Define and design a test-cross.
Definition: A test cross is a cross between an organism showing a dominant phenotype (whose genotype is to be determined) and a recessive parent. It was devised by Mendel to determine the genotypic composition of an organism, for example, whether a tall plant from F₂ has TT or Tt genotype, which cannot be predicted by simply looking at the phenotype.
Design: To perform a test cross, cross the organism with dominant phenotype with a homozygous recessive organism. For example, cross a tall plant (T_) with a dwarf plant (tt). The progenies of such a cross can easily be analysed to predict the genotype of the test organism. If the tall plant is homozygous (TT), all progeny will be tall (Tt). If it is heterozygous (Tt), the progeny will show a 1:1 ratio of tall (Tt) to dwarf (tt) plants.
Explanation
The textbook defines test cross in the context of Mendel's experiments where he needed to determine the genotype of plants showing dominant traits. The definition emphasizes crossing with a recessive parent (not self-crossing) to reveal the unknown genotype. The design involves analyzing progeny ratios to determine if the parent was homozygous or heterozygous.
Solution Steps
Step 1: Define test cross as crossing dominant phenotype organism with recessive parent
Step 2: Explain the purpose - to determine unknown genotype (TT vs Tt)
Step 3: Design the cross showing parent genotypes (T_ × tt)
Step 4: Describe expected progeny ratios for homozygous (all tall) vs heterozygous (1:1 ratio)