Here's why:

* Genes are segments of DNA. DNA is the blueprint of life, containing the genetic code for an organism.

* Genes contain the specific sequence of nucleotides (A, T, C, G) that code for a particular protein. This sequence is read in groups of three nucleotides called codons. Each codon corresponds to a specific amino acid.

* During protein synthesis, the genetic code within a gene is transcribed into messenger RNA (mRNA). This mRNA then travels to ribosomes, where it is translated into a protein.

So, genes are the key players in determining the amino acid sequence of a protein, and therefore its structure and function.