If there are more genotypes with code 3 (BB) than code 2 (AB), we omit
the genotypes with code==2 and call those with code==3 the BB
genotypes.
If, instead, there are more genotypes with code 2 than code 3, we omit
the genotypes with code==3 and call those with code==2 the BB genotypes.
Any chromosomes with class "X" (X chromosome) are changed to
class "A" (autosomal).