Final answer:
A 2:1 phenotypic ratio often suggests a recessive lethal allele is at play, where the homozygous recessive combination is lethal, leaving only the dominant and heterozygous combinations viable, such as in the case of the Curly allele in Drosophila.
Step-by-step explanation:
A 2:1 phenotypic ratio typically indicates the presence of a recessive lethal allele in a genetic cross. Specifically, this occurs when individuals heterozygous for a recessive lethal allele mate. The expected genotypic ratio is generally 1:2:1 for homozygous dominant, heterozygous, and homozygous recessive offspring, respectively. However, with a recessive lethal allele, the homozygous recessive genotype results in death, leading to a modified 2:1 ratio of surviving offspring.
This represents the wild-type homozygotes and heterozygotes that display the dominant phenotype. An example is the Curly allele in Drosophila, which causes aberrant wing shape in heterozygotes and is lethal in homozygous recessive individuals. As a result, the genotypic ratio observed in the offspring of such a cross would be 1:2:0.