14.
A protein coding gene in the mouse was mutated such that the resultant protein was non-
functional. The phenotypic effect of the mutation was observed in homozygous mutant
mice, in heterozygous mice that inherited the mutation from the mother, and in
heterozygous mice that inherited the mutation from the father. This suggests that:
a. The gene is imprinted
b. The mutation acts as a dominant negative
c. The gene possibly has more than two alleles
d. The gene is X-linked

Mouse Gene Mutation Phenotype in Homozygous and Heterozygous Mice: CSIR NET Solution

A mutation in a mouse protein-coding gene produces a non-functional protein, yet the phenotype appears in homozygous mutants and all heterozygous mutants regardless of parental origin. This indicates a dominant negative mutation effect.

Option Analysis

a. The gene is imprinted
Genomic imprinting silences one parental allele based on origin, so phenotypes differ between maternal and paternal heterozygotes. Here, equal effects from both parents rule out imprinting.​

b. The mutation acts as a dominant negative
Dominant negative mutations yield defective proteins that interfere with wild-type proteins from the normal allele, causing phenotype in heterozygotes. The non-functional protein disrupts function in both homozygous and heterozygous cases from either parent, matching the observation.​

c. The gene possibly has more than two alleles
Multiple alleles exist at population level but individuals carry only two; this scenario involves one specific mutation without evidence of additional alleles affecting dominance. It does not explain uniform heterozygous phenotypes.​

d. The gene is X-linked
X-linked genes show sex-specific inheritance: males express hemizygous traits, while heterozygous females often lack recessive phenotypes. No sex distinction or parent-sex patterns appear here.​

Correct Answer

b. The mutation acts as a dominant negative.​

The mouse protein coding gene mutation phenotype in homozygous heterozygous puzzle reveals key genetics principles for CSIR NET aspirants. When a mutation renders a protein non-functional yet triggers phenotypes across all genotypes, it points to dominant negative action over imprinting or X-linkage.

Dominant Negative Mechanism

Mutant proteins bind partners or compete with wild-type versions, blocking normal function even in heterozygotes. Unlike simple loss-of-function (recessive), this interference dominates.​

Why Not Other Options?

• Imprinting depends on parental origin, unlike the uniform effect here.​

• Multiple alleles don’t dictate single-mutation dominance.​

• X-linked shows male bias, absent in this data.​

CSIR NET Relevance

Such questions test inheritance patterns in molecular biology. Practice distinguishes dominant negative from haploinsufficiency.​