Add a \transposon macro to typeset transposable elements, and a
\foreigngene macro to typeset non-Drosophila genes.

Add a new macro to typeset gene alleles. Inside the \genotype
macro, the exponentiation character ^ is redefined as an alias to
that macro.

Remove all code that attempted to draw a rightdown hook to
represent a Y chromosome. This was an ugly hack that never
gave a satisfactory result.

Use the AMS \text macro to typeset gene names.

Homozygous chromosomes can now be entered without a final slash.

Rewrite the genotype macro so that it uses the FGChrPair macro to
typeset chromosome pairs.