Final answer:
The genotype of the tall pea plant in the cross with a short pea plant with homozygous recessive genotype (tt) must be heterozygous (Tt) because this results in 50% tall offspring, as per the observed data.
Step-by-step explanation:
In this genetics problem, we are examining a cross between a tall pea plant and a short pea plant to determine the genotype of the tall parent. The short pea plant is homozygous recessive (tt), and the offspring are 50% tall. Given that the phenotype of a tall pea plant can be due to two genotypes—homozygous dominant (TT) or heterozygous (Tt), and that the offspring have a 50% chance of being tall, the tall parent must be heterozygous (Tt). This is because a cross between a tt and TT parent would result in all tall (100%) offspring, while a cross between a tt parent and a Tt parent would result in a 50/50 split between tall and short offspring.
To further understand, we can use a Punnett square. If the tall parent were TT, all offspring would be Tt, which conflicts with the observed data of a 50% tall offspring. Therefore, the only logical conclusion is that the tall parent in the cross must have a genotype of Tt, as this would produce a phenotypic ratio of 1 tall (Tt) to 1 short (tt) in the offspring.