#19  Each F1 has two alleles (R r).  Once again during the process of   MEIOSIS,
           these alleles must undergo SEGREGATION with ONE (a number) allele(s) then
           heading for each F2 offspring. 
    #20  That means that the first F1 produces alleles  (R + r) which have separated
            (you show this separation by the + sign between them) and the second F1
            produces alleles  (R + r).