Botany · Ch 2 — Classical Genetics
Test Cross
Test Cross
A test cross is the cross of an individual of unknown genotype with a homozygous recessive individual, and its purpose is to reveal that unknown genotype. Mendel needed such a method because, while a homozygous recessive plant (tt, dwarf) can be identified just by its phenotype, a plant showing the dominant phenotype (tall) could be either homozygous dominant (TT) or heterozygous (Tt) - self-pollination alone would not immediately settle the question. So Mendel crossed dominant-phenotype F2 plants with a known homozygous recessive dwarf plant instead of self-pollinating them: if the tall parent was actually homozygous (TT), every offspring of the test cross is tall (no segregation); if the tall parent was heterozygous (Tt), the test-cross offspring split 1 tall : 1 dwarf, because the heterozygote produces two kinds of gametes in equal number …
What this figure shows. Two Punnett-square diagrams side by side comparing the two possible test-cross outcomes: a heterozygous tall plant (Tt) crossed with a homozygous dwarf plant (tt) gives offspring split 1 tall (Tt) : 1 dwarf (tt); a homozygous tall plant (TT) crossed with the same homozygous dwarf plant (tt) gives all-tall (Tt) …
What this figure shows. A flowchart showing the molecular basis of the tall/dwarf trait: the dominant Le allele codes for a functional enzyme that converts a gibberellin precursor molecule into active gibberellin (GA1), producing tall plants whether the genotype is LeLe or Lele, while the recessive le allele codes for a non-functional enzyme, so lele plants cannot make activ …