Dihybrid cross of completely linked gene refers to the inheritance pattern of two different traits that are controlled by genes located on the same chromosome. In a completely linked gene, the two genes are located on the same chromosome and are always inherited together. This means that there is no recombination or crossing over between the two genes during meiosis. The phenotypic and genotypic ratio of a dihybrid cross of completely linked gene can be determined using Punnett square.

Phenotypic Ratio
The phenotypic ratio refers to the ratio of different observable traits in the offspring resulting from a dihybrid cross of completely linked gene. The phenotypic ratio of a dihybrid cross of completely linked gene is always 1:1. This means that all the offspring will have the same combination of traits and there will be no variation in the phenotype.

Genotypic Ratio
The genotypic ratio refers to the ratio of different possible genotypes in the offspring resulting from a dihybrid cross of completely linked gene. The genotypic ratio of a dihybrid cross of completely linked gene is always 1:2:1. This means that there will be three possible genotypes in the offspring: homozygous dominant for both genes, heterozygous for both genes, and homozygous recessive for both genes. Each of these genotypes will occur in the ratio of 1:2:1.

Example
Let us consider a dihybrid cross between two individuals that are homozygous dominant for both genes (AA BB x AA BB). The Punnett square for this cross would be as follows:

A A
B AABB AABB
B AABB AABB

The offspring resulting from this cross would all be heterozygous for both traits (Aa Bb). The genotypic ratio would be 1:2:1 (1 AA BB : 2 Aa Bb : 1 aa bb). The phenotypic ratio would be 1:1, meaning that all the offspring would have the same combination of traits.

In conclusion, the phenotypic and genotypic ratio of a dihybrid cross of completely linked gene is determined by the nature of the genes involved and their location on the same chromosome. The phenotypic ratio is always 1:1, while the genotypic ratio is always 1:2:1.