POST MENDELIAN GENETICS

EXTENDING MENDEL Thomas Hunt Morgan:

MORGAN & Drosophila Morgan Revealed Extensions of Mendel’s Rules:

MORGAN’S WORK WITH Drosophila Red Eyes In Flies Is Normal (wild) White Eyes Is Rare (mutation) Figure 10.12  Exercise Label the phenotype that is considered wild type. Label the phenotype that is a rare mutant.

MORGAN & Drosophila To Explore How Eye Color is Inherited in Flies Morgan crossed: Red-eyed Female White-eyed Male X

MORGAN & Drosophila Next, Morgan crossed F1: Red-eyed Female Red-eyed Male X

THE DISCOVERY OF SEX CHROMOSOMES Nettie Stevens: Observed differences in chromosomes between male & female beetles Chromosomes named X and Y: Half male gametes contain Y, other half X All female gametes contain X

THE DISCOVERY OF SEX CHROMOSOMES Nettie Stevens: Developed hypothesis about sex determination: Male is formed when Female is formed when X X Y

NORMAL SEX CHROMOSOMES A Male

Gamete Formation In a Male Fly BACK TO MORGAN… Morgan guessed that D. melanogaster (like T. molitor) had chromosomes that differ between sexes: Gamete Formation In a Male Fly sperm sperm

MORGAN & Drosophila The X Linked Hypothesis: Morgan hypothesized that the gene controlling eye color

PRACTICE SEX LINKED PROBLEM Cross a heterozygous red-eyed female with a red-eyed male R = red eyes, r = white eyes Genotype of female parent = Genotype of male parent =

PRACTICE SEX LINKED PROBLEM PARENT: XR / Xr Red eyed female crossed with Red eyed male R = red r = white PARENT: XR / Y ** hint: gametes along top, “whole people” on inside

MORGAN & Drosophila Morgan’s work on Drosophila provided evidence that: The X chromosome contains genes the Y doesn’t: Inheritance patterns of sex-linked genes vary between sexes Genes are located on chromosomes

GENE LINKAGE Linkage: Autosomal Genes: Sex-Linked Genes: Reside on the autosomal chromosomes In humans: genes located on chromosome # Sex-Linked Genes: Found on sex chromosomes In humans: genes found on #

MORGAN & LINKED GENES First examples of linked genes were found on X chromosome of Drosophila: Morgan established that eye color & body color are linked traits Both found on X chromosome of fruit fly

MORGAN & LINKED GENES Morgan re-evaluated Mendel’s Principle of Independent Assortment Morgan predicted: Linked genes should be transmitted together during gamete formation

LINKAGE HYPOTHESIS rG Rg r r R R G G g g r r R R G G g g R R r r g G G Hypothesis (Morgan): When two genes occur on ONE chromosome, INDEPENDENT ASSORTMENT DOES NOT OCCUR LINKAGE HYPOTHESIS White eyes A Female (2X chromo) Red eyes Body Color: Gray body = wild type (G) Yellow body = mutant (g) r r R R Gray body G G g g Yellow body Eye Color: Red = wild type (R) White = mutant (r) Meiosis I r r R R G G g g Meiosis II R R r r Gametes g G G g rG Rg ONLY 2 gamete types

GENETIC RECOMBINATION: PRODUCTION OF NEW COMBINATION OF TRAITS Linked genes can become unlinked through cross over r R G g G g Crossing over during meiosis I r r R R G g G g Meiosis II Figure: 10.15c Caption: (c) To explain his results, Morgan proposed that crossing over occurred in a small percentage of the F1 females during meiosis I. The recombinant chromosomes that resulted would produce the unusual phenotypes observed in F2 males. r r R R Gametes G g G g rG rg RG Rg Recombinant chromosomes 19

RECOMBINATION PROBABILITY The farther apart two genes, the higher the probability they will be separated during crossover:

RECOMBINATION PROBABILITY A & D are more likely than B & C to become separated (unlinked) B & C more likely to be inherited together (stay linked)

GENE MAPPING Maps of genes can be constructed from recombination data Linkage map: genetic map based on recombination frequencies Crossing Over

GENE MAPPING % recombinant gametes Reflects distance between 2 loci Crossing over rarely occurs between adjacent loci, recombinations are rare. 0 : Yellow body 0 : Yellow body 0 : Yellow body Gene 2 1.4 : White eyes 1.4 : White eyes Gene 3 Gene 4 Map units Gene 5 Gene 6 Gene 7 20 : Cut wings Gene 8 Crossing over almost always occurs between distant loci, recombinations are frequent. Gene 9 Gene 10 Gene 11 Gene 12 Figure: 10.16 Caption: (a) Morgan hypothesized that chromosomes are composed of genes. (b) If Morgan's proposal is true, then the frequency of recombination between two loci should be a function of the physical distance between them. (c) Recombinants between the yellow-body locus and the white-eye locus occur 1.4% of the time. Therefore, these loci are 1.4 map units apart on the chromosome. (d) Recombinants between the yellow-body locus and another X-linked gene called the cut-wing locus occur 20% of the time. But recombinants between the white-eye locus and the cut-wing locus occur just 18.6% of the time. Therefore, the loci must be arranged as shown here. Linkage map Chromosomes are composed of genes The physical distance between loci determines the frequency of crossing over Frequency of cross over can be used to map physical between loci

GENE MAPPING If % of recombinant gametes is high, 2 genes are assumed to be far apart on a chromosome Map Unit refers to distance between 2 loci Function of % recombinants