11. In a heterozygous two recessive mutation at different site will give mutant phenotype when genes involved are
(1) Allelic and placed in cis
(2) Allelic and placed in trans
(3) Non-allelic and placed in cis
(4) Non-allelic and placed in trans

Core concept

“Two recessive mutations at different sites” means two different mutant alleles of the same locus (e.g., a¹ and a²).
A heterozygote can carry them in two ways:

• Cis: both mutations on the same chromosome (a¹a² / A⁺).

• Trans: one mutation on each homolog (a¹ / a²).

For a recessive gene, a wild‑type phenotype appears if there is at least one functional allele (A⁺) somewhere. A mutant phenotype appears only when no functional copy of that gene is present.

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Option-by-option explanation

1. Allelic and placed in cis

   • Genotype example: (a¹ a² / A⁺).

   • One chromosome carries both mutant sites, but the other homolog has a normal allele A⁺.

   • Because recessive mutations are masked by a functional A⁺, phenotype is wild type, not mutant.

2. Allelic and placed in trans – correct

   • Genotype example: (a¹ / a²).

   • Each chromosome has a different mutant allele; no chromosome carries a wild‑type copy.

   • Since both alleles at that locus are nonfunctional recessives, the heteroallelic combination behaves like homozygous recessive, giving a mutant phenotype.

   • This is the classical result of a complementation test within a cistron: failure to complement indicates allelism.

3. Non‑allelic and placed in cis

   • “Non‑allelic” means the two mutations are in different genes (say gene A and gene B) on the same chromosome.

   • In a heterozygote, each mutant gene is usually complemented by a wild‑type copy on the opposite homolog (A⁺ and B⁺), so functional products are still made.

   • The typical outcome is wild type, not guaranteed mutant.

4. Non‑allelic and placed in trans

   • Each chromosome has one mutant gene but at different loci; the other locus on that chromosome is wild type.

   • Together the two chromosomes supply at least one good copy of each gene (A⁺ and B⁺), so they complement and give wild type.

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Thus, in a heterozygous condition, two recessive mutations at different sites will still show a mutant phenotype only when they are allelic (same gene) and in trans, i.e., option (2).