Gregor Mendel's experiments with pea plants in the mid-19th century laid the groundwork for genetics. By breeding thousands of pea plant varieties, Mendel was able to deduce that traits are passed down through discrete units he called "factors," the modern equivalent of genes. His principles of inheritance established that offspring have a 50% chance of inheriting a dominant trait from a parent and a 25% chance of inheriting two copies of a recessive trait.