Due to presence of dominant and recessive trait.
In his experiment, Mendel discovered that all of the first generation progeny were tall when he crossed a true breeding pea plant (TT) with a true breeding short pea plant (tt) (Tt). However, the short pea plant returned in the second generation.
From this, he deduced that the allele for a certain trait, such as tallness, "T," is dominant over the corresponding recessive trait, "t," since when these two allele co-occur, the traits of "t" are not manifested. A recessive allele like "t" will only manifest itself when they are in a pair, i.e. tt.
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