Members of a species share most genomic information, while alleles, SNPs, recombination, organelle DNA, and population history create individual variation.
Coverage 2015–2025 · Updated 15 Jul 2026
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).
This objective is assessed through essay response, commonly using Compare / Discuss / Determine.
Compare / Discuss / Determine / Contrast / State / Distinguish / Calculate
Build the answer around this relationship: A genome includes all DNA in the organism, including organelle DNA where present.
Assuming every SNP changes the proteome rather than recognising that it always changes the genome.
Representative question
Discuss the role of genes and chromosomes in determining individual and shared character features of the members of a species.
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.
