In studying each pair of traits, Mendel mated two true-breeding individuals (the parental or P1 generation), each exhibiting one of the two contrasting forms of the characteristic. For example, if the characteristic was plant height, a dwarf plant was mated with a tall plant. The offspring of such a cross are known as the first filial or F1 generation. When F1's are allowed to self-fertilize (sometimes referred to as "self"ing), they produce an F2 generation.

Here's an example of what Mendel observed:

Mendel got the same result, regardless of which parent (tall or dwarf) provided the pollen, and which parent provided the ovum. (Therefore the characteristic was independent of sex.)

Mendel also observed the same result with the other characteristics:

In each case that he studied, Mendel found a 3:1 ratio in the F2 generation.

Mendel's Postulates

To explain his results, Mendel formulated three postulates:

1. Genetic factors exist in pairs in individual organisms. In the above example, there would be three possible pairwise combinations of the two factors.
2. When two unlike factors for a single characteristic are present, one factor is dominant to the other. (The other factor is considered recessive.) In the above example, the factor for tall is dominant to the factor for dwarf.
3. During gamete formation, factors segregate randomly so that each gamete receives one or the other with equal likelihood. The offspring of an individual with one of each type of factor has an equal chance of inheriting either factor.

Mendel didn't know the identity of his factors, because at that time cells were poorly understood. As we will see, Mendel's 'factors' are in fact genes.

Applying Mendel's postulates to the tall/dwarf example above, Mendel reasoned that the tall and dwarf parental plants contained identical pairs of factors, because the plants bred true. (We'll designate 'd' as the dwarf factor and 'D'as the tall factor.) So the tall plant was DD and the dwarf plant was dd. Therefore, the F1 plants received one tall factor and one dwarf factor, making them all Dd. These plants, heterozygous for the one characteristic being considered, are monohybrids. Tall is dominant to dwarf in this case, so all of the F1's were tall. F1 gametes would receive one factor or the other with equal probability, so self fertilization would produce four possible combinations of factors in the F2:

All would appear with equal frequency. Any of the plants with a D factor would be tall; only the dd would be dwarf. Therefore, 3/4 would be tall, and 1/4 dwarf.