Markers on one chromosome travel together through a meiosis, broken only
by crossovers. An early-generation hybrid's genotype is shaped by the
recombinant gamete(s) of its hybrid parent(s): one for a backcross, two
for an F2. The number of independently inherited pieces is about
meioses * (chromosomes + map length in Morgans). The default of one
meiosis suits backcrosses, the classes most affected; in the bear
pedigree truth set it let every known backcross be classified correctly
while animals with old introgression were still called later-generation.
Value
A number to pass to classify_hybrids() as effective_markers.