1Solutions to 7.012 Problem Set 7Question 1A female geneticist has difficulty characterizing flies by their eye color because she herself is red-greencolorblind (an X-linked recessive phenotype). She has one sister (colorblind) and one brother (notcolorblind).a) Sketch a pedigree that includes the geneticist, her sister, her brother, and their parents; indicategenotypes (clearly define your genotype symbols).*With random mating-it's as though there are a pool of gametes from which to choose. So imagine apool of X chromosomes, a mixture of Xs and Xcbs. To make a male, you pick one randomly, thereforethe chance ofXcb/Y = q.Females you pick 2 chromosomes randomly. p2 = frequency of: +/+2pq = frequency of cb/+;b) If 8% of males in a human population are red-green colorblind, what are the frequencies of the wild-type and colorblindness alleles?Let p= fraction X+Let q= fraction Xcb out of total pool of X chromosomes.To make a male, you take 1 X chromome:~ probability Xcb/Y = q andX+/Y = p therefore q = 0.08 and p = 0.92c) What % of females in this population should be red-green colorblind?~ probability= Xcb/Xcb = q2 = (0.8)2 = 0.64%d) What % of females in this population should be carriers for red-green colorblindness?14.7% or Xcb/X = 2pq =14.7%123451 father Xcb/Y 2 mother Xcb/X3 geneticist Xcb/Xcb4 sister Xcb/Xcb5 brother X/Ycb is colorblind allele(recessive phenotype)MIT Biology Department7.012: Introductory Biology - Fall 2004Instructors: Professor Eric Lander, Professor Robert A. Weinberg, Dr. Claudette Gardel2Question 2a) Clearly the intracellular and extracellular environments, along with ion permeability,influence the profile of an action potential. On the diagram below, draw what you think anaction potential would look in the following altered conditions. Label each action potentialwith the corresponding condition.i) The external Na+ concentration is lowered.Lowered peak, reduced rate of rise.ii) The external K+ concentration is lowered.More negative resting membrane potential.iii) Scorpion toxin which slows the inactivation of Na+ channels is added.Prolonged repolarizationNormali)ii)iii)3Question 2, continuedb) Action potential wave fronts can vary even within the same neuron. You find suchvariation when you generated action potential graphs from recordings taken in frog motorneurons. One of the graphs represents data recorded from the axon of a neuron, while theother represents data recorded from the cell body of the same frog motor neuron.Unfortunately, you forgot to label the graphs “taken from axon” or “taken from cell body”.Given the two action potentials below, label them properly, and explain how you know yourlabels are correct.Explain:The action potential recorded from the cell body is smaller and prolonged with respect to that seen in theaxon. This is because the axon is myelinated and therefore less current is leaked.0 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 80 1 2 3 4 0 2 4 6 8Time (ms)Figure by MIT OCW.4Question 3The sensory neurons synapse onto the interneurons and release excitatory neurotransmittersto continue the action potential through to the motor neurons.• At which of the remaining synapses would you expect an inhibitory neurotransmitter(such as GABA) to be released? Clearly label either on the diagram or write explicitlywhich synapses you refer to.You would expect an inhibitory neurotransmitter (such as GABA) to be released at the synapse fromthe motor neuron serving the flexors to the flexor muscle.Alternatively, You would expect an inhibitory neurotransmitter (such as GABA) to be released atSynapse 7• At which synapse would you expect an excitatory neurotransmitter (such asacetylcholine)? Clearly label either on the diagram or write explicitly which synapsesyou refer to.You would expect an excitatory neurotransmitter (such as ACh) to be released at the synapse fromthe motor neuron serving the quadriceps to the quadricep muscle.Image removed due to copyright reasons.5Question 4You work in a mouse lab, studying a colony of mutant mice with a depressed phenotype: theyshow little interest in eating, don’t interact with other mice, and don’t play with toys that yougive them.You decide to examine the brains of some of these mice, and look at a region of the brainpreviously shown to be important for ‘good feelings’.You look at synapses in this region and find that the presynaptic axon terminals of cells in themutant brain are filled with vesicles. You compare the number of vesicles in your mutantmouse with the number in a normal mouse, and find that there are significantly more vesiclesin the mutant cell.a) What process may be affected by this mutation?Exocytosis or Synaptic vesicle fusionb) What neurotransmitter is likely used to transmit signals in the network of neurons you arestudying?Seratoninc) Considering the neurotransmitter in (b), how does a mutation that affects the process in (a)explain the mutant mouse’s behavior?If seratonin is not released to the postsynaptic cells in this region of the brain, then the postsynaptic cellsare not stimulated to fire. If the postsynaptic cells are not firing, the end result of “feeling good” doesnot occur.d) Could a drug that inhibited the reuptake of the neurotransmitter at the synaptic cleft relievethe mouse’s depressed symptoms? Why or why not?No. If the neurotransmitter is never released, inhibiting reuptake will not solve the problem.OrYes. If a little neurotransmitter is released, inhibiting reuptake could prolong the effect of thatneurotransmitter and relieve the
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