Recombination-Raven8-no3pt-large

Recombination-Raven8-no3pt-large - We have seen that...

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Unformatted text preview: We have seen that Mendelian characters are determined by genes located on chromosomes and that the independent assortment of Mendelian traits reflecm the independent assortment of chromo— somes in meiosis. This is fine as far as it goes, but it is still incom- plete. Of Mendel’s seven traits in figure 12.4, six are on different chromosomes and two are on the same chromosome, yet all show independent assortment with one another. The two on the same chromosome should not behave the same as those that are on dif— ferent chromosomes. In fact, organisms will generally have many more genes that assort independently than the number of chro— mosomes. This means that independent assortment cannot be due only to the random alignment of chromosomes during meiosis. [’1 I Mendel did not examine plant height and pod shape in his dihybrid crosses. The genes for these traits are very close together on the same chromosome. How would this have changed Mendel ’3 results? 242 part genetir and molecular biology The solution to this problem is found in an observationi that was introduced in chapter 11: the crossing over of homo- logues during meiosis. In prophase I of meiosis, homologuesr appear to physically exchange material by crossing over (figure 13.5). In chapter 11, you saw how this was part of the mecha4 nism that allows homologues, and not sister chromatids, to dis— I join at anaphase I. Genetic recombination exchanges alleles on homologues Consider a dihybrid cross performed using the Mendelian‘ framework. Two true—breeding parents that each differ with re- spect to two traits are crossed, producing doubly heterozygous F1 progeny. If the genes for the two traits are on a single chromo- some, then during meiosis we would expect alleles for both loci to segregate together and produce only gametes that resemble the two parental types. But if a crossover occurs between the loci, then each homologue would carry one allele from each ‘~ t and produce gametes that combine these parental traits figure 13.5). We call gametes with this new combination of es recombinant gametes as they are formed by recombining parental alleles. The first investigator to provide evidence for this was gan, who studied three genes on the X chromosome of phila. He found an excess of parental types, which he ex- ed as due to the genes all being on the X chromosome and ore coinherited (inherited together). He went further ting that the recombinant genotypes were due to cross— :‘0VC1' between homologues during meiosis. . Experiments performed independently by Barbara Mc- “-e o k and Harriet Creighton in maize and by Curt Stern « opbila, provided evidence for this physical exchange of I No crossing over during prophase I Meiosis ll Parental :; re 1 3 . 5 I CROSSING OVER EXCHANGES ALLELES ON HOMOLOGUES. —i When a crossover occurs between two loci, it leads to the r production of recombinant chromosomes. If no crossover occurs, I then the chromosomes will carry the parental combination of alleles. . . .ravenbiology.com l genetic material. The experiment done by Creighton and Mc- Clintock is detailed in figure 13.6. In this experiment, they used a chromosome with two alterations visible under a microscope: a knob on one end of the chromosome and a part of a different chromosome attached to the other end. In addition to these cytological markers, this chromosome also carried two genetic markets: a gene that determines kernel color and one that de- termines kernel texture. chromosome extension marker I knob marker 6 C dominant for color ‘ recessive for texture — starchy Parental c gametes Wx Recombinant gametes WX dominant for M - - WX eIOSIs texture — waxy Progeny with recombinant phenotypes carry physically recombinant chromosomes. figure 13.6 THE CREIGHTON AND MCCLINTOCK EXPERIMENT. This experiment first demonstrated that chromosomes physically exchange genetic material during recombination. The experimental design was to use chromosomal differences visible in the microscope and two unrelated genes on the same chromosome. When plants heterozygous for visible and genetic markers were testcrossed, progeny that are genetically recombinant have also exchanged visible markers. This shows th: the chromosomes have physically exchanged genetic material. tbzzpter chromosomes, mapping, and the meiosis-interirame tannertian 2‘ The longer chromosome, which had the knob, carried the dominant colored allele for kernel color (C) and the recessive waxy allele for kernel texture (wx). Heterozygotes were produced with the altered chromosome paired with a normal chromosome carrying the recessive colorless allele for kernel color (5) and the dominant starchy allele for kernel texture (Wx) (see figure 13.6). These plants appeared colored and starchy because they were heterozygous for both loci, and they were also heterozygous for the two visibly distinct chromosomes. A testcross was performed with these F1 plants and color— less waxy plants. The progeny were analyzed for both physical recombination (using a microscope to observe chromosomes) and genetic recombination (by examining the phenotype of progeny). The results were striking: All of the progeny that ex- hibited the recombinant phenotype also now had only one of the chromosomal markers. That is, physical exchange was accompa- nied by the recombinant phenotype. Recombination is the basis for genetic maps The ability to map the location of genes on chromosomes us— ing data from genetic crosses is one of the most powerful tools of genetics. The insight that allowed this technique, like many great insights, is so simple as to seem obvious in retrospect. Morgan had already suggested that the frequency with which a particular group of recombinant progeny appeared was a reflection of the relative location of genes on the chro— mosome. An undergraduate in Morgan’s laboratory, Alfred Sturtevant put this observation on a quantitative basis. Stur- tevant reasoned that the frequency of recombination observed in crosses could be used as a measure of genetic distance. That is, as physical distance on a chromosome increases, so does the probability of recombination (crossover) occurring between the gene loci. Using this logic, the frequency of re- combinant gametes produced is a measure of their distance apart on a chromosome. Linkage data To be able to measure recombination frequency easily, inves— tigators used a testcross instead of intercrossing the F1 prog— eny to produce an F2 generation. In a testcross, as described earlier, the phenotypes of the progeny reflect the gametes produced by the doubly heterozygous F1 individual. In the case of recombination, progeny that appear parental have not undergone crossover, and progeny that appear recombinant have experienced a crossover between the two loci in question (see figure 13.5). When genes are close together, the number of recombi— nant progeny is much lower than the number of parental prog- eny, and the genes are defined on this basis as being linked. The number of recombinant progeny divided by total progeny gives a value defined as the recombination frequency. This value is converted to a percentage, and each 1% of recombi— nation is termed a map unit. This unit has been named the centimorgan (CM) for T. H. Morgan, although it is also called simply a map unit (m.u.) as well. 244 partIII genetic and molecular biology b recessive allele (black body) b1“ dominant allele (gray body) vg recessive allele (vestigial wings) vg* dominant allele (normal wings) *D vgvg Parental ‘ I generation i 3 ii Cross-fertilization F1 generation Parental male gametes r 415 parental wild type (gray body. normal wing) F1 generation female possible gametes I 92 recombinant ‘ (gray DOdY. vestigial wing) ‘ 88 recombinant (black body, ‘ normal wing) 405 parental ‘ mutant type (black body, vestigial wing) 180 + 1000 = 0.18 total recombinant offspring 18% recombinant frequency 18 CM between the two loci figure 13.7 TWO-POINT CROSS T0 MAP GENES. Flies homozygous for long wings (vg*) and gray bodies (12*) are crossed to flies homozygous for vestigial Wings (71g) and black bodies (12). Both vestigial wing and black body are recessive to the normal (wild type) long wing and grey body. The F1 progeny are then testcrossed to homozygous vestigial black to produce the progeny for mapping. Data are analyzed in the text. 'ng maps :1 cting genetic maps then becomes a simple process of ' g testcrosses with doubly heterozygous individuals counting progeny to determine percent recombination. is best shown with an example using a two—point cross. - Drarophz'la homozygous for two mutations, vestigial wings Eand black body (h), are crossed to flies homozygous for the “type, or normal alleles, of these genes (vg+ If“). The doubly ,, ozygous Fl progeny are then testcrossed to homozygous ive individuals (vg h/vg h), and progeny are counted (fig— ’ 3.7). The data are shown below: z '31 wings, black body (vg h) .; wings, gray body (vg+ 11*) 'gial wings, gray body (vg 17+) 92 (recombinant) g wings, black body (vg+ h) 88 (recombinant) Progeny 1000 405 (parental) 415 (parental) : The recombination frequency is 92 + 88 divided by 1000, in distance separating loci increases, the probability of recom— "on occurring between them during meiosis also increases. thappens when more than one recombination event occurs? 1 If homologues undergo two crossovers between loci, then . parental combination is restored. This leads to an under- : ate of the true genetic distance because not all events can mated. As a result, the relationship between true distance on a: omosome and the recombination frequency is not linear. {5- as a straight line, but the slope decreases; the curve '15 off at a recombination frequency of 0.5 (figure 13.8). At long distances, multiple events between loci become uent. In this case, odd numbers of crossovers (1, 3, 5) pro- e recombinant gametes, and no crossover or even num- PfiySiea’lD o u,- wmm. m n a Chromosome ' 13.8 7 RELATIONSHIP BETWEEN TRUE DISTANCE AND RECOMBINATION FREQUENCY. As distance on a chromosome increases, the recombinants are not all detected due to double . crossovers. This leads to a curve that levels off at 0.5. ' .ravenbiology.c0m bers of crossovers (0, 2, 4) produce parental gametes. At large enough distances, these frequencies are about equal, leading to the number of recombinant gametes being equal to the num- ber of parental gametes, and the loci exhibit independent as- sortment! This is how Mendel could use two loci on the same chromosome and have them assort independently. What would Mendel have observed in a dihybrid cross the two loci were 10 EM apart on the same chromosome? 1: this likely to have led him to the idea of independent assortment? chapter thramoromer, mapping, and the meiosis—interitanee connection 245 ...
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This note was uploaded on 04/18/2008 for the course BIO 102 taught by Professor Haworth during the Spring '08 term at Marquette.

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Recombination-Raven8-no3pt-large - We have seen that...

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