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How it works: four-leaf clovers

Legend says they are good luck; science says they're a good look

Published: Thursday, March 15, 2012

Updated: Friday, March 16, 2012 02:03

Conditions must be just right for this rare form of clover to sprout.

Wikimedia Commons

Conditions must be just right for this rare form of clover to sprout.

  St. Patrick's Day is right around the corner. Will you be sporting a clover on your lapel? Legend has it St. Patrick used the three-leaf clover to demonstrate the Holy Trinity when he introduced Christianity to Ireland.

  But do you ever wonder what causes four-leaf clovers? Scientists have only recently been able to answer that question.

  The plant we usually associate with St. Patrick's Day is called white clover, or Trifolium repens. There are about 300 species of clover—and as you can probably guess by the name, "Trifolium," they're known for having three leaves.

  It's estimated that one clover in 10,000 is a four-leafer, though you can find patches that yield dozens of four-leaf specimens. Need help finding them? Ask Edward Martin. He's the world record holder, with a collection of 160,000 four-leaf clovers.

  What makes clovers sprout that extra leaf is a combination of genetics and environmental factors.

  You probably remember genotypes and phenotypes from high school biology. All plants and animals get one allele from two parents. The combination of those two alleles is called the genotype. How that genotype is expressed is called the phenotype.

  Phenotype expression, though, can be complicated by a lot of things.

  In 2010, scientists studying the genome of white clover found that there's a gene that creates clovers with more than three leaves, but there's another gene that suppresses it.

  This is a good example of what geneticists call epistasis (eh-PEE-STAY-sis), or interaction between genes. One gene is modified by other genes. Those modifier genes can alter the gene they're influencing or keep it from being expressed at all.

  When you see a three-leaf clover, that's epistasis at work. If you find a four-leaf clover, it means the modifier gene that keeps the leaf count at three is broken, probably due to a mutation that won't let it do its job.

  The gene that creates four-leaf clovers can also lead to clovers with five, six or more leaves. The highest count to date is 21 leaves on a single clover. Shigeo Obara, a Japanese man who cultivates multiple-leaf clovers, found it in his garden in 2008. He expects he'll find clover plants in his garden with even more leaves.

  To complicate matters even more, the expression of these genes is also influenced by environmental factors like pollution, soil composition, or temperature. It's an example of incomplete penetrance.

  A gene with complete penetrance will always be expressed (one example is hemophilia: if you have the gene, you'll always have the disease). A gene with high penetrance is likely but not guaranteed to express itself and a gene with low penetrance will express itself rarely. Both are examples of incomplete penetrance.

  It's not clear why those factors affect expression of the genes, though. We still don't know enough about how the genes work together or how those environmental factors affect them. So there's no way to predict exactly which patches will bear four-leaf clovers.

  What we do know is that because the gene that creates clovers with more than three leaves has low penetrance, even if a clover plant has the right genetics to produce four leaves, it might not if it's not in the right kind of soil or it's the wrong time of year. It's one of the reasons it took as long as it did to understand the genetics behind clover leaves.

  So if you go hunting for a four-leaf clover, count yourself lucky if you find one. A lot of things had to go just right for that fourth leaf to pop up.

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