Q.In pea plants, round seed shape ($R$) is dominant over wrinkled seed shape ($r$). A heterozygous round-seeded plant ($Rr$) is self-pollinated. Using a Punnett square, calculate what fraction of the offspring is expected to have wrinkled seeds, and what fraction is expected to be true-breeding round-seeded.

A monohybrid cross studies the inheritance of a single character - for example plant height in garden pea - and so tracks just the two alleles of one gene through a cross. Crossing a pure-breeding tall parent (TT) with a pure-breeding dwarf parent (tt) gives an F1 generation that is uniformly tall (Tt), the dwarf character completely masked; selfing this F1 gives an F2 generation split 3 tall : 1 dwarf (Mendel's own data: 787 tall to 277 dwarf among 1,064 F2 plants), the recessive dwarf trait reappearing because segregation reconstitutes some homozygous recessive (tt) individuals. This pattern is exactly what the Law of Dominance and the Law of Segregation, taken together, predict, and Mendel additionally showed by reciprocal crossi …