Which one of the following genetic law is responsible for 3 : 1 expression of the phenotypic ratio in $F_2$ generation?

Law of dominance

The correct answer is Option (1) → Law of dominance

Law of Dominance:

(i) Characters are controlled by discrete units called factors.

(ii) Factors occur in pairs.

(iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).

The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.