The shamrock, or clover, is a standard symbol of St. Patrick's Day. The most common version has just three leaves. But according to popular lore, the variety that carries that extra bit of "luck of the Irish" is the four-leaf clover.

But what makes the four-leaf variety so rare? CBC Radio science columnist Torah Kachur looks at the genetics of clover.

Why do most clovers have three leaves?

It's because of something called developmental patterning. When the leaves are forming, they start out as a series of overlapping areas that eventually turn into only three leaves.

This is typical of lots of different plant structure development — the number of petals are on a flower, for example, are determined by very similar genes.

So what makes a four-leaf clover?

That was a mystery for a long time — and still is to some extent

The problem is that plants are notorious for having complicated genetics, compared to humans. We only have two copies of every gene — we get one from each parent, and that makes it relatively easy to do genetic analysis for humans.

But many plants have multiple copies of their entire genome. In fact, Trifolium — the scientific name for clover — has four copies of every gene. So that makes it really hard to tease apart what gene is doing what in the plant.

Research from the University of Georgia, published in 2010, finally pieced together the genetic puzzle of the multi-leaved clover. But even that research left a few mysteries.

The University of Georgia scientists thought that they had finally found one of the possible genes involved in leaf development. Then, something mysterious happened.

St. Patrick's Day 2015 Ireland

The clover has become a symbol of St. Patrick's Day, and good luck. But the genetics that produce a four-leaf clover are still a bit of a mystery. (Cathal McNaughton/Reuters )

Two separate experiments conducted in summer and winter of the same year found the gene involved in creating a four-leaf clover — but the two experiments mapped it to two different places in the genome, which is impossible. 

So to this day, it's still a mystery where exactly this four-leaf clover gene is actually located, and how it really works. 

What we do know is that it's a genetic trait, like most other things in a plant.

In fact, researchers are interested in breeding plants that have more than three leaves because they'd provide more fodder for cattle that graze on clover.

And they're thinking beyond four leaves — the world record holder for most clover leaves, according to Guinness, is a clover plant found with 56 leaves.

That's the effect of getting rid of one of the genes that normally prevents too many leaves on a clover. If it gets mutated or stops working, the clover starts making multiple leaves.The result doesn't look like a flat clover with four leaves, but is layered, like a peony flower.

What are the chances of finding a four-leaf clover?

Pretty small — Scientific American, for example, quotes an estimate that only one in 10,000 clover plants has four leaves.

So you'd have to get pretty lucky indeed to find one.