Biology Topic 8

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What is a nerve?
A nerve is a bundle of the axons of many neurones surrounded by a protective covering.
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What does the central nervous system consist of?
The brain and spinal cord.
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What does the peripheral nervous system consist of?
Sensory neurones that carry sensory information from the receptors to the CNS and motor neurones that carry motor commands from the CNS to the effectors.
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What can the peripheral nervous system be subdivided into?
The autonomic nervous system and the somatic nervous system.
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What is the autonomic nervous system?
It is involuntary and stimulates smooth muscles, cardiac muscle and glands.
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What is the somatic nervous system?
It is voluntary and stimulates skeletal muscle.
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What can the autonomic nervous system be subdivided into?
The sympathetic nervous system and the parasympathetic nervous system.
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What is the sympathetic nervous system?
It prepares the body for 'fight or flight' responses (e.g. speeds up heart rate).
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What is the parasympathetic nervous system?
It prepares the body for 'rest and digest'.
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Where are the cell bodies of motor neurones always situated?
The cell body of motor neurones is always situated within the central nervous system and the axon extends out, conducting impulses from the CNS to effectors.
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What is the fatty insulating layer around the axon called?
Myelin sheath
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What is the myelin sheath made up of?
Schwann cells
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What is the advantage of reflex pathways?
They produce rapid responses; important for protection and survival.
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What is a reflex?
A rapid, involuntary response to stimuli.
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What is a stimulus?
A change in the environment; can be internal or external.
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What controls the size of the pupil?
The iris.
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What are the antagonistic muscles contained in the iris?
Radial and circular muscles.
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Why are the radial and circular muscles in the iris known as antagonistic?
When one contracts the other relaxes.
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What controls the radial muscles?
The radial muscles are controlled by a sympathetic reflex.
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What controls the circular muscles?
The circular muscles are controlled by a parasympathetic reflex.
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Which of the two sets of muscles (radial and circular) cause the pupil to dilate?
Radial.
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How does the pupil constrict?
High levels of light hit photoreceptors in the retina causing a nerve impulse to pass along the optic nerve to CNS; impulse is sent from the CNS along a parasympathetic motor neurone to the circular muscles of the iris; they contract, radial relax.
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What is the purpose of the pupil reflex?
To prevent damage to the retina from high-intensity light; in dim light it ensure maximum light reaches the retina.
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The pupil reflex response to light is very rapid. Why does this need to be the case?
To protect the eye from sudden flashes of bright light.
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Why are neurone cell membranes said to be polarised?
The inside of the axon is more negative than the outside.
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What is the value of the resting potential?
-70mV
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Why is the resting potential -70mV?
At -70mV, the electrical gradient exactly balances the chemical gradient; there is no net movement of K+ and hence a steady state exists, maintaining the potential difference at -70mV.
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Is the membrane permeable to sodium and potassium ions?
The membrane is virtually impermeable to sodium ions but it permeable to potassium ions.
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Why are neurones referred to as electrically excitable cells?
The potential difference across their cell surface membrane changes when they are conducting an impulse.
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What are relay neurones?
They are found mostly within the CNS. They can have a large number of conenctions with other nerve cells.
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What are relay neurones also known as?
Relay neurones are also known as connector neurones and interneurones.
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What is the effect of stimulating a neurone by passing a small electric current through it?
If an electrical current above the threshold level is applied to the membrane, it causes massive changes in the p.d. The p.d. across the membrane is locally reversed (inside positive outside negative). This is known as depolarisation.
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What level does the potential difference reach in depolarisation?
+40mV
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What is the process of returning to the resting potential called?
Repolarisation
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Why is it important that a neurone is returned to its resting potential as soon as possible?
It is important that the membrane is returned to its resting potential as soon as possible in order that more impulses can be conducted.
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What is an action potential?
The large change in voltage across the membrane is known as an action potential.
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What are the three stages in the generation of an action potential?
Depolarisation, repolarisation and restoring the resting potential.
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What causes an action potential?
Once threshold stimulation occurs an action potential is caused by changes in the permeability of the cell surface membrane to Na+ and K+ due to the opening and closing of voltage dependent Na+/K+ channels.
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What is the all-or-nothing property?
This means there is no way of controlling the degree of depolarisation of the membrane; action potentials are either there or they are not.
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Why are sodium ion channels referred to as voltage dependent?
The channel's opening is dependent on changes in the voltage.
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Will it be possible for an action potential to be triggered in a dead axon?
No (unless ATP is added); the polarisation of the membrane is maintained by the concentration gradients achieved by the action of energy-requiring Na+/K+ pumps; membrane integrity is lost in a dead axon.
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What does the refractory period do?
It ensures that impulses only travel in one direction.
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How long does the refractory period last for?
5 milliseconds; until all the voltage dependent sodium and potassium channels have returned to their normal resting state (closed) and the resting potential is restored.
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How does the refractory period ensure that an action potential will not be propagated back the way it came?
A new action potential will only be generated at the leading edge of the previous one as the membrane behind it will be incapable of transmitting an impulse;the membrane has to be repolarised (-70mV) before another action potential can be generated.
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Will a very strong light produce the same size action potential in a neurone coming from your eye as does a dim light?
Yes; the size of the stimulus has no effect on the size of the action potential, it just has to be above threshold level.
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What does the size of the stimulus affect?
The frequency of impulses and the number of neurones in a nerve that are conducting impulses.
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What are strong stimuli normally associated with?
A high frequency of firing and the firing of many neurones.
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When is an action potential generated?
When the stimulus is above threshold level.
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How is an impulse passed along an axon?
When a nerve is stimulated the action potential generated triggers a sequence of action potentials along the length of the axon. (propagation of an impulse along an axon).
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How does the diameter of the axon affect the speed of nervous conduction?
In general, the wider the diameter, the faster the impulse travels.
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How does the myelin sheath affect the speed of nervous conduction?
The myelin sheath acts as an electrical insulator along most of the axon, preventing any flow of ions across the membrane.
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What are the gaps in the myelin sheath called?
Nodes of Ranvier, they occur at regular intervals along the myelinated axon.
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Where is the only place depolarisation can occur along a myelinated axon?
The nodes of Ranvier.
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What is saltatory conduction?
The 'jumping' conduction; depolarisation happening at the nodes sets up a circuit which reduces the p.d. of the membrane at the next node, triggering an action potential.
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How does an impulse travel along an unmyelinated axon?
The impulse will travel along a wave of depolarisation.
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What is a synapse?
Where two neurones meet; the cells don't actually touch - there is a small gap, the synaptic cleft.
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What are neurotransmitters contained in?
Synaptic vesicles.
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What causes the release of neurotransmitters into the synaptic cleft?
The arrival of an action potential.
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Does it require energy to release neurotransmitters?
Yes; the presynaptic cell expands a considerable amount of energy to produce neurotransmitter and package it into vesicles ready for transport out of the cell.
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What is the name of the first neurotransmitter to have ever been discovered?
Acetylcholine.
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What are the three stages leading to the nerve impulse passing along the postsynaptic neurone?
Neurotransmitter release; stimulation of the postsynaptic membrane; inactivation of the neurotransmitter.
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What series of events causes neurotransmitter to be released?
When presynaptic membrane is depolarised the membrane becomes more permeable to Ca2+ and this diffuses into the cell down a concentration gradient. Increased Ca2+ causes synaptic vesicles to be fuse with the membrane and release their contents.
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What are the three stages leading to the nerve impulse passing along the postsynaptic neurone?
Neurotransmitter release, stimulation of the post synaptic membrane and inactivation of the neurotransmitter.
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How is the neurotransmitter acetylcholine removed from the synaptic cleft?
The enzyme acetylecholinesterase breaks down the acetylcholine so that it can no longer bind to receptors.
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What are the two roles of synapses?
Control of nerve pathways, allowing flexibility of response; integration of information from different neurones, allowing a coordinated response.
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What do excitatory synapses do?
They make the postsynaptic membrane more permeable to sodium ions.
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What is spatial summation?
Impulses from several different neurones produce an action potential in the postsynaptic neurone.
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What is temporal summation?
Several impulses along one neurone produce an action potential in the postsynaptic neurone.
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What do inhibitory synapses do?
Make the postsynaptic membrane more permeable to chloride ions so that it becomes hyperpolarised.
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How do auxins increase plant growth?
Auxins bind receptors in zone of elongation; produce second messenger molecule;change in gene expression; auxins cause acidification of cell well (indirectly acitvate proton pump) low pH cellulose bonds break; cell expands; increase ion uptake
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What is the role of the conjunctiva?
Protects the cornea
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What is the role of the cornea?
Bends light
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What is the role of the lens?
Focuses light on the retina
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What is the role of the retina?
Controls the amount of light entering the eye
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What is the role of the sclera?
Protective layer
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What is the role of the choroid?
Prevents internal reflection of light
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What is the role of the ciliary muscle?
Alters thickness of lens for focusing
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What is happening in your eyes in the dark?
Na+ diffuses into rod cell through non-specific cation channel; Na+ actively pumped out of the inner segment of the rod cell; rod cell becomes depolarised; glutamate released, binds to ganglion cell and prevents it from depolarising
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What is happening to your eyes in the light?
Rhodopsin breaks down into retianal and opisn; opsin causes non-specific cation channel to close; Na+ actively pumped out of the inner segment of the rod cell; hyperpolarised; glutamate is not released so ganglion cell can become depolarised
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What is dark adaptation?
The reforming of rhodopsin that has been broken down
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What are plant photoreceptors called?
Phytochromes
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How do you convert Pr into Pfr and vice versa?
Absorption of red light converts Pr into Pfr and absorption of far red light converts Pfr into Pr. Sunlight contains a lot of red light so there is more conversion of Pr into Pfr. At night, any Pfr present is slowly converted to Pr.
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What type of light inhibits germination and what type of light triggers germination?
Red light is effective at triggering germination and far red light inhibits germination.
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What are long day plants?
They only flower when day length exceeds a critical value. They need Pfr to stimulate flowering.
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What are short day plants?
They need long hours of darkness to convert all Pfr into Pr. Pfr inhibits flowering in short day plants.
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What is greening?
When a shoot has broken through soil into sunlight and the plant undergoes profound changes in both its form and biochemistry.
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What is the frontal lobe responsible for?
Decision making, reasoning, planning and consciousness of emotions. It is also concerned with forming associations and with ideas.
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What is the temporal lobe responsible for?
Processing auditory information, also involved in memory.
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What is the parietal lobe responsible for?
Orientation, movement, sensation, calculation, some types of recognition and memory.
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What is the occipital lobe responsible for?
Processing information from the eyes, including vision, colour, shape recognition and perspective.
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What is the thalamus responsible for?
Routing all the incoming sensory information to the correct part of the brain, via the axons of the white matter.
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What does the hippocampus do?
Involved in laying down long term memory.
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What is the cerebellum responsible for?
Balance, coordinating movement.
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What is the medulla oblongata responsible for?
Regulates body processes that we do not consciously control; heart rate, breathing and blood pressure
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How does fMRI work?
Radio signals are not absorbed by oxyhaemoglobin; more active areas need more oxygen; less radio signal absorbed the higher the level of activity in that region. Can produce 4 pics a second.
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What is a critical period?
Periods of time during postnatal development when the nervous system must obtain specific experiences to develop properly.
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Describe two animal studies on visual development.
Monocular deprivation in newborn monkeys and kittens; Monkeys - one eye did not receive any stimuli for 6 months. The monkey became blind in that eye. Kittens - before 3 weeks and after 3 months monocular deprivation had no effect, at 4weeks it did.
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What is happening in the monocular deprivation studies?
There is a lack of stimulation in one eye;axons from non-deprived eye pass impulses to cells in the visual cortex; their synapses are strengthened; axons from deprived eye don't pass impulses to the visual cortex; their synapses are eliminated
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How do we make judgements about the distance of objects less than 30m from us?
We depend on cells in the visual cortex that obtain information from both eyes at once. The visual field is seen from two different angles, and cells in the visual cortex allow us to compare the view from both eyes-Stereoscopic vision.
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How do we make judgements about the distance of objects further than 30m from us?
For objects >30m from us the images on our two retinas are very similar so visual clues and past experiences are used when interpreting the images.
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Is depth perception innate or learned?
the visual cliff - babies and newborn animals refused to crawl across a 'cliff' which suggests it is innate; carpentered world hypothesis-those who live in a world with more right angles tend to perceive depth cues very differently to those who don't
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How are memories created?
Can be created by altering the pattern of connections and the strength of synapses.
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What is habituation?
A type of learning - desensitised to stimuli
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What advantages can habituation provide?
Allows animals to ignore unimportant stimuli so that limited sensory, attention and memory resources can be concentrated on more threatening or rewarding stimuli.
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What is happening during habituation?
With repeated stimulus Ca2+ channels become less responsive so less Ca2+ crosses the presynpatic membrane, less neurotransmitter is released, less depolarisation of postsynaptic membrane so no action potential.
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What are the ethics of using animals in medical research?
Consent, animal rights, animal welfare, animal suffering and experiencing pleasure
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What are the main symptoms for Parkinson's disease?
Stiffness of muscles, tremor of the muscles, slowness of movement, poor balance, walking problems.
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What are the treatments for Parkinson's disease?
Drugs that slow the loss of dopamine from the brain; treating symptoms with drugs - Ldopa, precursor for dopamine (can be converted); dopamine agonists (directly activate dopamine receptor); gene therapy; surgery
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What are the drug treatments for depression?
Selective serotonin reuptake inhibitors - blocks the uptake of serotonin - prozac
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How does ecstasy affect synapses?
Increases the concentration of serotonin in the synaptic cleft; binds to molecules in the presynaptic membrane that are responsible for transporting serotonin back into the cytoplasm.
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What is a genome?
All the DNA of an organism (or species) including the genes that carry all the information for making the array of proteins required by the organism (or species).
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What is a SNP?
Single-nucleotide polymorphisms - a DNA sequence variation that involves a change in a single nucleotide.
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What is candidate gene?
A gene that may cause a particular disease.
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What is a drug target?
A specific molecule that a drug interacts with to bring about its effect.
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What are the ethical dilemmas of the human genome project?
Testing for genetic predispositions - should insurers have this information? Who should decide about the use of genetic predisposition tests, and on whom they should be used? Making and keeping records of individual genotype - confidentiality
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