Monohybrid Cross and the Law of Dominance

A monohybrid cross is a genetic cross in which the parents differ in, and the experimenter therefore follows, just a single pair of contrasting characters at a time, keeping every other character constant. When Mendel crossed a pure-breeding tall pea plant with a pure-breeding dwarf pea plant, the entire first filial (F1) generation grew up uniformly tall — not a single dwarf plant, and not a single plant of intermediate height, appeared among the F1 offspring, even though the dwarf parent had contributed exactly as much genetic material to each F1 offspring as the tall parent had. Mendel called the character that showed itself in the F1 generation — tallness, in this cross — the dominant character, and the character that appeared to vanish completely — dwarfness — the recessive character; he stressed that the recessive character was not destroyed or permanently lost but merely masked, since it would reappear, unaltered, in a later generation (as indeed it did in the F2, described in the next section).

This single observation is formalised as Mendel's Law of Dominance, one of the three foundational laws of classical genetics: when an organism is heterozygous, carrying one dominant allele and one recessive allele of a given gene, only the dominant allele is expressed in the observable phenotype, while the recessive allele's own potential expression is completely suppressed and hidden from view. Modern genetic notation follows a convention Mendel himself essentially originated: the dominant allele of a gene is written using a capital letter — T for the tall-determining allele in this example — while its recessive counterpart is written using the SAME letter in lower case, t for the dwarf-determining allele. Under this notation, a true-breeding tall parent plant is homozygous dominant, genotype TT (both alleles are the dominant T); a true-breeding dwarf parent plant is homozygous recessive, genotype tt (both alleles are the recessive t); and every single F1 offspring of the TT × tt cross necessarily receives one T allele from its tall parent and one t allele from its dwarf parent, making it heterozygous, genotype Tt — yet, by the Law of Dominance, every Tt plant is phenotypically indistinguishable from a TT plant, because the dominant T allele alone determines the observed tall phenotype. …