B, Mendel thinks that YyRr plants can produce four kinds of female gametes and four kinds of male gametes, which are yr, yR, Yr, YR respectively, and the ratio is1:1.B is correct;
C, the variation of alleles on homologous chromosomes cross-exchanged by non-sister chromatids in tetrad stage belongs to gene recombination, and C is wrong;
D the chromosome number of human cells is 2N in the process of subtracting Ⅰ, n in the early, middle and late stages of subtracting Ⅱ, and 2N in the late stage of subtracting Ⅱ. D is wrong.
Therefore, choose: B.