WHAT is phenotypic ratio of crossing mutant 1 to mutant 2 in a haploid
The correct answer and explanation is:
Correct Answer:
The phenotypic ratio of crossing mutant 1 × mutant 2 in a haploid organism depends on whether the mutations are in the same gene (allelic) or in different genes (non-allelic). Assuming you are referring to a complementation test in haploid organisms:
- If the mutations are in the same gene (allelic): All offspring will show the mutant phenotype → Phenotypic ratio = 100% mutant
- If the mutations are in different genes (non-allelic): Offspring will show the wild-type phenotype (due to complementation) → Phenotypic ratio = 100% wild-type
Explanation (≈300 words)
In haploid organisms (like many fungi and some algae), each gene is present as a single copy. When crossing two mutants, each carrying a mutation that causes a similar phenotype, scientists often want to determine whether these mutations affect the same gene or different genes. This is the principle behind the complementation test.
Let’s say mutant 1 and mutant 2 both exhibit a specific mutant phenotype (e.g., inability to synthesize an amino acid). You cross mutant 1 to mutant 2. Since haploid organisms cannot mask a mutation with a second allele (as in diploids), any gene product must come from a single functional allele.
- If the mutations are in the same gene (allelic), the resulting haploid offspring will still have a non-functional version of that gene — the defect persists. Thus, no complementation occurs, and the phenotype remains mutant in all progeny.
- If the mutations are in different genes (non-allelic), each mutant provides a functional copy of the gene that the other lacks. Together, the two complement each other’s deficiencies, resulting in a wild-type phenotype. Thus, all offspring will appear wild-type due to complementation.
This makes haploids ideal for complementation testing since the results are clear and not complicated by dominance or heterozygosity.
So, the phenotypic ratio in a haploid mutant cross is:
- 100% mutant if mutations are in the same gene.
- 100% wild-type if mutations are in different genes.
This is a binary outcome — no segregation ratios like 3:1 or 9:7, as seen in diploid organisms.