Question

Cis-trans or complementation test results may be ambiguous, if-
A. The mutations are dominant
B. There is intragenic complementation
C. The mutations belong to different genes with different phenotype
D. There is polar mutation
E. There is gene interaction
Choose the correct answer from the options given below

• A. A-B-C-D
• B. B-C-D-E
• C. A-B-D-E
• D. A-C-D-E

Hint:

Remember that the "Cis" test is the control to show that two mutations on the same chromosome produce a mutant phenotype, while the "Trans" test determines allelism.
Intragenic complementation is the most common biological exception encountered in laboratory genetics.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the conditions under which the cis-trans (complementation) test fails to provide a clear result regarding whether two mutations are allelic (on the same gene) or non-allelic (on different genes).
The complementation test is a fundamental tool in genetics used to determine if two recessive mutations occur in the same functional unit or gene.
If the mutations complement each other (wild-type phenotype produced), they are generally in different genes.
If they fail to complement (mutant phenotype produced), they are in the same gene.

Step 2: Detailed Explanation:

• Dominant Mutations (A): The complementation test is designed for recessive mutations.
If a mutation is dominant, the mutant phenotype will appear even in the presence of a wild-type allele.
Thus, in a trans-heterozygote, the dominant mutant phenotype will always be expressed regardless of whether the mutations are on the same gene or different genes, making the test results ambiguous.

• Intragenic Complementation (B): This occurs when two different mutant alleles of the same gene can cooperate to produce a partially functional protein.
It typically happens in proteins that form multimeric complexes (homodimers or tetramers).
Mutants that should fail to complement (because they are allelic) actually show a wild-type or near-wild-type phenotype, leading to the false conclusion that they are in different genes.

• Polar Mutations (D): These are mutations (often nonsense or frameshift) that affect the expression of downstream genes in an operon or a polycistronic mRNA.
A mutation in one gene might prevent the translation of subsequent genes, leading to a loss of function in those genes as well.
This creates a situation where a mutation in one gene appears to be allelic to mutations in downstream genes because they fail to complement.

• Gene Interaction (E): Complex interactions such as epistasis or inter-allelic suppression can mask or alter the expected phenotypic outcomes.
If the product of one gene is required for the expression or function of another, the phenotypic ratio and complementation pattern may not follow standard Mendelian expectations.

• Different Phenotypes (C): If mutations belong to different genes and have different phenotypes, the complementation test is usually very clear, as the wild-type phenotype for both traits will be restored. This does not lead to ambiguity.
Final Answer:
Based on the analysis, conditions A, B, D, and E cause ambiguity in complementation tests.
Therefore, option (C) representing A-B-D-E is the correct choice.