How many genes have been identified with roles in production of red flowers?


Genetics and Evolution

Time allowed: 1 hour and 30 minutes

Total marks available for this paper: 50

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  1. Briefly outline the experiment that used fission yeast to identify the human cdc2​ (5 marks)

Temperature sensitive ​cell division cycle​ (cdc​) mutants were isolated in fission yeast ­ at the permissive temperature these conditional mutants had a wild­type phenotype, but at the restrictive temperature these mutants arrested at a specific point of the cell cycle. The cdc2 gene was identified as a key regulator of the cell cycle because cdc2 mutants arrest prior to entry into M phase. cdc2 mutants were grown in permissive conditions (1 mark) and transformed with a yeast plasmid library of human genes (1 mark). The transformed cells were shifted to a restrictive temperature (1 mark). Only the cells transformed with copy of the human cdc2 genes grew in the restrictive conditions (1 mark). These plasmid rescue experiments provided the cdc2 mutants with a functional copy of a cdc2 gene. This complementation occurred because the cdc2 genes in yeast and humans are highly conserved (1 mark).

  1. Wild­type ​Antirrhinum​ plants have red flowers. Five white flowered mutants were identified by mutagenesis. The mutants were crossed to each other and wild­type. The flower colour of the progeny was recorded and presented in the table below.
    1. Why were all the mutants also crossed to wild type? (1 mark)

To test whether or not the mutations were recessive

  1. Write down the complementation groups you have identified. (2 marks)
  • c e)
  • d)
  1. How many genes have been identified with roles in production of red flowers? (1 mark)


  1. What has comparative genomics revealed about ​ coli​? (4 marks)

Comparative genomics has revealed that E. coli​ has a core genome of about 3000, although there may be as many as 13000 genes represented in the genomes of a diverse set of strains (1 mark). Many of these genes have unknown functions and are likely to include important virulence factors (1 mark). It has also revealed that some genes, or gene clusters, are restricted to one or few strains of E.coli (1 mark). Other genes, or clusters of genes, are shared with more, but not all strains (1 mark).

  1. Briefly describe the origin and evolutionary history of the globin gene family (4 marks)

Answer: the origin of the this family is ancient (~800MYr) (1 mark) and it is a diverse family (neuroglobin, cytoglobin and myoglobin, alpha­globins and beta­globins) (1 mark). Multigene families that have diversified via successive rounds of gene duplication and divergence, and whole genome duplication (2 marks). Clusters of related globin genes have followed different evolutionary trajectories (1 mark) ­ this evidenced by a human beta globin gene being an inactive pseudogene in other mammals, or more complicated gene inversion or fusion events.

  1. What were the major findings from Barbara McClintock’s work? (3 marks)

Answer: McClintock also showed that some regions of the genome are more

likely to suffer chromosome breaks than others

She concluded that mobile elements within the chromosomes must be responsible for these breaks

She showed that these elements could move to a new position and the position of the chromosome break would move with them

  1. Define the following concepts:
    1. Complex trait (1 mark)

Answer: a phenotype influenced by multiple genes as well as the environment

  1. Phenotypic plasticity (1 mark)

Answer: the ability of one genotype to produce more than one phenotype when exposed to different environments

  1. Heritability (1 mark)

Answer: the proportion of the total variation due to genetic differences among individuals

  1. a) How did Ernst Mayr define a biological species? (2 marks)

A species is a group of actually or potentially i​nterbreeding ​natural populations that are reproductively i​solated​ from other such groups.

  1. Give an example of a type of organism for which species cannot be defined in this way, and explain why not. (2 marks)

Those that reproduce only asexually, such as dandelions, because the definition depends on recombination.

  1. Compare allopatric and parapatric modes of speciation. (3 marks)

In allopatric speciation, a population is split by a barrier and the two​ isolated populations evolve independently ​until, when they meet again, they are too divergent to merge. In parapatric speciation, m​utants within a population are able to colonise an adjacent environment ​that is not available to the parental population. There is ​no physical barrier, but a hybrid zone​ with unfit hybrids.

  1. a) What has been the evolutionary fate of the genes of the endosymbiotic bacterium that became a mitochondrion? (3 marks)

(1) Many were lost because they were not needed; (2) many moved to the nucleus where they encode proteins that are imported into the mitochondrion;

(3) some remained in the mitochondrial genome.



  1. b) We believe that diatoms have a secondary photosynthetic symbiosis.

What does this mean, and what is the evidence? (3 marks)

The ancestor of diatoms became photosynthetic by engulfing another eukaryote (a red alga), which had previously acquired a chloroplast by primary endosymbiosis with a cyanobacterium. The diatom chloroplast has four membranes, nuclear­encoded chloroplast proteins have both signal and transit peptides, the phylogeny of chloroplast proteins suggests a red algal origin.

  1. What was the Cambrian explosion, and when did it happen? (3 marks)

The first appearance in the fossil record of ​most modern animal phyla​ (and others) over ​a period of 5­10 million years​ after the start of the Cambrian period ​541 Mya​.

  1. A population geneticist collected 100 fruit flies that were hovering over the bins at the back of a pub, and found that, for a particular molecular marker called ​Mar​, there were 25 ​Mar​­1/Mar​­1 homozygotes, 5 Mar​­2/Mar​­2 homozygotes and 70 ​Mar​­1/​Mar​­2 heterozygotes.
    1. Calculate the frequencies (i.e. proportions) of the two alleles, Mar​­1 and Mar​­2, in this population. (1 mark)

Frequency of ​Mar​­1 = ((25 x 2) + 70)/200 = 0.6. Frequency of ​Mar​­2 = 0.4.

  1. Calculate the numbers of the three genotypes that would be expected in this sample if this locus were in Hardy­Weinberg equilibrium. (3 marks)
(0.6)​2​ x 100 = 36 ​Mar​­1/​Mar​­1
2 x 0.6 x 0.4 x 100 =

(0.4)​ 2​ x 100 =

48 Mar​­1/Mar​­2

16 Mar​­2/​Mar​­2

  1. Suggest a possible reason for the difference between the observed and the expected genotype frequencies. (1 mark)

The excess of heterozygotes might reflect selection against one or both homozygotes.


  1. Sketch a rooted phylogenetic tree showing the evolutionary relationships among modern humans, chimpanzees, gorillas, Neanderthals, orangutans, bonobos and a suitable named outgroup species. Indicate the last common ancestor of humans and chimpanzees, and give its approximate date. (6 marks)

1 mark for something that looks like a phylogeny; 1 mark for a sensible outgroup in the right place; 3 marks for the correct sister groups; 1 mark for the correct common ancestor; 1 mark for any date(s) within the range 3­9 Mya (that is 7 marks, allowing full marks for a slight imperfection).

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