The hereditary material stored in chromosomes is called DNA (Deoxyribonucleic Acid).

Here's a breakdown:

* Chromosomes: These are thread-like structures found in the nucleus of every cell. They are made up of DNA and proteins.

* DNA: This is a long, complex molecule that contains the genetic instructions for building and maintaining an organism. It's organized into genes, which are units of heredity that determine specific traits.

So, chromosomes are essentially the packaging units for DNA, and DNA is the actual blueprint containing the genetic information passed down from parents to offspring.