I remember my kindergarten teacher explaining to us that four-leaf clovers were rare and brought luck to those who found them. She let us spend time several afternoons searching the lawn behind our classroom for all the luck we could find. She then taught us to preserve them by pressing them in various books in the classroom.
It has been estimated that there are approximately 10,000 three-leaf clovers for every four-leaf clover, however this probability has not deterred collectors who have reached records as high as 160,000 four-leaf clovers. It is debated whether the fourth leaflet is caused genetically or environmentally. Its relative rarity suggests a possible recessive gene appearing at a low frequency. Alternatively, four-leaf clovers could be caused by somatic mutation or a developmental error of environmental causes. They could also be caused by the interaction of several genes that happen to segregate in the individual plant. It is possible all four explanations could apply to individual cases.