Which component is NOT considered a building block of heredity?

The component that is not considered a building block of heredity is phenotype. Phenotype refers to the observable physical and physiological traits of an organism, which result from the interaction between its genotype (the genetic makeup) and the environment. While heredity indeed influences phenotype, it does so through the genetic information encoded in DNA, the structures of chromosomes, and the reproductive cells known as gametes. These three components—DNA, chromosomes, and gametes—are directly involved in the transmission of genetic information from one generation to the next. In contrast, phenotype is the end result of these genetic factors interacting with environmental influences, rather than a fundamental building block of heredity itself.