# Understanding the meaning of $s$ and $t$ in a population genetics equation

Sewall Wright in this article (1937) at the end of page 313 gives the equation:

$$\Delta q = (s+tq)q(1-q) \space\space\space\space\space\space\space\space\space(1)$$

This equation is an approximation who holds only when $t$ and $s$ are small. $q$ is the frequency of allele A. $s$ and $t$ define the fitness of the three possible genotypes AA, AB and BB. I am not sure which genotype has fitness equal to 1 and I don't how $t$ and $s$ define the genotype fitnesses.

Let's assume for example that the genotype BB has fitness of 1. Then, the fitness of the genotypes AB and AA might be given by $1-t$ and $1-s$ respectively or $1-s$ and $1-t$ respectively or eventually by $1-t$ and $1-(s+t)$ (or variants) or even $s$ and $t$ (and opposite) or $st$ and $s$ (and variants). The genotypes fitness might as well be given by $1-st$ and $1-s$ but it would not be make much sense given the assumptions used for the approximation.

My question is: do you know how is the fitness of the three genotypes defined in equation (1)?