Ans: B: $(ax+by)^2$

Based on $\frac {1}{4} : \frac {1}{2} : \frac {1}{4}$ ratio of TT : Tt : tt, it can be mathematically condensed to the form of a binominal expression $(ax + by)^2$, that has gametes bearing genes T or t in equal frequency of $\frac {1}{2}$.

The expanded form of the expression is:

$(\frac{1}{2}T + \frac {1}{2} t)^2 = (\frac {1}{2}T + \frac {1}{1}t) \times (\frac {1}{2}T + \frac {1}{2}t)$

$= \frac {1}{4} T^2T + \frac {1}{2} T t^2 + \frac {1}{4} tt$