Mendel came up with a simple but revolutionary explanation for the results he saw in the F₂ generation. He concluded that within an individual, hereditary came in paired units, with one unit derived from each parent. Each simple physical trait, such as stem height, was determined by the combined action of a single pair of units. Each unit could come in either a dominant form, which he denoted with a capital letter “A,” or a recessive form, which he denoted with a lowercase “a.” Two units with two possible forms gave four possible combinations: AA, Aa, aA, and aa; since Aa and aA were equivalent, there were really only three functional combinations. Because “A” is dominant over “a,” both AA and Aa produced plants with the same physical Only “aa” produced a plant that showed the recessive

 

    Mendel realized that the results he saw in the F₂ generation could only be explained if, during the formation of reproductive cells, paired units are separated at random so that each gamete contains only one of the two units. This postulate is now known as the law of segregation