At the time of Mendel's pea plant experiments, no one knew how organisms formed gametes. As Mendel studied the inheritance of two different characters, he wondered how the alleles for the two characters segregated into gametes. Mendel had two hypotheses for how this might work.

one of the hypothesis put forward by Gregor Mendel states that when two different alleles are inherited together, one may be expressed, while the effect of the other may be masked.

the second hypothesis states that when gametes are formed, the two alleles of each gene are segregated.