Final answer:
When offspring of two homozygous recessive individuals express the mutant phenotype, it indicates the mutation is in the same gene because they inherited the non-functional allele from both parents.
Step-by-step explanation:
In a complementation test, if mating two homozygous recessive individuals results in offspring that exhibit the mutant phenotype, you can conclude that the mutation is in the same gene. This is because the recessive trait is expressed when there are two copies of the mutant allele, which implies that both parents contributed the same non-functional allele to the offspring.
The logic behind the complementation test is based on Mendel's principles of inheritance, where for a characteristic that follows dominant and recessive patterns, the dominant allele will mask the presence of the recessive allele in a heterozygous organism, resulting in the expression of the dominant trait. However, when an organism is homozygous recessive, having two copies of the recessive allele, this recessive trait is unmasked and expressed in the phenotype. Therefore, the observed phenotype in the offspring indicates that the recessive alleles at the same locus from both parents are non-functional.