Answer: The correct answer for the blank is- Gene.
Gene can be described as the segment of DNA ( deoxyribonucleic acid) that codes for a particular protein, which inturn determines the specific trait of an individual.
They contain genetic instruction ( in the form of nucleotide sequences ) that control particular trait. Genes are considered as the unit of heredity because they are passed from one generation to the next generation in the form of DNA.
Inherited traits are those that are passed from parents to their offspring.
Thus, code / genetic instruction for these traits is found in genes.