Here's why:

* Genes are the blueprints: Genes are segments of DNA that contain the instructions for building and maintaining an organism. They act like blueprints for traits like eye color, hair color, height, and even certain predispositions to diseases.

* Passing down the blueprints: During sexual reproduction, each parent contributes half of their genetic material (in the form of chromosomes) to their offspring. This means the offspring inherits a combination of genes from both parents.

* Variations in traits: Slight variations in these genes, called alleles, lead to the diversity we see in the biological characteristics of individuals within a species.

Therefore, the inheritance of biological characteristics is indeed primarily determined by the genes passed down from parents to offspring. This is a fundamental principle of genetics.