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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

Repair Species Boundaries with HL Evidence

HL only
A compact contrast panel. Left side: bacteria exchanging genes by transformation, plasmids, and recombination, showing why interbreeding is the wrong test. Right side: horse and donkey producing an infertile mule, with a note that chromosome-number mismatches can block fertile hybrids.
Bacterial horizontal gene transfer by transduction, transformation and conjugation.

The biological species concept is based on sexual reproduction, so it does not work well for asexual organisms or for bacteria that exchange genes horizontally.

Bacteria gain diversity through mutation and horizontal gene transfer, including transformation, plasmid transfer and recombination. Genes can therefore cross lineage boundaries without the organisms breeding as animals or plants do.

In many sexually reproducing species, haploid and diploid chromosome numbers are shared traits because meiosis and fertilization conserve chromosome number across generations. If closely related parents have different chromosome numbers, homologous pairing in a hybrid's meiosis can fail, reducing fertility.

A horse and donkey can produce a mule, but mismatched parental chromosome sets disrupt homologous pairing during meiosis, so the hybrid is usually infertile.

Chromosome number can support a species distinction but is not a universal definition: different species can share a chromosome number, and bacterial boundaries require evidence other than interbreeding.

Chromosome number as shared trait

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Suggest.

Command terms

State / Suggest

What earns marks

Build the answer around this relationship: Haploid chromosome number is usually fixed for a species.

Watch for

Confusing haploid, diploid, and autosome counts.

Representative question

Question 1

[Maximum number: 1]

Sex is determined in the same way in pangolins as in humans. State how many autosomes there are in somatic cells of M. pentadactyla.

Boundaries And Identification

HL only

The HL extension connects two problems: species boundaries and species identification. Bacteria challenge the biological species concept because they are asexual and exchange genes horizontally. Chromosome number can help define boundaries in many sexual species because meiosis conserves it and mismatches can produce infertile hybrids. Dichotomous keys identify visible organisms using paired choices. DNA barcoding and eDNA identify species using short sequences, PCR, and databases when visible traits are limited.

  • Bacterial boundaries are blurred by mutation and horizontal gene transfer.
  • Chromosome number is usually conserved within sexual species by meiosis.
  • Different chromosome numbers can prevent fertile hybrids, as in mules.
  • Dichotomous keys use observable traits and paired choices.
  • DNA barcoding/eDNA use PCR and database comparison for damaged, partial, hidden, or difficult specimens.
  • Local plant or animal species can be used to build and test dichotomous keys.

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.