Enzymatic explanation of genetic ratios

• Genetic ratios, Genetic interactions and dominance can be explained simply by one gene one enzyme hypothesis or enzymatic relationship.

Let us take an example of 9:7 ratio (Complementary gene action) in F2 generation in dihybrid cross of flower hybrid.

Parents : AABB ( Purple)  x aabb ( white)

F1 : AaBb ( Purple)

F2: 9 AB, 3 Abb, 3 aaB ,1 aabb ] 9:7

The ratio can be explained by considering following biochemical pathway:

White precursor (Substrate, colorless) ——Enzyme A, Gene A———–> White precursors (product, colorless, chromogen) —Enzyme B, Gene B———-> Purple pigment (Color pigment, Anthocyanin)

• The purple color of flower is dependent on two non-allelic complementary genes A and B.
• Gene A produces an enzyme that catalyzes the formation of colorless chromogen for the formation of anthocyanin pigment.
• Gene B controls the production of an enzyme which catalyzes the transformation of this chromogen into anthocyanin. Hence, these two genes are complementary to each other.

Conclusion:

• If Plant possess dominant gene A and B, it produces purple flower.
• If plant possess dominant A and recessive bb then substrate is changed into chromogen, but not to anthocyanin which leads to white color.
• Similarly, presence of B and absence of A results no further progress of reaction and ultimately results in white color.
• If both gene A and B are absent, then no biochemical reaction occurs and results white color.
• The meaning of dominant and recessive also becomes clear by enzymatic pathways.