A3.1.9—Diversity of eukaryote genomes
Eukaryote genomes differ greatly in gene number, sequence, and DNA composition, but gene count alone does not rank evolutionary success.
- Syllabus
- First assessment 2025
- Objective
- A3.1.9
- Level
- HL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2022
Most common paperPaper1
Typical marks1
Recent exam appearances
May 2022Paper1 ["HL"] · TZ110[ 1 ]A3.1.9—Diversity of eukaryote genomes
Practice this objective
Coverage 2022–2022 · Updated 15 Jul 2026
Genome variation within and between species
A genome is all the genetic information of an organism: protein-coding genes, non-coding DNA and, where present, non-nuclear DNA such as plasmids or organelle genomes.
Members of the same species share most genes and genome organisation but differ in variants. A single-nucleotide polymorphism (SNP) is a one-base difference that can act as a genetic marker. Variation between species is generally much greater than variation within one species, although the two distributions can overlap.
Across eukaryotes, genomes differ in total size, base sequence and chromosome organisation. A larger genome does not automatically mean a more complex organism because much DNA is non-coding and genome size can be affected by repeated sequences or polyploidy. In comparisons, state both the level (within or between species) and the feature being compared (sequence, size or organisation).
Diversity of eukaryote genomes
Assessment in practice
1 marksHow it is assessed
This objective is assessed through multiple choice.
What earns marks
Build the answer around this relationship: Eukaryote genomes vary in gene number and DNA sequence.
Representative question
Question 1
The table shows the estimated total number of genes in several organisms.
| Species | Estimated number of genes |
|---|---|
| Saccharomyces cerevisiae (a yeast) | 6000 |
| Escherichia coli (a bacterium) | 3200 |
| Drosophila melanogaster (fruit fly) | 14000 |
| Canis familiaris (domestic dog) | 19000 |
| Oryza sativa (rice) | 51000 |
| Homo sapiens (human) | 25000 |
What can be deduced from the information in this table?
A
Throughout evolution, the number of genes increases.
B
The domestic dog is more closely genetically related to the fruit fly than to the human.
C
The number of genes does not determine evolutionary success.
D
Humans produce about half as many proteins as rice.
SL Transfer: Classify With Evidence
For SL, A3.1 is really one evidence map. Variation explains why individuals differ. Morphology and binomial nomenclature help group and name organisms. The biological species concept uses interbreeding and fertile offspring, but boundaries can be difficult during gradual speciation. Chromosomes and karyograms add cellular evidence. Genomes, SNPs, genome size, and whole genome sequencing add molecular evidence. The skill is choosing the right evidence for the question.
- Variation supports natural selection.
- Morphology groups by structure; binomial nomenclature names species universally.
- Biological species concept uses interbreeding and fertile offspring.
- Karyograms compare chromosome number and structure.
- Genome evidence includes SNPs, between-species differences, genome size, and sequencing uses.
Concept essentials
- Eukaryote genomes vary in gene number and DNA sequence.
- Gene count alone does not determine evolutionary success.
- Genome comparisons require interpretation beyond one numerical measure.
- Not all DNA in a eukaryotic genome codes for proteins.