AAMC FL Practice Exam

The principle of a 3:1 ratio in a genetic cross indicates what about the original crossed parents?

1. They are homozygous

2. They are diploid

3. They are heterozygotes

4. They are polyploid

The correct answer is: They are heterozygotes

A 3:1 phenotypic ratio in a genetic cross typically arises in the offspring when two heterozygous parents are crossed for a single trait governed by simple dominance. In this scenario, the dominant allele can mask the presence of the recessive allele, leading to a situation where three individuals display the dominant phenotype for every one individual showing the recessive phenotype. When two heterozygous individuals (each with one dominant and one recessive allele, often represented as Aa) are crossed, the possible genotypes of the offspring include one homozygous dominant (AA), two heterozygous (Aa), and one homozygous recessive (aa). This results in a 3:1 ratio of dominant to recessive phenotypes among the offspring. The other options, while touching on various genetic concepts, do not lead to this specific ratio. For instance, if the parents were homozygous, there would be no variability in the offspring phenotypes to create a 3:1 ratio. Diploid refers to the number of chromosome sets in an organism, but it does not directly relate to this ratio. Polyploidy, which involves having more than two sets of chromosomes, is also unrelated to the specific 3: