Muscular Dystrophy

I sit here and stare
at a pedigree tree
If the two parents of the couple
were brother and sister,
what is the probablitiy
that the couple's first child
will be an affected boy?
I think and I think
I write and erase
I daddle and doodle
and I finally realize,
who the fuck
is sick enough
to mate
consanguineously