The four-leaf clover is a rare mutation of the common three-leaf clover. It is a common activity for children to look for these and it is considered good luck to find one.
People who search for four leafed clovers note that some patches of ground appear more likely to contain four leafed examples and that this may be partly due to differing growing conditions such as pollution, soil composition, and other environmental factors. A countervailing hypothesis is that the varieties themselves are more accustomed and that the mutation may be a phenotype rather than a genotype.
The good luck attribution to four-leafed clover may arise in part because they are so difficult to find.
