A3.1.13 (HL)—Chromosome number as shared trait
Chromosome number is usually shared within a species, but fusion, fission, polyploidy, and meiotic changes can alter numbers during evolution.
- Syllabus
- First assessment 2025
- Objective
- A3.1.13
- Level
- HL
Exam analysis
Chance of appearing4%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1
Common command terms
- State
- Suggest
Scoring notes
- Common mistake
- Confusing haploid, diploid, and autosome counts.
Recent exam appearances
November 2025Paper2 ["HL"] · TZ17(c)[ 4 ]A3.1.13 (HL)—Chromosome number as shared trait
May 2023Paper2 ["HL"] · TZ24(c)(ii)[ 1 ]A3.1.13 (HL)—Chromosome number as shared trait
May 2023Paper2 ["HL"] · TZ24(c)(i)[ 1 ]A3.1.13 (HL)—Chromosome number as shared trait
November 2020Paper1 ["HL"] · TZ036[ 1 ]A3.1.13 (HL)—Chromosome number as shared trait
May 2017Paper1 ["HL"] · TZ217[ 1 ]A3.1.13 (HL)—Chromosome number as shared trait
Practice this objective
Coverage 2017–2025 · Updated 15 Jul 2026
Concept essentials
- Haploid chromosome number is usually fixed for a species.
- Somatic cells contain diploid chromosomes, while gametes contain haploid chromosomes.
- Chromosome fusion and fission can alter chromosome number over evolutionary time.
- Polyploidy can contribute to reproductive isolation and speciation.
- Incorrect chromosome pairing during meiosis can reduce fertility.