6 Ordered Tetrads Sordaria, Neurospora Each chromatid - one spore
7 Ordered Tetrads Sordaria, Neurospora Each cell from meiosis is represented by two identical spores
8 Tetrad Analysis Evaluate by direct examination or tetrad dissection Sordaria fimicola tan, black gray, white
9 Classify Ascus ( ordered, unordered ) - Spore Phenotypes Parents crossed: tan x gray t g zygote Parental ditype (PD) also t g t g, g t t g, etc. Nonparental ditype (NPD) also b w w b, b w b w, etc. Tetratype (TT) also b w g t, b t w g, etc.
10 Determining Number of Genes Involved Parents crossed: tan x gray# genes for color?
16 Distance Between Gene and Centromere Only possible to determine with Ordered Tetrads MI segregation MI ascus MII segregation MII ascus No CO CO between Gene and Cen
17 Distance Between Gene and Centromere Only possible to determine with Ordered Tetrads Distance Gene to Cen = 1/2 (# MII asci) x 100 total
18 Distance Gene and CEN - Sample Problem Where did the crossovers occur? Distance: Aa to Cen? Bb to Cen?
19 Fungal Genetics - Sample Problem Classify PD, TT, NPD Linked? PD>>NPD? Distance: Kk - Ll Kk and Cen? Ll and Cen?
20 Determining Gene Order MII means CO has occurred between gene and centromere Relative frequency: Lowest DCO; Intermediate SCO
21 Determining Gene Order Possible OrdersCOs required for MII both genes 2 CO 1 CO
22 Determining Gene Order When MII for both is lowest frequency, genes are on opposite sides.
23 Determining Gene Order When MII for both is intermediate, genes are on the same side. Which gene is farthest away?
24 Determining Gene Order MII for both genes Intermediate frequency MI for inner, MII for outer Intermediate frequency
25 Determining Gene Order - Sample Problem Number of CO?
26 Ordered Tetrad Analysis - Sample Problem 1. Classify as PD, NPD, TT. 2. Classify as MI, MII for each gene. 3. Determine linkage, gene order. 4. Calculate distances between genes, each gene and centromere.
27 Ordered Tetrad Analysis - Tracking Crossovers Where did crossovers occur?
28 Mechanism - Homologous Recombination Point to point pairing, breakage and rejoining How does this occur?
29 Mechanism - Homologous Recombination Model must explain: 1) Conversion Asci 2) CO intermediates observed
30 Holliday Model - Homologous Recombination First step, nicks in backbones of two ds DNAs