The code for heredity is carried on
in each organism's DNA.

The code for heredity is carried on genes in each organism's DNA.

Explanation:

Genes are portions of the genome that codes for a protein or an RNA, Hereditary information that is contained in nucleotide sequence of DNA.

nucleic acid molecules are incredibly complex, containing the code that guarantees the four different nucleotide units comprise dna, the nucleic acid of interest to the genealogist. and one of four nitrogen-containing molecules called nucleotides