Why Do Some Clovers Have Four Leaves?

Four-leaf clovers are a rare variation on the usual three-leafed kind caused
by a genetic mutation. Credit: stock.xchng.

From Live Science:

The leaves of clover plants are said to hold the luck o' the Irish when they sport four leaves. This myth likely arose because four-leaf clovers are rare finds — the result of an equally rare genetic mutation in the clover plant.

There are about 300 species in the clover genus Trifolium, or trefoil, so named because the plants usually have three leaves, or technically, leaflets. The ones you typically find in North America are white clover (Trifolium repens).

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