8. Mutation at two different loci of the same gene X results in altered functions. These two mutated versions of gene X are called
(1) alleles
(2) Complementation group
(3) interrupted genes
(4) linkage group

Concept and reasoning

A gene can exist in multiple sequence variants.
If gene X acquires a mutation at one site (say position 100) and another version has a different mutation at another site (say position 250), both forms still represent gene X but with altered DNA sequences and often altered functions. Such distinct sequence forms of the same gene are termed alleles.

So “two mutated versions of gene X” = two different alleles of gene X.

Option‑wise explanation

1. Alleles – correct

   • Alleles are different forms of the same gene occupying the same locus on homologous chromosomes, differing in DNA sequence and often function.

   • Wild‑type and each mutant version are all alleles of that gene; here, the two mutants are distinct mutant alleles of gene X.

2. Complementation group

   • A complementation group is a set of mutations that fail to complement each other because they lie in the same gene.

   • It describes a group of mutations, not the individual mutant versions themselves. The question asks what the two versions are called, not what group they belong to.

3. Interrupted genes

   • Refers to genes whose coding sequences are split by introns (eukaryotic “split genes”).

   • This is about gene structure (exons/introns), unrelated to having two different point mutations in the same gene.

4. Linkage group

   • A linkage group is a set of genes located on the same chromosome that tend to be inherited together.

   • It concerns multiple genes on a chromosome, not multiple mutant forms of one gene.

Therefore, two different mutant versions of the same gene X are correctly termed alleles.