The threadlike structures that control the traits of organisms are chromosomes.

Here's why:

* Chromosomes are made of DNA: DNA contains the genetic instructions for building and maintaining an organism.

* Genes are segments of DNA: These segments hold the specific instructions for individual traits, like eye color or height.

* Chromosomes are organized structures: They package the long strands of DNA into a manageable form for cell division and function.

So, while the DNA itself is the blueprint, chromosomes are the organized structures that carry and transmit those instructions from one generation to the next.