Chapter 7 Linkage, Recombination, and Eukaryotic Gene Mapping PDF

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Outline of Chapter 7 Linkage, Recombination, and Eukaryotic Gene Mapping...

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Chapter 7: Linkage, Recombination, and Eukaryotic Gene Mapping 7.1 Linked Genes Do Not Assort Independently - Independent Assortment: two alleles at a locus separate independently of alleles at the other loci - independent assortment -> recombination or new combinations - Recombination -> when f1 progeny reproduces, combination of gametes may differ from combinations in the gametes of its parents - genes located close together on the same chromosome are linked genes - linked genes travel together in meiosis and are NOT expected to assort independently 7.2 Linked Genes Segregate Together while Crossing Over Produces Recombination Between Them - genes occasionally switch from one homolog. chromosome to the other through crossing over - Crossing over RESULTS in recombination -> breaks up association of genes - linkage keeps genes together - crossing over mixes them up -> new combinations Complete Linkage Compared with Independent Assortment - AaBb x aabb - alleles present in the gametes contributed by heterozygous will be expressed in progeny (b/c homozygous is recessive) - Heterozygous will determine what progeny looks like since homozygous ONLY contributes ab gamete - gametes that contain only original combination of alleles present in parents are nonrecombinant gametes - ex: Aa Bb - nonrecombinant: AB, ab - recombinant: Ab, aB - nonrecombinant progeny: progeny displaying original combinations of traits present in P generation - recombinant progeny: progeny with new combination of traits formed from recombinant gametes Crossing Over Between Linked Genes - Genes that exhibit crossing over are said to be incompletely linked - For each meiosis in which a single crossover occurs, there will be two nonrecombinant gametes and two recombinant gametes - frequency of recombinant gametes is half the frequency of crossing over - maximum proportion of recombinant gametes is 50% Calculating Recombination Frequency - Frequency of recombination = (number of recombinant progeny/total number of progeny)

Coupling and Repulsion - arrangement of alleles on homologous chromsomes is critical to determining the outcome of cross - coupling/cis configuration: wild types on one chromosome, mutants on the other - repulsion/trans configuration: one side contains wild-type and mutant allele Predicting the Outcomes of Crosses with Linked Genes - knowing the arrangement of alleles on a chromosome allows us to predict the types of progeny that will result from a cross with linked genes - can figure out which one will be most numerous - proportions => recombination frequency - Tt Dd x tt dd - if 16% will be recombinant -> 16% will have tt Dd or Tt dd - Other 84% will be Tt Dd or tt dd Constructing a Genetic Map with a Three-Point Testcross - three-point test cross -> three traits - crossing in P-generation = homozygous dominant x homozygous recessive - then use F1 x homozygous recessive 1) Look at the one with the most abundant and least abundant progeny - most abundant = non recombinant - least abundant = double cross!! 2) Of the least abundant -> look for the gamete determined by heterozygous parent (expressed gamete) - which one sticks out? (ex. X+ Y+ Z/ X Y Z) (X Y Z+/X+ Y+ Z) - Z sticks out!! 3) the one that sticks out = middle guy! - order is probably (X Z Y) 4) recombination frequency? -> will be between two of the 3 alleles - (ex. X-Z frequency or Z-Y recombination frequency) - add it up and divide by total!! - distance: you kno what 2 do - interference: one crossover interfering with additional cross over - to figure it out: - coefficient of coincidence: (observed double cross overs/expected) - interference = 1- COC Effects of Double Crossovers - can include 3 or even 4 of chromatids - two-strand double crossover = no new combination of alleles - three-strand double crossover = 50% recombination - four-strand double crossover = 100% recombination

- map distance based on recombinaton rate underestimate true distance since some crossovers are not detected among progeny - when they are very close together -> crossover unlikely...