’ gene causes leaves and petals to fuse together in the plant Arabidopsis. 

‘Hothead’ is a recessive gene so when two mutated versions of the

gene are present it is just about inevitable, according to basic inheritance principles, that its offspring will also be mutated. 

This mutant plant has a malfunctioning gene that prevents its flowers from opening. Purdue University scientists found that 10 percent of the offspring of two such mutant plants don't have this malformation, but rather are like the normal grandparents. This chain of inheritance defies accepted scientific beliefs. 

But in an article published in Nature (24 March 2005) titled ‘Genome-wide non-Mendelian inheritance of extragenomic information in Arabidopsis’ it was reported that, in  a surprisingly large number of cases (10%!!!), the offspring had leaves and petals that separated normally. Essentially the offspring had not inherited the mutated versions of the ‘Hothead’ gene as was expected. This goes against the entire theory of inheritance. 

This offspring of parents carrying two mutant alleles has normal looking flowers and carries non-mutant (wild type) "Hothead" gene.  

"Our genetic training tells us that's just not possible. This challenges everything we believe”, Pruitt said. 

Previously, researchers who had noticed similar breeding quirks assumed that someone had contaminated the plants with pollen carrying a normal gene, says Pruitt. "We've done a lot of experiments, described in this paper, that show none of the simple explanations account for this skipping of generations by an inherited trait." 

The scientists kept the plants in isolation so they couldn't accidentally crossbreed with plants that didn't have the mutated gene, called hothead, that causes organ fusion like that seen in the flowers. The researchers used molecular markers - bits of DNA that help identify and locate genes in organisms - to determine whether a plant carried normal or mutant copies of the genes. 

Through a battery of tests, the researchers failed to find any copies of the normal gene in unexpected parts of the parents' DNA.  Apparently mutant parent plants have hidden templates containing genetic information from the preceding generation that can be transferred to their offspring, even though the traits aren't evident in the parents, according to Purdue University researchers. This discovery flies in the face of the scientific laws of inheritance first described by Gregor Mendel in the mid-1800s and still taught in classrooms around the world today. 

Invisible RNA?

To explain the phenomenon the researchers proposed that there was invisible RNA inherited from the plant’s grandparents. This proposed invisible RNA somehow replaced the faulty section of the mutated gene. This theory relied on there being an invisible RNA cache that is yet to be detected. 

Once scientists understand more about the mechanism, they then may be able to manipulate it to modify genes already in plants and animals in order to correct mutations that cause diseases and abnormal growth. 

"This means that inheritance can happen more flexibly than we thought in the past," said Robert Pruitt. "While Mendel's laws that we learned in high school still are fundamentally correct, they're not absolute”. 

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References: 

Lolle, S.J. . . . and R.E. Pruitt. 2005. Genome-wide non-mendelian inheritance of extra-genomic information in Arabidopsis. Nature 434(March 24):505-509. Abstract available at http://dx.doi.org/10.1038/nature03380. 

 
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