13. A cis-trans complementation test is carried out to identify
(1) if two mutations are allelic in nature.
(2) if two genes interact with one another.
(3) the number of genes influencing phenotype.
(4) to understand the dominance / recessjve relationships between alleles.

Core idea

A cis–trans (complementation) test is a classical genetic test in which two mutations that produce a similar phenotype are combined in:

• Cis: both mutations on the same DNA molecule.

• Trans: each mutation on a different homolog.

By examining whether the trans‑heterozygote shows wild type or mutant phenotype, geneticists determine whether the mutations affect the same gene (allelic) or different genes (non‑allelic).​

• If the trans combination is mutant, the mutations fail to complement and are considered allelic (same gene).

• If the trans combination is wild type, the mutations complement each other and are non‑allelic (different genes).

Thus, the primary purpose is to test allelism.

Option-wise explanation

1. If two mutations are allelic in nature – correct

   • The cis–trans test is explicitly designed to decide whether two mutations belong to the same functional gene (allele series) or to different genes that independently affect the same trait.​

2. If two genes interact with one another

   • Gene interaction (epistasis, modifier effects) can influence interpretation, but the cis–trans test is not primarily a test for interaction; it is for whether mutations share a locus.

3. The number of genes influencing phenotype

   • Complementation data from many mutants can be grouped into complementation groups to infer the minimum number of genes involved in a pathway, but the single cis–trans test itself is focused on pairwise allelism, not directly counting genes.

4. To understand dominance/recessive relationships between alleles

   • Dominance is usually inferred from simple heterozygote phenotypes (mutant/wild‑type crosses). The cis–trans test assumes mutations are recessive for straightforward interpretation; it is not primarily for classifying dominance.

Therefore, a cis–trans complementation test is carried out mainly to determine whether two mutations are allelic (in the same gene).