In a species of sweet pea, flower colour is determined by a biochemical pathway controlled by two independently assorting genes, $A$ and $B$:

$$Colorless\; Precursor\; 1\underrightarrow{Enzyme\; A}Colorless\; Precursor\; 2\underrightarrow{Enzyme\; B}Red\; Pigment$$

• Enzyme A is coded for by the dominant allele $A$. The recessive allele $a$ produces a non-functional enzyme.
• Enzyme B is coded for by the dominant allele $B$. The recessive allele $b$ produces a non-functional enzyme.

If two plants heterozygous for both genes ($AaBb$) are crossed, what is the expected phenotypic ratio of red-flowered to white-flowered offspring?