Tom Maravola, age 15, of Columbia, Tenn., for his question:

WHAT ARE MENDEL'S LAWS?

Mendel's laws are a group of principles of hereditary transmission of physical characteristics that were formulated in 1865 by an Augustinian monk named Gregor Johann Mendel.

Mendel experimented with seven contrasting characteristics of pure breeding garden peas. He discovered that by crossing tall and dwarf parents, for example, he got hybrid offspring that resembled the tall parent rather than being a medium height blend. To explain this he conceived of hereditary units, now called genes, which often expressed dominant or recessive characteristics.

Formulating his first principle (the law of segregation), Mendel stated that genes normally occur in pairs in the ordinary body cells, but segregate in the formation of sex cells (eggs or sperm), each member of the pair becoming part of the separate sex cell. When eggs and sperm unite, forming a gene pair, the dominant gene (tallness) masks the recessive gene (shortness).

Mendel formulated his second principle (the law of independent assortment) when he found that the expression of a gene for any single characteristic is usually not influenced by the expression of another characteristic.

Mendel's laws became the theoretical basis for modern genetics and heredity.