Office hours:Monday 5-6pm (door closes @ 5:15p)Friday 4-5 pm11 Koshland HallAre mutant A and mutant B alleles? YES: A&B are alternative forms of the same gene lozengea and lozengeb NO: A&B are alternative forms of different genes lozengea and lozengelikebAlleles: DIFFERENT (alternative) forms of the same geneWhat does a geneticist mean by “gene” ?…revealed by how a geneticist tests for “allelism”mutant a (lozenge-shaped eyes) vs. wildtypemutant b (lozenge-shaped eyes) vs. wildtypeGiven:First must ask:(1) are each of these mutant phenotypes likely to be caused by single-gene differences from the wildtype?(2) do the mutants map to the same general region of the genome?Before we ask:Are mutant a and mutant b alleles?mutant a vs. wildtypemutant b vs. wildtypeGiven:(sequencing mutant A and mutant B genomes would not necessarily answer either question!)(1) are each of these mutant phenotypes likely to be caused by a single-gene differences from the wildtype?(2) do the mutants map to the same general region of the genome? (1) a/a X +/+ --> a/+ --> a/a & a/+ & +/+ only(2) m1-m2-,a+ X m1+m2+,alz --> trihybrid --> least frequent gamete classes: m1- m2- alz & m1+m2+a+mutant a vs. wildtypemutant b vs. wildtypeGiven:hence order: m1 a m2 least frequent classtwo outside parental, inside nonparental:low resolution segregation tests:3-factor cross rule:Are mutant a and mutant b alleles (i.e. genetic alternatives)?Do we go with Mendel or (Lewis)/Benzer?high-resolutionsegregation testfor recombinationbreedingbehavior of hybridscomplementation testfor functionphenotypeof hybridsmutant amutant bmutant amutant bORa ++ bab?ORa ++ bab?Are a & b alternativeunits of segregationduring meiosis?i.e. are they segregational alleles?Are a & b alternativecis-acting units of functionduring development?i.e. are they functional alleles?Are mutant a and mutant b alleles (i.e. genetic alternatives)?high-resolutionsegregation testfor recombinationbreedingbehavior of hybridscomplementation testfor functionphenotypeof hybridsmutant amutant bmutant amutant bmust first establish:that a & b arerecessive?a ++ b=a=b=w.t.=?+ +a ballelesnot allelesgametesORa ++ bab?ORa ++ bab?wildtypemutant b= w.t.phene.abONLY(hence one functional copy sufficient)mutantPhenotype(don’t complement)wildtypePhenotype(complement)allelesnot allelesmutant awildtype= w.t.phene.Are mutant a and mutant b alleles (i.e. genetic alternatives)?complementation testfor functionphenotypeof hybridsmutant amutant bORa ++ bab?mutantphenotype(don’t complement)wildtypephenotype(complement)allelesnot alleleswildtypemutant b= w.t.phene.mutant awildtype= w.t.phene.What aboutthe cis/transtest?a & bin transWhere is the cis controlcomplementationmutant a mutant b + +a & bin cisAre mutant a and mutant b alleles (i.e. genetic alternatives)?complementation testfor functionphenotypeof hybridsmutant amutant bORa ++ bab?mutantphenotype(don’t complement)wildtypephenotype(complement)allelesnot allelesWhen do we need the cis controlmutant a mutant b + +--- are we being misledby the apparent failure of theserecessive mutants to complement in trans?…could mutants be recessive individually and in different cis-acting units of genetic function (i.e. not be functional alleles),but interact in combination toappear dominant together)If so, the cis control will also be mutant!Are mutant a and mutant b alleles (i.e. genetic alternatives)?complementation test for functionphenotypeof hybridsmutant amutant bmutantphenotypedon’t appear to complementbut can we believe it?TRANSwildtypephenotype… we can believe it!BUT if mutantphenotype…we’ve been misled!!!CIS phenotype ! TRANS phenotype, hence functional allelescis vs. trans matters!CIS phenotype = TRANS phenotype, hence NOT functional allelescis vs. trans doesn’t matter!mutant a mutant b + +CIS controlAre mutant a and mutant b alleles (i.e. genetic alternatives)?complementation test for functionphenotypeof hybridsmutant amutant bmutantphenotypedon’t appear to complementrecessive mutantsin TRANSwildtypePhenotype… we can trust the trans resultmutant a mutant b + +same recessivemutants in CISCIS/TRANS testBut nobody bothers with the cis control for recessive mutantswhen doing complementation testsdespite what all the textbooks say (not even Benzer did it, as we will see)…then why am I wasting your time with this?It is too hard in most cases, and in most cases unnecessary.Are mutant a and mutant b alleles (i.e. genetic alternatives)?CIS/TRANS testforfunctional allelismmutant amutant bTRANSCIS phenotype ! TRANS phenotype, hence functional allelesmutant a mutant b + +CISThe true cis/trans test will allow us to determine allelismeven if one or both of the mutants are not recessive!…and all that matters is whether the cis vs. trans phenotypes arethe same or different, not whether either one is wildtype.Remember: the “complementation test” per se is limited to recessive mutants. Most mutants are recessive, but some of the most useful & interesting are not.CIS phenotype = TRANS phenotype, hence NOT functional allelesAre mutant a and mutant b alleles (i.e. genetic alternatives)?high-resolutionsegregation testfor recombination:alleles as alternativeunits of segregation(recombination)complementation testfor functionalleles as alternativecis-acting units of functionmutant amutant bmutant amutant bMendel said that genes are the units of segregation, which led to the “beads on a string” model of genes & chromosomes:a & b may appear to be allelicby the functional test(failure to complement)…yet NOT allelicby the functional test(nonparental alleles recoveredby meiotic recombination)results mayconflictgenetic map(linear like chromosome)Are mutant a and mutant b alleles (i.e. genetic alternatives)?high-resolutionsegregation testfor recombination:alleles as alternativeunits of segregation(recombination)complementation testfor functionalleles as alternativecis-acting units of functionIf one can get recombination between functional alleles (alternative forms of a gene),then how do the genetic maps one can therefore construct within single genes compare to the genetic maps that can be (had been) constructed between genes?--- WHAT IS THE NATURE OF GENETIC FINE STRUCTURE?…and what is the basic
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