352SLecture08_S11 - Gene$cs and Evolu$on: Lecture #8...

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Unformatted text preview: Gene$cs and Evolu$on: Lecture #8 Today’s Topics: 1.  Linkage and recombina$on 2.  Mapping 2 genes 3.  Mapping 3 genes from 2 ­point crosses •  Readings: Pierce, pgs. 161 ­ 170; 173  ­ 175 Linkage and Recombination I Mendel Revisited: •  Law of Segrega$on (1st Law) •  Law of Independent Assortment (2nd Law) •  2nd law results in recombina$on: sor$ng of alleles into new combina$ons. •  Figure 7.2 1 Genes on Chromosomes: •  Walter SuQon: Chromosome Theory of Heredity -  1903 -  Genes are located on chromosomes -  Chromosomes are inherited during reproduc$on. Genes on Chromosomes: •  Figure 7.3 illustrates a cross that violates the 2nd Law. -  Traits: flower color (purple, white) and pollen shape (round, long) -  Dihybrid cross -  F2 ra$os differ from 9:3:3:1 •  Meiosis explains Mendel’s 1st and 2nd Laws. •  More genes than there are chromosomes: -  Many genes per chromosome -  Genes on same chromosome won’t assort independently!! -  Viola$ons to the 9:3:3:1 ra$o 2 Linkage: •  Figure 7.3 (con$nued). -  Too many “parental” phenotypes -  Not enough “recombinant” phenotypes Linkage: •  The effect of crossing over. •  In theory, linkage groups should be inherited as a unit •  Crossing over results in recombinaEon •  •  •  •  Explana$on: Linkage. Linked genes: genes located on the same chromsome. Linkage groups: groups of linked genes # linkage groups = (1N – 1) + # sex chromosomes -  Humans = 22 autosomes + 2 sex chromosomes = 24 •  Figure 7.4. •  Linked genes do not assort independently!! NotaEon: •  Two linked genes – one with A, a – other with B, b •  AA BB x aa bb •  Draw the chromosomes: A B B Or A Or AB/AB and ab/ab 3 NotaEon: •  AA BB x aa bb •  F1 genera$on: •  Incorrect nota$ons: NotaEon: Or: Or: AB/ab •  These individuals are also genotype AaBb, but the posi$ons of the genes and alleles don’t display correct linkage rela$onships! Note: All F1 individuals have genotype AaBb Complete Linkage: •  Complete Linkage: genes are so close crossing over never occurs between them. -  Rarely occurs in the real world -  Useful in showing effect of linkage on crosses Complete Linkage: MD/MD x md/md F1: MD/md F1 is then testcrossed to the md/md parent •  Figure 7.5a: results under complete linkage. •  Figure 7.5b: results under independent assortment. •  Example: 1.  Leaf type – dominant allele M for normal, recessive allele m for moQled. 2.  Plant height – dominant allele D for tall, recessive allele d for dwarf. 4 Complete Linkage: •  Things to note from Figure 7.5: -  Under complete linkage, only “parental” types are seen in F2 -  No “recombinant” types seen in F2 -  Under independent assortment, “parental” and “recombinant” types occur in equal propor$ons in F2 Incomplete Linkage: •  Incomplete linkage: some crossing over occurs between the linked genes. -  Usually the case for linked genes •  Figure 7.6 -  -  -  -  Crossing over occurs in prophase I of meiosis Non ­sister chroma$ds exchange gene$c material Other chroma$ds unchanged – nonrecombinants Recombinants formed by crossing over 5 Incomplete Linage: 1.  Recombinants are seen. 2.  Recombinants are the result of crossing over. 3.  Nonrecombinants are most common. Recombination frequency = #recombinant progeny/total # progeny)x100 =[(8 + 7)/(55 + 53 + 8 + 7)]x100 = 12.2% Chromosome ConfiguraEons: •  Arrangements of genes and alleles are cri$cal to predic$ng outcomes of crosses with linked genes. •  Example: genotype AaBb •  Or Chromosome ConfiguraEons: •  Note: -  Dihybrid configura$on determined by parental ones -  Dihybrid configura$on determines common phenotypes in the testcross -  Crossover rates are the same for cis and trans configura$ons •  Figure 7.8 •  cis (coupling) configuraEon: both mutants (or wild ­types) on the same chromosome •  trans (repulsion) configuraEon: recessive and wild ­type on same chromosome. 6 Chromosome ConfiguraEons: •  If you know: -  Chromosome configura$ons (cis or trans?) -  Recombina$on rates between the linked genes under study •  You can predict the outcomes of the test cross •  Try Figure 7.10 for fun. •  2 ­point cross: cross between dihybrid and the homozygous recessive. -  Used to tell if 2 genes are linked or assor$ng independently Frequency of RecombinaEon: •  Crossing over: 1.  Random event during synapsis in prophase I 2.  Probability of a crossover between 2 linked genes is a func$on of how far apart the genes are on the chromosome. 3.  Crossover probability will determine the frequency of recombina$on. •  An analogy. •  Note: -  Crossover frequency increases with distance between genes -  linked genes that are very far apart may approximate independent assortment. -  2 ­point cross may not always detect linkage. Chromosome Mapping: •  Chromosome mapping: determine the posi$on of genes on chromosomes. •  Assume: •  Crossing over only occurs between homologous sites •  Crossing over is random at sites along the chromosomes •  Crossing over between chroma$ds is random with respect to which alleles are present. 7 Chromosome Mapping: •  Two ways to measure the distance between 2 genes: 1.  Distance inferred from recombina$on = map distance 2.  # of base pairs between gens = physical distance •  Units of measurement: –  1 map unit = 1 cen$morgan (cM) = 1% recombina$on -  Map distance = (#recombinant gametes/total # gametes)x100 Chromosome Mapping: •  Mapping 3 linked genes: –  Draw 3 chromosomes – put different gene in the middle of each one –  Write map distances from the three 2 ­point crosses –  Only 1 chromosome will make mathema$cal sense. •  Example. –  –  –  –  Genes A and B are 15 cM apart Genes A and C are 10 cM apart Genes B and C are 5 cM apart Draw the map! Next Time: •  Lecture 9: Linkage and Recombination II, Review •  Readings: -  Pierce, pgs. 175 – 179; 183 8 ...
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