According to Mendel’s hypothesis, the gametes combine by chance with certain fixed probabilities. In selfing between plants of F1 generation (all hybrid), both the parents are heterozygous, i.e., have Tt genes. These genes separate in the gametes. Therefore 50% gametes having T and other 50% gametes having t from both parents. During fertilization, male gamete (pollen) with gene T or t may combine with female gamete (ovule) with gene T or t. So the probability of having gene T or t from both male and female gametes is 50% each.

The four possible combinations of genotype are : TT, Tt, tT and tt because :
(i) Gene T from male gamete and gene T from female gamete = TT = 1/4.
(ii) Gene T from male gamete and gene t from female gamete =Tt = 1/4.
(iii) Gene t from male gamete and gene T from female gamete = tT = 1/4.
(iv) Gene t from male gamete and gene t from female gamete = tt = 1/4.

If (ii) and (iii) points considered as Tt then 1/4 + 1/4 = 1/2. 
Therefore, F2 ratio of TT : Tt : tt is 1/4 : 1/2 : 1/4, which is equal to 1 : 2 : 1, one homozygous dominant (TT), two heterozygous or hybrid (Tt) and one homozygous recessive (tt).