In order to write genetics, I need something like $\frac{A}{a}$ but with
a double horizontal bar -- and left( and right)
I tried this, but there is too much white around it.
\newcommand\gen[2]{ %genotype
$
\left(
\begin{array}{c} #1 \\
\hline \hline #2\\
\end{array}
\right)
$