The fundamental principles of biology that are acceptable to this day include cell theory, gene theory, evolutionary theory, homeostasis, and energy.
Cell theory
Cell theory is a scientific theory proposed by the scientists, Theodor Schwann, Matthias Jakob Schleiden, and Rudolf Virchow. It is formulated to refute the old theory, Spontaneous generation. It suggests the following tenets: (1) All living things are made up of one or more cells, (2) the cell is the structural and functional unit, (3) cells come from a pre-existing process of division, (4) all cells have the same chemical composition, and (5) energy flow occurs within the cell. (Ref.2)
Gene theory
In Gene theory, the gene is considered as the fundamental, physical, and functional unit of heredity. (Ref.3) It is located on the chromosome and contains DNA. The gene stores the genetic code, i.e. a sequence of nucleotides that determines the structure of a protein or RNA. A gene is a unit of heredity because it is transmitted across generations. It is through which the phenotypic trait of an organism is based upon.
Gregor Johann Mendel was one of the main pioneers that established the science of genetics. As such, he is regarded as the father of the said field. He was able to determine the occurrence of unit factors (now referred to as genes) that were passed down from one generation to the next. He described these unit factors as occurring in pairs. One of the pairs will be dominant over the other (recessive). He formulated the Mendelian laws to elucidate how heredity occurs.
These laws include Law of Segregation, Law of Independent Assortment, and Law of Dominance. The inheritance pattern that follows these laws is referred to as Mendelian inheritance. Conversely, an inheritance pattern that does not conform to these laws is described as Non-Mendelian.