In Mendel's experiment ratio 3:1 is known as .................. ratio.

The monohybrid cross-refers to inheritance of singe contrasting trait by two parents at a time. For example, a cross of tall (T) and dwarf (t) pea plants is a monohybrid cross as it exhibits the inheritance of plant height (TT x tt). A monohybrid cross between two pure breeding varieties obtains all dominant $F_1$ generation which in turn produces $F_2$ generation upon selfing and gives the monohybrid ratio of 3 (dominant):1 (recessive). The dihybrid cross-refers to the inheritance of two pairs of contrasting characters by two parents at the same time. For example, a cross of two pea plants, one with yellow and round seeds (YYRR) and other with green and wrinkled seeds (yyrr). Selfing of $F_1$ hybrids give dihybrid ratio of 9:3:3:1 in $F_2$ generation. Likewise, the inheritance of three traits by two parents at a time refers to the trihybrid cross. Example, inheritance of plant height, seed shape, and seed color (TTYYRR x ttyyrr). The phenotypic ratio for a trihybrid cross is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. Thus, the correct answer is option A.