Question

In the ${F_2}$ generation of a cross, progeny having different traits are produced in the ratio 3:1. State whether it is a monohybrid cross or dihybrid cross? Give one example of such a cross.

Answer 1

Monohybrid cross refers to a genetic cross occurring between two homozygous individuals having different composition of allele for a particular trait whereas dihybrid cross is a genetic cross that occurs in individuals differing in two traits.

Complete answer:
Example of monohybrid crosses:
${F_1}$ generation
When a tall pea plant (TT) is crossed with a dwarf pea plant (tt). Here tall pea plant is the dominant characteristic. The seeds obtained from this cross resulted in ${F_1}$ generation. This ${F_1}$ generation was self pollinated resulting in the ${F_2}$ generation.

| T| t
---|---|---
T| Tt (Tall) | Tt (Tall)
t| tT (Tall)| Tt (Tall)

${F_1}$ generation consists of all heterozygous tall pea plants with genotypes Tt.
${F_2}$ generation
When this ${F_1}$ Generation is selfed.

| T| t
---|---|---
T| TT (Tall)| Tt (Tall)
t| tT(Tall)| Tt (dwarf)

In the resulting ${F_2}$ generation, the phenotypic ratio of tall to dwarf plants obtained is 3:1 concluding that 3/4th of the plants were tall while 1/4th were dwarf.
Thus, ${F_2}$ generation consists of 1 homozygous tall (TT), 2 heterozygous tall (Tt) and 1 homozygous dwarf (tt) but the phenotypic ratio of the cross remains 3:1 (Tall: dwarf).
So, in monohybrid cross, ${F_2}$ generation shows a phenotypic ratio of 3:1 whereas in a dihybrid cross, ${F_2}$ progeny shows a phenotypic ratio of 9:3:3:1. Therefore the given cross is a monohybrid cross.

Note: Monohybrid cross is a cross which was used by Mendel for crossing two homozygous parents having different allele concentration to determine the distribution of traits in the second filial generation.