Final answer:
Cis-effect mutations occur within or close to a specific gene and only influence that gene's expression. Trans-effect mutations affect a gene product that regulates multiple genes, potentially across different DNA molecules. In merozygote experiments, only trans-effect mutations can be complemented.
Step-by-step explanation:
Mutations in DNA can have various effects, ranging from silent to significant changes in the organism's functionality. Cis-effects and trans-effects are two ways mutations can influence gene expression.
A cis-effect mutation occurs within or close to a particular gene, affecting only that gene's expression. Examples would include promoter mutations or enhancer region mutations that control the expression of a particular gene on the same DNA molecule. Because these mutations affect the gene's regulation in a position-dependent manner, they are not transferable to other genes or separate DNA molecules.
In contrast, a trans-effect mutation occurs when a regulatory gene product, such as a transcription factor, is affected. This mutated gene product can diffuse throughout the cell and affect the expression of target genes on different DNA molecules, hence having a distant effect.
Regarding a merozygote experiment, where a bacterial cell contains two copies of a gene (one on the bacterial chromosome and one on an extrachromosomal element such as a plasmid), only mutations with trans-effects can be complemented. This is because the normal product from the unaffected gene copy can restore normal function. Cis-effect mutations cannot be complemented in such experiments as the defective regulation affects only the local site of the gene.