Final answer:
When crossing a heterozygous long-stem pea plant (Aa) with a homozygous short-stem plant (aa), the progeny will have a 1:1 ratio of long to short stems. Different genotypes can result in the same phenotype when one allele is dominant over the other.
Step-by-step explanation:
In the context of pea plants and Mendelian genetics, when a heterozygous long-stem plant (Aa) is crossed with a homozygous short-stemmed plant (aa), the offspring are expected to exhibit a 1:1 ratio of the traits.
Here, a long stem (A) is dominant, and a short stem (a) is recessive. If a homozygous tall pea plant has the genotype TT, it will be tall.
Similarly, a heterozygous pea plant with genotype Tt will also be tall, showing that different genotypes can result in the same phenotype due to the presence of the dominant allele. However, homozygous recessive plants (tt) will be short as both alleles are recessive.
Mendel observed that crossing two heterozygous pea plants would result in approximately a 3:1 ratio of dominant to recessive phenotypes due to the random segregation of alleles.