The development of an organism — from a fertilized egg, through embryonic and juvenile stages, to adulthood — requires the coordinated expression of sets of genes at the proper times and in the proper places. Studies of several bizarre mutations in the fruitfly, Drosophila , provided keys to understanding the molecular basis of large-scale developmental plans. Early embryonic genes express proteins that set up the orientation and define the body segments of the fly embryo. Then "homeotic" genes act on the segments to make the body parts distinct to each segment.

The derivation for this equation is shown in the yellow box, below. Notice that Equation 11 is in a similar form to the Henderson-Hasselbach equation presented in the introduction to the Experiment (Equation 16 in the lab manual). Equation 11 does not meet the strict definition of a Henderson-Hasselbach equation, because this equation takes into account a non-acid-base reaction ( ., the dissociation of carbonic acid to carbon dioxide and water), and the ratio in parentheses is not the concentration ratio of the acid to the conjugate base. However, the relationship shown in Equation 11 is frequently referred to as the Henderson-Hasselbach equation for the buffer in physiological applications.