USA Biology Olympiad (USABO) Practice Exam

During the breeding of laboratory rats, what characteristic indicates homozygous dominant individuals?

1. Presence of black fur only

2. Aggressive behavior

3. Indistinguishable from heterozygotes

4. Larger body size

The correct answer is: Indistinguishable from heterozygotes

In genetics, homozygosity refers to having two identical alleles for a particular gene, and the dominant phenotype typically manifests when at least one dominant allele is present. In the case of laboratory rats, the homozygous dominant individuals would express a phenotype that corresponds to the dominant allele, which may appear similar or indistinguishable from heterozygous individuals who also have at least one dominant allele. This characteristic is particularly true in situations where the traits of interest do not exhibit a clear visual distinction between the homozygous dominant and heterozygous phenotypes. For instance, if black fur is associated with the dominant allele, both homozygous dominants (BB) and heterozygotes (Bb) will display black fur, making it impossible to tell them apart based solely on their external appearance. Other traits or variables, such as aggressive behavior or larger body size, might be influenced by multiple factors, including environmental influences or polygenic inheritance, which do not serve as reliable indicators of homozygous dominance. Therefore, the indistinguishable nature of homozygous dominant individuals from heterozygotes accurately reflects the inheritance patterns and traits being studied, solidifying the link between genotype and phenotype in genetics.