Answer:
For the expression of gene, the DNA has to be transcribed into mRNA. mRNA contains the sequence of nucleotide which codes for amino acids.
Amino acid is coded by the triplet codon present on the mRNA. The amino acids are joined together and make protein which expresses itself in phenotype.
The process by which mRNA make proteins is called translation and this conversion is called gene expression because the codon sequence is expressed in the form of amino acid or proteins. So formation of mRNA is directly related to gene expression.