Eukaryote gene mapping - lect notes

Linkage

• Each characters coded by a geneon different chromosomes: law of independent Assortment

  • Same chromosome? Linked genes/syntenicutilizing test cross and genetic markers (mutation w/ distinct phenotype)

  • First evidence of linkage: William Bateson, Edith Saunders, Reginal Punnett; 1905

    • Experiment — do the genes for flower color & pollen shape in sweet peas assort indepdently?

  • Notation for genetic linkage: Thomas Hunt Morgan. Drosophila flies; several X-linked mutations —> some genes are kinked

    • for individuals with white eye (w) and minature wing (m) notation is: w m/ w m ,, where backslash signifies a pair of homologous chromosomes where genes on either side of the slash (w and m) are linked

    • Wild-type males, notation w+ m+/Y

  • Morgan’s Fly Experiments

    • Why are males with recessive alleles?

    • They must get Y chromosome from father, mom only has recessive alleles

    • In F2 generation, Morgan noticed most offspring had parental phenotypes, but only 37% were recombinant

    • 50% recombinants are expected if independent assortment is true*** will be asked about on final exam

  • Morgan Proposes crossing over

    • Alleles of some genes assort together because they lie near each other on the same chromosome

    • Crossing over can occur at breaks in between genes = recombinant progeny

    • In drosophila — crossing over will occur less frequently in between the alleles to produce recombinant phenotypes (if there is a random break, it will happen less frequently in between the genes than in an area above or below the break)

  • Creighton & McClintock

    • Proving crossing over

      • 1931; found a heterozygote in corn with a translocated chromosome—part of chromosome 8 had broken off and attached to one homolog of chromosome 9 & a darkly stained knob on the same homolog

      • Found that recombinant chromosomes carried knob with C allele, and translocated chromosome

Linkage & testcrosses

• How to determine if genes are linked?

• testcrosses cross individual with homozygous recessive allels with heterozygous individual

  • Ex. a+/a b/b+ * a/a b/b

  • If genes NOT linked = 1:1:1:1 ratio = a+b+ a+b a b+ a b

Coupling & Repulsion

• w+m+/w m two wild-type allele on one homolog, two recessive alleles on other = coupling

• w+m/ w m+

Map Units

• 1913, Alfred Sturtevant suggested that recombination frequencies could be used to calculate distances between genes on a genetic mao

• Defined map unit (mu) as interval in which 1% of crossing over takes place—-aka centimorgan

• for linked genes, crossover frequency does NOT equal recombination frequency

• Crossover frequency: frequenc of physical exchange between chromosomes in between genes of interest

• Recombination frequency: frequency of recombination of genetic markers (alleles) in a cross—determined by offspring phenotypes

Two-point testcrosses

• Two-points are two genes—want to obtain recombination frequencies

• A double heterozygote is crossed with a homozygous recessive indivisual to obtain recombination frequencies

• In every case, we should obtain equal # of parental and recombinant phenotypes

recomb freq: (# recombinants/# of testcross frequency * 100)

• Ex. 20% recom freq

  • cross a+ b+/b with itself 20% of genes will be recombinatns, either a+ b or a b+

  • In any testcross, the recombo freq CANNOT exceed 50%

    • is expected for unliked genes, only linked genes will be less than 50%

• Genes can be unlinked if on different chromosomes, or far apart fon the same chromosome

Three-point testcross

• (parent 1) cross triple heterozygote with a (parent 2) triple homozygous recessive

• Results: classes 1 and 2 are parental phenotypes with no crossing over

• other classes are recombinant—possibly one crossover or a double crossover

• double crossover: two crossovers, one between each pair of linked genes

• In general, double crossovers are more rare than single; doubles occur at lowest freq

• p r and j must be arranged in a way that the center gene changes from parental claass to classes 7 & 8

• Gene order = p j r (or r j p) to account for double crossovers

• p — j distance = added progeny for classes 3 and 4 + 7 and 8 and divide by total progeny

• j — r distance = classes 5 and 6 + 7 and 8; divide by total progeny

• Recombo is estimated for regions I and II and then try to draw a gene map, showing distances between genes p, j, r

• (52 + 46) + (4 + 2)/500 = 0.208 ~ 20.8%

• ***will always be the bottom most numbers on table = smallest # = rare

• which allele differs from the others on the table? those are the numbers you will calc