The University of Texas has some cool stuff about Gregor Mendel who came up with a few LAWS of heredity based upon statistical evaluation of the traits of pea plants.
Segregation occurs during meiosis, the formation of the sex cells.
Breeding experiments with the fruitfly , Drosophila melanogaster, showed different traits being transmitted on different chromosomes. This was the birth of modern genetics. The variation within one species alone is tremendous. Take the species Canis familiaris, there are over 400 recognized breeds within this one species, the dog. Think of all the different types of dogs.