If am individual cross between two true breed mice the probability of offspring with heterozygous condition is 100 percent in such cross. True breed is an individual organism with homozygous for the particular character either dominant or recessive.
So, if mice with RR (purebred or homozygous dominant) bred with mice with rr (homozygous recessive), all the offspring will be heterozygous in genotype which is Rr.
Thus, the correct answer is : option B. (Punnet square is attached)
