The genotype–phenotype distinction is drawn in genetics. “Genotype” is an organism’s full hereditary information. “Phenotype” is an organism’s actual observed properties, such as morphology, development, or behavior. This distinction is fundamental in the study of inheritance of traits and their evolution.
It is the organism’s physical properties which directly determine its chances of survival and reproductive output, while the inheritance of physical properties occurs only as a secondary consequence of the inheritance of genes. Therefore, to properly understand the theory of evolution via natural selection, one must understand the genotype–phenotype distinction. The genes contribute to a trait, and the phenotype is the observable expression of the genes (and therefore the genotype that affects the trait). Say a white mouse had the recessive genes that caused the genes that cause the color of the mouse to be inactive (so “cc”). Its genotype would be responsible for its phenotype (the white color).