In Gregor Mendel's work with pea plants, he found that when a tall pea plant, with genes TT, was crossed with a short pea plant, with genes tt, that % of the first generation of offspring plants were .

In Gregor Mendel's work with pea plants, he found that when a tall pea plant, with genes TT, was crossed with a short pea plant, with genes tt, that _100_ % of the first generation of offspring plants were tall pea plant_.

Explanation:

After crossing the two types of plants, Mendel observed that among the F1 there were only tall variants, while the other variant (short) disappeared. Mendel named "dominant" the expressed variant. Mendel then let these new plants auto pollinate and observed the results in the second generation, F2. He saw that the short variant that had previously disappeared, reappeared again. Both tall and short plants were present in the F2. Mendel named "recessive" the second alternative variant.

Mendel explained his results by arguing that discrete factors were responsible for these phenotypes. These factors should have been present in the F1 in pairs. One of them came from one parental plant, and the other factor came from the other plant. These factors then separated again when sex cells were produced, giving two types of gametes, each with only one factor.

Mendel concluded that each plant had a pair of factors -which he named alleles-, and that each factor coded for one trait -tall and short-. He thought that these factors -T and t- separate -segregate- during gamete formation. This conclusion is known as the segregation principle (First Mendels´ Low).