Model organism fact check: How many chromosomes (haploid count) does the nematode Caenorhabditis elegans have?

Introduction / Context:
Caenorhabditis elegans is a premier genetic model, with a compact, well-mapped genome. Knowing its chromosome number is foundational for interpreting linkage maps, genome assemblies, and sex determination in this organism.

Given Data / Assumptions:

• We are asked for the chromosome count characteristic of C. elegans.
• The standard laboratory strain is hermaphroditic (XX) with males (XO) occurring at low frequency.

Concept / Approach:
C. elegans has six chromosomes in the haploid set: five autosomes (I–V) and one sex chromosome (X). Hermaphrodites are XX (2n = 12), males are XO (2n = 11). The neat, small karyotype supports clear genetic analysis and fully sequenced reference assemblies per chromosome.

Step-by-Step Solution:

Verification / Alternative check:
Genome browsers and reference papers list the six chromosome scaffolds by name (I–V, X), confirming the haploid number.

Why Other Options Are Wrong:

• 3, 9, 12: do not match known karyotype; 12 would be the diploid number in hermaphrodites, not haploid.
• 5: omits the X chromosome.

Common Pitfalls:
Confusing haploid with diploid number, or mixing up autosomes and sex chromosomes.

Final Answer:
6