Final answer:
If a testcross results in half tall and half dwarf progeny, the tall parent is heterozygous (Tt). A Punnett square predicts a 1:1 phenotypic ratio in a cross between a dwarf (tt) and a heterozygous tall (Tt) pea plant. Crossing two heterozygous tall plants (Tt) leads to a 3:1 phenotypic ratio of tall to dwarf offspring.
Step-by-step explanation:
Understanding the Testcross and Phenotypic Ratios
When a testcross produces half tall and half dwarf progeny from a tall parent, it indicates that the parent is heterozygous for the trait, with one dominant and one recessive allele (Tt). In this scenario, the genotypic ratio of the offspring would be 1:1 for tall (Tt) to dwarf (tt).
Using Mendelian genetics principles, we can predict the offspring in a cross between a dwarf pea plant (homozygous recessive) and a tall pea plant (heterozygous) by creating a Punnett square. This cross would result in a 1:1 phenotypic ratio of tall to dwarf offspring. The phenotype of a pea plant that is homozygous dominant (TT) for the tall trait is tall, just as the phenotype of a pea plant that is heterozygous (Tt) is tall.
When two heterozygous tall pea plants (Tt) are crossed, we would use a Punnett square to predict a genotypic ratio of 1 TT: 2 Tt: 1 tt, where the tall phenotype is dominant, leading to a 3:1 phenotypic ratio of tall to dwarf offspring.