Diversity of organisms: exam questions

Answer: C – genus first with a capital letter, species name lowercase, in italics [1]

Answer: B – chromosomes in diploid cells are in homologous pairs [1]

Answer: B – members of a species share most of their genome; SNPs give some diversity [1]

• (a) a group of organisms that can breed and produce fertile offspring [1]
• (b) speciation is the splitting of one species into two or more [1]
• (b) speciation usually happens gradually / not by a single act [1]
• (b) non-interbreeding populations diverge / become more and more different in their traits [1]
• (b) no clear point at which they become separate species / intermediate stages exist [1]
• (b) interbreeding cannot be tested if populations are geographically separated OWTTE [1]
• max 4

• banding pattern of human chromosome 2 matches chimpanzee chromosomes 12 and 13 (2A and 2B) end to end [1]
• telomere sequences found near the middle of chromosome 2 / at the fusion point [1]
• remains of a second (inactive) centromere on chromosome 2 [1]
• gene order (largely) matches that on the two chimpanzee chromosomes [1]
• fusion explains reduction from 48 to 46 / 24 to 23 pairs [1]
• the hypothesis is testable / made predictions that could have been falsified [1]
• evidence supports but does not prove the hypothesis / fusion itself was not observed OWTTE [1]
• max 4

• (a) 149,000 ÷ 3,100 = 48 (times) [1] Accept 48.1
• (b) supported: simplest organisms / bacterium and yeast have the smallest genomes [1]
• (b) not supported: plants such as wheat / Paris japonica have larger genomes than humans [1]
• (b) Arabidopsis and Drosophila have genomes of a similar size / same order of magnitude despite being very different organisms OWTTE [1]
• (b) conclusion: no close correlation between genome size and complexity / hypothesis not supported overall [1]
• (b) only one or a few species per group / small sample of species [1]
• (c) more non-coding / repetitive DNA [1] Accept extra copies of whole chromosome sets / polyploidy
• max 5

• sequencing is increasingly fast and cheap, so many genomes can be compared [1]
• current: research into evolutionary relationships between species / comparing numbers of base differences [1]
• current: finding SNPs / variation linked to traits or diseases [1]
• future: personalised medicine [1]
• choosing drugs / doses / predicting disease risk from an individual's genome [1]
• max 3

• the biological species concept defines a species as organisms that can breed and produce fertile offspring [1]
• asexually reproducing organisms do not interbreed, so the concept cannot be applied [1]
• each clone / individual would be reproductively isolated, making every one a species OWTTE [1]
• bacteria reproduce asexually / by binary fission [1]
• bacteria exchange genes by horizontal gene transfer / plasmids [1]
• genes pass between different species, so species boundaries are blurred [1]
• interbreeding cannot be tested for fossils / geographically separated populations [1]
• members of a species have the same chromosome number [1]
• hybrids of parents with different chromosome numbers are unlikely to be fertile [1]
• chromosomes cannot pair in meiosis, so viable gametes are not produced [1]
• e.g. horse (64) × donkey (62) gives an infertile mule (63) [1]
• max 6

• (a) each step has two contrasting descriptions / paired choices [1]
• (a) uses features that are easy to observe [1]
• (a) uses features that do not vary within a species / not size or colour [1]
• (a) each choice leads to another numbered step or a name [1]
• (b) water sample taken and DNA extracted [1]
• (b) barcode region copied by PCR and sequenced [1]
• (b) sequences compared with a reference database of barcodes to identify species [1]
• max 4