With the help of Punnett square describe monohybrid experiment.

Explanation

Punnett square was developed by a British geneticist Reginald C. Punnett.
= It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
= The possible gametes are written on two sides usually the top row and left columns.
= All possible combinations are represented in boxes below in the squares, which generates a square output form.
= The punnett square shows the parental tall TT (male) and dwarf tt (female) plants, the gametes produced by them and the `F_1` .TP progeny.
= The `F_1` plants of genotype Tt are self-pollinated.
= The symbols and are used to denote the female (eggs) and male (pollen) of the `F_1` generation respectively.
= The `F_1` plant of the genotype Tt when self-pollinated, produces gametes of the genotype .T. and .t. in equal proportion.
= When fertilisation take place, the pollen grains of genotype .T. have a 50 percent chance to pollinate eggs of the genotype .T. as well as of genotype .t..
= Also pollen grains of genotype .t. have a 50 percent chance of pollinating eggs of genotype .T. as well as of genotype .t
= As a result of random fertilisation the resultant zygote can be of the genotypes TT, Tt or tt.
= From the punnett square it is easily seen that 1/`4^(th)` of the random fertilisation lead to TT, 1/2 lead to Tt and 1/`4^(th)` to tt.
= Though the F, have genotype of Tt but the phenotypic character seen is .tall..
= At `F_2 3//4^(th)` of the plants are tall, where some of them are TT while others are Tt.
= Externally it is not possible to distinguish between the plants with the genotypes .TT. and Tt.
= Hence within the genophytic pair "Tt. only one character .T. tall is expressed.
= Hence the character .T. or .tall. is said to dominate over the other allele .t. or dwarf character.
= It is thus due to the dominance of one character over the other that all the `F_1` are tall (though genotypically 1/2 are Tt and only` 1//4^(th)` are TT).

= This leads to a phenotypic ratio of `3//4^(th)` tall : [1/4 TT + 1/2 Tt) and 1/4th tt i.e. a 3:1 ratio but a genotypic ratio of 1:2:1.
= The 1/4: 1/2 : 1/4 ratio of TT : Tt : tt is mathematically condensable to the form of the binomial expression (ax + `b y]^2`, that has the gametes bearing genes T or t in equal frequency of 1/2.
= The expression is expanded as given below: `[1//2T + 1//2t]^2 = [1//2T + 1//2t] xx ` [1/2T + 1/2t] = 1/4TT + 1/2Tt + 1/4tt