Explain, using examples, how the environment may affect the phenotype of an organism. (8)
- phenotypic variation results from interaction of genotype & environment / Vp = Vg + VE
- environment may limit expression of gene(s)
- e.g.: size, mass, height
- because, food / nutrients / ions, in short supply / malnutrition
- environment may, trigger gene (must be stated)
- ref. low temperature & change in animal colour
- ref. high temperature and, curled wing in Drosophila / gender in crocodiles
- ref. UV light & melanin production
- ref. wavelength of light &, flowering / germination / fruit colour
- environment effect usually greater on polygenes
- environment may induce mutation affecting phenotype
Describe, using examples, the difference between continuous & discontinuous variation of the phenotype. (8)
Discontinuous
- one / few, genes control a phenotype
- qualitative
- discrete categories / no intermediates
- different alleles at a single gene locus have large effect on phenotype
- different genes have different effects
- little / no contribution by environment to phenotype
- e.g.: albinism / sickle cell anaemia / haemophilia / Huntington’s disease
Continuous
- several genes control a phenotype
- quantitative
- range of categories / many intermediates
- different alleles at a single gene locus have small effects
- environment has considerable influence on phenotype
- example: height / mass
Explain the genetic basis of continuous & discontinuous variation. (6)
Continuous variation
- > 3 genes / many genes / polygenes
- many alleles
- different alleles have small effects on character
- different genes have same effect on character
- additive effect
- large environmental effect
Discontinuous variation
- one / few genes
- few alleles
- different alleles have large effects on character
- different genes have different effects on character
- different genes may interact
- small environmental effect
State the general theory of evolution & explain the process of natural selection in evolution. (7)
- Individuals in population produce many offspring / more than is necessary to maintain population.
- Numbers in population remain roughly constant.
- Variation (in phenotype) in members of population / within a species
- Due to genetic variation / caused by mutation
- Individuals compete for survival / survival of the fittest
- Ref. to selection pressure(s) / environmental factors / named factor (biotic or abiotic)
- (cause) many fail to survive / do not reproduce
- those better adapted to survive / have advantageous alleles / have selective advantage
- (reproduce to) pass on (advantageous) alleles to offsprings
- genetic variation leads to change in phenotype
- ref. changes in, gene pool / allele frequency
- over time produces evolutionary change
- new species arise from existing ones / ref. to speciation
- directional / stabilising, selection
Describe why variation is important in natural selection. (6)
- ref. continuous / discontinuous variation
- genetic / inherited variation
- variation in phenotype / characteristics
- (can be due to) interaction of genotype & environment
- e.g. of characteristic that influences survival
- ref. intraspecific competition / struggle for existence
- those with favourable characteristics survive
- pass on favourable characteristics to offspring
- those with disadvantageous characteristics die
Explain how speciation can occur due to geographical separation. (8)
- named geographical barrier; e.g. river / mountain / sea / lake
- population, separated (into two)
- no, mating / breeding / gene flow, between , populations / groups
- different (named), selection pressures / environment / conditions
- different mutations
- individuals with beneficial alleles, are selected for / reproduce / have selective advantage
- genetic drift / founder effect
- change in, allele frequency / gene pool
- over a long time / many generations
- can no longer (successfully), interbreed / reproduce / produce fertile offspring, with original, population / species
- allopatric (speciation)
Explain why mutations for antibiotic resistance spread so rapidly among bacteria. (6)
- (most bacteria) reproduce rapidly
- frequent DNA replication
- chances for, mutation increases
- no / fewer, editing enzymes
- mutation passed to large number of descendants / ref. vertical transmission
- mutation may be on plasmid
- transferred via horizontal transmission
- even to different species
- conjugation / process described
- transformation / transduction / process described
- ref. selection (of resistant strains by antibiotic as selection pressure)
Outline how artificial selection differs from natural selection. (6)
Artificial selection:
- selection (pressure by) humans
- genetic diversity lowered
- inbreeding common
- loss of vigour / inbreeding depression
- increased homozygosity / decreased heterozygosity
- no isolation mechanisms operating
- (usually) faster
- selected feature for human benefit
- not for, survival / evolution
Natural selection:
- Environmental selection pressure
- Genetic diversity remains high
- Outbreeding common
- Increased vigour / less chance of inbreeding depression
- Decreased homozygosity / Increased heterozygosity
- Isolation mechanisms do operate
- (usually) slower
- Selected feature for organism’s benefit
- Promotes, survival / evolution
Explain, using a specific plant or animal example, how selective breeding is used to produce disease-resistant varieties. (9)
- plant with resistance to fungal disease
- crossed
- with non-resistant parent with desired traits
- offspring matured & tested by exposing them to the fungal disease
- those that survive & display desired traits are selected & backcrossed (with parents)
- process repeated for several generations
- backcrossing increases percentage of
- background genes so the plant adapts well to the environment
- animal / plant, with resistance to named disease
- non-resistant parent with desired trait(s) selected
- crossed with selected parent with resistance
- two practical details of cross (e.g. plant: male sterility / removal of anthers / bagging / pollination / offspring seeds sown, animal: AI, IVF, embryo transplant, surrogate mother)
- offspring inspected (e.g. plant challenged by the pathogen)
- those showing resistance AND desired traits selected
- (selection &) interbred / backcrossed continued for, several generations / repeats
- comment on pbs of inbreeding
- reasons for backcrossing - increase percentage of background genes to adapt to environment
- ref. background genes
- how numbers of chosen product increased
- ref. gene pool used (primary, secondary, etc.)
- ref. gene bank / landrace / rare breed / wild type
Describe the process of selective breeding in a named animal. (8)
- named animal
- e.g. of trait selected for
- parents chosen for trait(s)
- and general fitness
- ref. progeny testing to identify suitable parent
- especially for selection of sex-limited trait
- ref. heritability / Vg
- ref. background genes to suit conditions
- ref. AI to maximise offspring from suitable male
- & to allow long-distance mating
- ref. embryo transplantation to maximise offspring from suitable female
- idea selection over many generations
- ref. avoiding inbreeding
Outline how selective breeding (artificial selection) has improved the yield of crops, such as wheat & maize. (7)
- choose parents with good features
- breed them
- repeat for many generations
- introduction of disease resistance
- named crop disease
- dwarf varieties
- (dwarf varieties) mutant alleles for gibberellin synthesis
- (dwarf varieties) more energy put into grain than into height (of plant)
- (dwarf varieties) less susceptible to being knocked over by weather
- inbreeding leads to uniformity
- named example; e.g. standard height / cobs ready to harvest at same time
- hybridisation leads to hybrid vigour
Discuss, with reference to specific examples, the harmful effects of inbreeding. (12)
- Tends to increase homozygosity
- Recessive alleles
- Deleterious / genetic disorders
- Decrease to, vigour / fitness
- Inbreeding depression
- Gene pool restricted / genetic uniformity
- Alleles may be lost
- Appropriate use of 2 examples, harmful effects
- No way of adapting
- To environmental change
- E.g. environmental change
- To new strain, pest / disease
- (In plants problem greatest with natural outbreeders) (animals are normally outbreeders)
- (Still applied to natural inbreeders)
- Artificial selection
- For desirable, phenotype
- Also selects for ‘invisible’ traits
- Especially if selection pressure great
Explain the role of isolating mechanisms in the evolution of new species. (9)
- ref. to definition of species
- ref. allopatric
- geographical isolation
- ref. to examples e.g. islands / lakes / mountain chains / idea of barrier
- ref. to example organism
- ref. to populations prevented from interbreeding
- isolated populations subjected to different selection pressures / conditions
- over time sufficient differences to prevent interbreeding
- ref. sympatric
- ref. to reproductive isolation
- ref. behavioural barriers (within a population)
- e.g. day active / night active
- correct ref. to gene pool
- change in allele frequencies
Describe the bottleneck effect. (3)
- Occurs when a previously large population suffers a dramatic fall in numbers.
- (A major environmental event can greatly reduce the number of individuals in a population which in turn) reduces genetic diversity in the population as alleles are lost.
- Surviving individuals breed & reproduce with close relatives.
Describe the Founder effect. (4)
- A small number of individuals from a large parent population start a new population.
- Founder effect occurs as a result of chance (e.g. a storm may separate a small group of individuals from the main population).
- Only some of the total alleles from the parent population will be present.
- (Because the population that results from the founder effect is very small), it is more susceptible to the effects of genetic drift.
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Chapter 12 Respiration Essay QS21
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Chapter 13 Photosynthesis Essay QS14
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Chapter 14 Homeostasis Essay QS11
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Chapter 15 Control & Coordination Essay QS13
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Chapter 16 Inheritance Essay QS31
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Chapter 17 Selection & Evolution Essay QS15
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Chapter 18 Biodiversity Essay QS19
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Chapter 19 Genetic Technology Essay QS15
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Statistical Tests28
