Answer: The correct answer for the blank is -
Genes.
Step-by-step explanation:
Genes can be described as segment of DNA (deoxyribonucleic acid) that code for specific protein, which ultimately determines the trait or characteristic of an organism (such as eye color, hair color).
They are considered as the unit of heredity as they are passed from one generation to the next generation through chromosomes.
Thus, genes is the right answer for the given blank.