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

• (a) [1]
  • 1.8 (times);

  Working: 14.4 ÷ 8.0.

• (b) [2 max]
  • the sequence shows whether a patient carries the SNP in *CYP2C19*, and in how many copies;
  • patients carrying the SNP could be given a different drug that does not need activation by this enzyme / a higher dose;
  • patients without the SNP can be given clopidogrel with confidence, as they activate it normally, OWTTE;
  • (personalized medicine:) the treatment is chosen to suit the individual's genome, so fewer patients receive a drug that will not work for them;
• (c) [2 max]
  • 8.0 % of patients without the SNP still had a further heart attack or stroke;
  • (so) other factors affect the outcome, e.g. other genes/SNPs, lifestyle (diet, smoking, exercise) or age;
  • a genome sequence gives the probability of an outcome, not a certainty, OWTTE;
  • an alternative drug may also fail in some patients / has its own side effects;

• (fetal) cells are obtained, e.g. by amniocentesis / chorionic villus sampling;
• chromosomes are stained to show banding patterns and photographed;
• chromosomes are arranged in (homologous) pairs by length, centromere position and banding pattern;
• the sex chromosomes show the sex of the baby (XX female, XY male);
• to detect an abnormal number or structure of chromosomes, e.g. trisomy 21 / Down syndrome, OWTTE;

Beyond this subtopic. Award if given; the marks above can be earned without it.

• D2.1 cells are cultured and arrested in (metaphase of) mitosis, when chromosomes are condensed;

• (a) [1]
  • 6.5 (pulses s⁻¹);

  Units not required. Accept 24.9 − 18.4 = 6.5.

• (b) [2]
  • not (shown to be) significant / cannot be concluded to be significant;
  • because the ranges of mean ± SE overlap / 12.3–12.9 mm and 12.7–13.3 mm overlap, OWTTE;

  Award the second mark only with reference to overlap of the standard-error ranges (or equivalent). Do not award for 'the numbers are close' alone.

• (c) [2 max]
  • the morphological concept groups organisms by observable physical traits / appearance;
  • the populations do not differ (significantly) in body measurements such as femur length / look alike, OWTTE;
  • the trait that differs most (song pulse rate) is behavioural, not morphological, so it would not have been used;
• (d) [1]
  • 38 (%);
• (e) [2 max]
  • the biological species concept requires interbreeding to produce fertile offspring;
  • some interbreeding does occur between the populations (34–38 % of pairings mated and 41–45 % of eggs hatched), so they are not fully isolated;
  • (but) very few hybrid offspring are fertile (9–12 % compared with 94–95 % within populations), so by the concept the populations are (almost) separate species, OWTTE;

  Accept either conclusion if supported by the fertility data. Do not award the third mark for a conclusion based on the mating percentages alone.

• (f) [2 max]
  • speciation is gradual, and the two populations are part-way through diverging, OWTTE;
  • some interbreeding and some fertile hybrids still occur, so there is no clear boundary between the populations;
  • the level of hybrid fertility (or of divergence) used to define separate species is a human choice / different species concepts give different answers;
• (g) [3 max]
  • within N the sequences would be almost identical, because members of one population share most of their genome;
  • but they would still differ at a few positions / single-nucleotide polymorphisms (SNPs), since no two individuals have identical genomes;
  • between N and S there would be more differences than within N;
  • because few hybrid offspring are fertile, genes rarely pass between N and S, so new mutations/SNPs build up separately in each population;
  • the differences would still be few compared with long-separated species, because N and S have diverged only recently / still interbreed a little, OWTTE;

  Content pivot. Award the third mark only with a reason from the points that follow it.

• comparing the (whole) genomes of different species to investigate evolutionary relationships (more base-sequence data gives more reliable trees);
• sequencing the genomes of many individuals to study variation within a species (e.g. within humans);
• identifying mutations/alleles associated with (genetic) disease;
• personalized medicine: predicting an individual's disease risk / selecting treatments or drug doses suited to the individual's genome, OWTTE;

• (a) [3 max]
  • as genome size increases, the percentage of non-coding DNA also increases, from 12 % in *Escherichia coli* to over 99 % in *Paris japonica*;
  • the increase is not proportional: the human genome is about 20 times larger than that of the fruit fly but the percentage of non-coding DNA rises only from 82 % to 98 %, and it cannot exceed 100 %;
  • the number of protein-coding genes rises far less steeply than genome size: about 5 times from *E. coli* to human, while the genome is about 670 times larger;
  • this means that almost all of the extra DNA in a large genome does not code for protein, so genome size gives little information about how many genes an organism has;

  A value quoted from the table is required for the first mark. Both a description of the data and an interpretation are needed.

• (b) [3 max]
  • in favour: the eukaryotes in the table all have larger genomes than the prokaryote, and eukaryotic cells have more organelles and more cell types, so genome size does rise with complexity in broad terms;
  • in favour: the multicellular organisms have more protein-coding genes than yeast or *E. coli*, and gene number is loosely related to genome size;
  • against: *Paris japonica* has a genome about 50 times larger than the human genome yet is not a more complex organism, so the relationship breaks down badly;
  • against: most of a large genome is non-coding and much of it is repeated sequence, so extra DNA need not add any function;
  • conclusion: genome size is a poor indicator of complexity; the number of genes, and especially how their expression is regulated, is far more informative, OWTTE;

  Content pivot. A conclusion is required for the final mark.

• (a) [2]
  • sample 1 correctly labelled (matches Atlantic cod);
  • samples 2 and 3 are mislabelled (matched to a different species: haddock, tilapia), OWTTE;
• (b) [2]
  • 81 % identity is low: within-species barcodes are normally >98–99 % identical, so an 81 % match is not convincing evidence it is red snapper;
  • it may be a related species missing from the reference database (or degraded/contaminated DNA), so the identification is unreliable / needs a better reference or repeat, OWTTE;
• (c) [1]
  • barcoding can identify processed/fragmented material (fillets, larvae, eggs) that has lost the visible features needed for morphological identification, OWTTE;
• (d) [2]
  • a query sequence can only be identified by matching it to a known sequence, so the true species must already be in the database;
  • if the species is absent or wrongly entered, the sample will be misidentified (or unidentifiable), OWTTE;

• members of one species share (most of) their characteristics but are not identical / vary genetically;
• between species, differences are (usually) larger and include (base) sequences of DNA/genomes;
• the more distantly related two species are, the more their (base/amino-acid) sequences differ;
• every individual (except identical twins/clones) has a unique genome, OWTTE;

Beyond this subtopic. Award if given; the marks above can be earned without it.

• D1.3, D2.1 sources of variation within a species: mutation / sexual reproduction (meiosis and fertilization);