(b) (i) how gene sequencing has allowed for genome-wide comparisons…

The idea

Sequencing whole genomes means the entire DNA content of an organism can be read and compared, not just single genes. Between individuals of the same species, sequencing reveals variation such as single nucleotide polymorphisms and other differences, which can be linked to traits, disease susceptibility or ancestry. Between species, comparing whole genome sequences shows how conserved or divergent genes and non-coding regions are, allowing evolutionary relationships to be inferred: the more similar the sequences, the more recently the species are likely to have shared a common ancestor. Genome-wide comparison also identifies genes shared across many organisms and genes unique to particular lineages. The key idea is that sequencing at genome scale provides the raw data for comparing many genes and regions at once, giving a broader and more reliable picture than comparing isolated genes.

What earns the marks

• Whole-genome sequencing provides the complete DNA sequence, allowing many genes and regions to be compared at once rather than single genes.
• Comparing genomes of individuals reveals variation such as single nucleotide polymorphisms that may relate to phenotype, disease or ancestry.
• Comparing genomes of different species shows degrees of similarity, and greater similarity indicates a closer evolutionary relationship and more recent common ancestry.
• Genome-wide comparison can identify conserved genes shared across species and genes unique to particular lineages.
• Large sequence datasets require computational alignment and analysis to make comparisons meaningful.

Common mistakes and how to put them right

• Thinking genome comparison only looks at protein-coding genes.

  Put it right: Whole genomes include non-coding regions, which are also compared.

• Assuming any sequence difference proves a close or distant relationship on its own.

  Put it right: Relationships are inferred from the overall degree of similarity across the genome.

• Believing sequencing alone gives conclusions without analysis.

  Put it right: Computational alignment and comparison are needed to interpret the data.

Exam tips

• State clearly whether the comparison is between individuals or between species, since the purpose differs.
• Use the principle that greater sequence similarity indicates closer evolutionary relationship and more recent common ancestry.
• Refer to genome-wide data covering many genes and regions, not just a single gene, when explaining the advantage of whole-genome sequencing.