As first noted by Gregor Mendel, genetic characters, or differentiating traits, are discrete entities. Their visible manifestations (such as plant height, seed shape, etc.) may be masked in some organisms, but the characters are nevertheless present.
Mendel was able to arrive at his second law because the traits he studied were located on separate chromosomes and were thus unlinked characters. Later work revealed the phenomenon of genetic linkage.
The inheritance and segregation observed by Mendel is characteristic of organisms with two sets of chromosomes (diploid) which specify the characters examined. Some characters in these organisms are inherited in other ways. That kind of inheritance is called non-Mendelian inheritance.
This is page 11111 of Molecular Genetics by Ulrich Melcher, ©1997, 1998