Midterm#1 comments. Overview- chapter 6. Recombination. Recombination 1 st sense

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1 Midterm#1 comments So fr, ~ 10% of exms grded, wide rnge of results: 1 perfect score, 1 score < 100pts rtil credit is given if you get prt of the nswer right Tests will e returned next Thursdy Some of you pper to hve reding comprehension prolems- you nswered mny questions I did not sk! (nd not just #10!) Some mistkes cn e voided if you ttempt to drw out the possile or lterntive solutions Suggestion: If you hven t done so lredy, form study group nd work prolems to relly lern the concepts ecomintion Increses the genetic vrition of popultion ecuse new llelic comintions re produced Essentil to evolutionry dpttion: since the environment chnges, orgnisms must respond to new forces unlike those experienced y their prents Muttion cretes the new lleles, nd recomintion rings them together in new rrngements to crete new genotypes nd phenotypes ( ex: disese resistnce, tolernce) Overview- chpter 6 In, recominnt products with new comintions of prentl lleles re generted y: independent ssortment (segregtion) of lleles on nonhomologous chromosomes. crossing-over in premeiotic S etween nonsister homologs. In dihyrid, 50% recominnts indictes either tht genes re on different chromosomes or tht they re fr prt on the sme chromosome. ecomintion frequencies cn e used to mp gene loci to reltive positions; such mps re liner. Crossing-over involves formtion of D heteroduplex. ecomintion 1 st sense The first wy in which this word is used y geneticists refers to independent segregtion of heterozygous genes on different chromosomes, ecuse chromosomes sort independently in ( due to rndom ttchment of spindle fiers to one homolog or the other). Outcome? Mtings etween two geneticlly different individuls cn led their offspring to produce germ cells tht re unlike those they got from either prent.

2 ecomintion 2 nd sense The second wy in which this word is used y geneticists refers to the inheritnce of new comintion of lleles on single chromosome, due to the rekge of the D strnds, strnd swpping, nd seling of roken ends y D repir enzymes. Outcome? Mtings etween two geneticlly different individuls cn led their offspring to produce gmetes tht hve different llelic comintions long the sme D strnd, compred to the originl prentl input. ecomintion (1) fundmentl consequence of independent ssortment (independent segregtion) crossing-over etween homologous chromtids Yields hploid products with genotypes different from oth of the hploid genotypes prentls tht originlly formed the diploid meiocyte 2 different genotypes recominnts ecomintion (2) ringing together of two or more pirs of lleles into new comintions / / prentl genotypes / / prentl () genotypes / / recominnt () genotypes

3 Gol of recomintionl nlysis? To detect nd quntify the output of meiotic products this is done indirectly y mking crosses nd then counting phenotypes. The numer of unusul or new phenotypes (nd inferred genotypes) is then compred to the input (prentl) genotypes. If the frequency of different output genotypes cn e counted, it is possile to deduce whether the trits of interest re on the sme or re on different chromosomes. Geneticists construct the crosses so tht genotypes cn e deduced from phenotypes. Independent ssortment (1) lso known s independent segregtion Consequence of independent lignment of chromosomes in meiotic ivlents / ; / / ; / / ; / ; ; ; ; O lternte ivlnts Independent ssortment (2) For genes on different (nonhomologous) pirs of chromosomes, recominnt frequency is lwys 50% / ; / / ; / / ; / / ; / / ; / / ; / ; ; ; ; 50% recominnts ; ; ; ; Dihyrid testcross (1) Determines genotype of dihyrid y crossing to homozygous recessive tester rentl / ; / / ; / F 1 / ; / / ; / testcross F 1 gmetes ; tester gmetes ; / ; / progeny proportions progeny phenotypes 1:1:1:1 rtio ; ; / ; / / ; / ; / ; /

4 Dihyrid testcross (2) est wy to study recomintion is in dihyrid testcross only dihyrid produces recominnt genotypes ll homozygous recessive tester gmetes like Typicl 1:1:1:1 rtio result of independent ssortment in dihyrid Ech genotype in progeny hs unique phenotype Oserved y Mendel in testcrosses with two pirs of trits Dihyrid selfing Cross etween two / ; / dihyrids recomintion occurs in oth memers of cross recomintion frequency is 50% ; ; ; ; ; / ; / / ; / / ; / / ; / ; / ; / / ; / / ; / / ; / ; / ; / / ; / / ; / / ; / ; / ; / / ; / / ; / / ; / tio: 9 / ; / 3 / ; / 3 / ; / 1 / ; / roduct rule Multiply proilities of independent occurrences to otin proility of joint occurrence E.g. rnched tree or grid methods For mting / ; / / ; / Segregtion t, gives ¾ / nd / in progeny Segregtion t, gives ¾ / nd / in progeny ¾ / / ¾ / 9/16 / ; / 3/16 / ; / / 3/16 / ; / 1/16 / ; /

5 Independent ssortment: multiple loci Clcultions cn e mde for ny gene comintion using predicted outcomes t single loci nd the product rule 1 / ; / ; C/c ; D/d 2 / ; / ; C/c ; D/D # gmetes 1 2 x 2 x 2 x 2 = 16 # gmetes 2 1 x 2 x 2 x 1 = 4 # genotypes in F 1 2 x 3 x 3 x 2 = 36 # phenotypes in F 1 2 x 2 x 2 x 1 = 8 Frequency of / ; / ; C/ ; D/ ½ x ¾ x ¾ x 1 = 9/32 Deducing genotypes from rtios Genetic nlysis works in two directions predict genotypes in offspring determine genotypes of prents in cross Specific expecttions, e.g., 1:1:1:1 nd 9:3:3:1 cn e used to deduce genotypes Testcross exmple: henotype # oserved / ; / 310 / ; / 295 / ; / 305 / ; / 290 The oserved results re close to 1:1:1:1, llowing the deduction tht the tested genotype ws dihyrid. Crossing-over (CO) rekge nd rejoining of homologous D doule helices Occurs only etween nonsister chromtids t the sme precise plce Visile in diplotene s chismt Occurs etween linked loci on sme chromosome cis: recessive lleles on sme homolog (/) trns: recessive lleles on different homologs (/) Cis trns crossing-over cis trns meiotic crossing-over / / / / Drwing shows only chromtids engged in crossing-over Effect is to switch etween cis nd trns