A3.1 Diversity of organisms. Practice questions with markscheme.

• A: a shared genus implies close relationship; they remain separate species (their rare hybrids are (mostly) infertile);

• A: species differ in genome size and in base sequence; within a species individuals share genome size/organization but differ in base sequence (except identical twins/clones);

• C: karyotyping images condensed (metaphase) chromosomes, usually arranged in homologous pairs by size/banding, revealing number and large-scale abnormalities;

• A: gametes are haploid (n = 23): one chromosome from each homologous pair;

• a species is a group of organisms that share (a set of) observable / physical / morphological traits;
• an organism is assigned to a species by comparing its traits with those of (described members of) the species, OWTTE;
• members of one species resemble each other more closely than they resemble members of other species;
• limitation: variation within a species (e.g. between males and females / juveniles and adults / colour forms) can make members of one species look different;
• limitation: members of separate species can look almost identical / cryptic species / similarity due to convergent evolution;
• limitation: the amount of difference needed to separate two species is a subjective judgement;

Award [2 max] for the concept and [1 max] for a limitation. Accept 'appearance' for morphology.

• (a) [1]
  • 3.2 × 10⁶ / 3 200 000 / about 3 million (positions);

  Accept 3 × 10⁶. Working: 3.2 × 10⁹ × 0.001.

• (b) [1 max]
  • all the genetic information / all the DNA of an organism (or cell);
  • includes all the chromosomes (and mitochondrial/chloroplast DNA), OWTTE;

  Do not accept 'all the genes' alone, since the genome includes non-coding DNA.

• (c) [2 max]
  • a SNP is a position in the genome where a single base differs between individuals / where alternative bases occur;
  • SNPs arise by mutation and are inherited (from parents), so they spread through the population;
  • a SNP can alter the amino acid sequence of a protein / gene expression, giving differences in traits;
  • each individual has a different combination of (millions of) SNPs, so no two individuals (except identical twins) are genetically identical, OWTTE;

• (a) [1]
  • 100 000 / 10⁵ (US$);

  Accept 80 000–150 000.

• (b) [2]
  • 95 000 000 / 1500 (or 10⁸ / 10³);
  • ≈ 60 000 times (accept 50 000–100 000);

  Award [2] for a correct answer in the accepted range without working.

• (c) [2 max]
  • cost fell throughout the period / negative correlation with time;
  • the fall was slow before 2007 and very steep between 2007 and 2011 (about 1000-fold in four years);
  • the fall slowed again after 2013 / cost levelled off near 1000 dollars;

  Accept any two.

• (d) [2 max]
  • sequencing an individual patient's genome to identify disease-causing alleles / personalised medicine;
  • sequencing the genomes of many species to establish evolutionary relationships / phylogenetics;
  • sequencing of pathogen genomes to track the spread / evolution of an outbreak;
  • identifying mutations in a tumour to choose a treatment;
  • research into the function of genes / comparative genomics;

  Content pivot. Accept any two.

• the concept defines a species by interbreeding to give fertile offspring;
• asexual organisms (e.g. many bacteria) never interbreed, so the concept cannot be applied;
• some distinct species do hybridize, and some hybrids are (partially) fertile, blurring boundaries;
• allopatric populations cannot be tested for interbreeding (in nature);
• speciation is gradual, so populations exist at intermediate stages of divergence;
• (hence) other evidence, morphology, base/amino-acid sequences, ecology, is used alongside it, OWTTE;