During fertilization, egg and sperm combine at random with respect to their genetic content. Genes on different chromosomes assort independently of each other during meiosis. Genes on the same chromosome tend to be inherited together. Based on this tendency, geneticists have learned to map the locations of genes on chromosomes. Genes interact with each other and with the environment and determine the eventual physical makeup of the individual.
Genes reside on chromosomes - a chromosome is a string of genes, much like a string of beads.
The gene for each trait can exist in two or more alternative forms. Linked genes lie on the same chromosome and tend to be packaged into gametes together. A plant geneticist mates a corn plant with smooth yellow seeds to a corn plant with wrinkled purple seeds. She then mates the F1 plants to each other to produce the F2. In the F2, the geneticist finds a phenotypic ratio of 9 purple smooth to 3 purple wrinkled to 3 yellow smooth to 1 yellow wrinkled.