TETRAD ANALYSIS IN FUNGI Image removed to due to copyright considerations. THE PRODUCTS OF A SINGLE MEIOSIS ARE PACKAGED IN A SAC (ASCUS)AAaaTHE PRODUCTS OF A SINGLE MEIOSIS A a Haploid Diploid 1st division Meiosis X a A A a a A Image removed to due to copyright considerations. A a a AMendel 1. Segregation: Equal numbers of A and a •The phenotype resulting from a mutation in a single gene will segregate exactly 2A : 2a. •Question in Tetradspeak: Does the phenotype segregate2:2? Yes. A and a are alleles of a single gene.HOW DO WE KNOW WHETHER TWO GENES ARE LINKED? baX BA A A a a B B b b A B A B a b a b BBbbA A a a or A b A b a B a B Parental Ditype (PD) Non-Parental Ditype (NPD)How do you determine linkage? You cross AB x ab and find in 100 tetrads: 48 PD 52 NPD Are A and B linked?Mendel 1. Segregation: Equal numbers of A and a •The phenotype resulting from a mutation in a single gene will segregate exactly 2A : 2a. •Question in Tetradspeak: Does the phenotype segregate 2:2? Yes. A and a are alleles of a single gene. 2. Independent Assortment (linkage): AB x ab •Tetradspeak: Are PD = NPD Yes No Two genes are unlinked Two genes are linkedIndependent Assortment (linkage): AB x ab •Tetradspeak: Are PD = NPD Two genes are unlinked Two genes are linkedComplete Linkage of Two Genes AB X ab AB AB ab ab AB AB ab ab Parental ditype AB AB ab abThe Products of a Single Crossover A B a b X A B AB Parental AB Ab Recombinant a b aB Recombinant ab ab Parental X TETRATYPE2. Independent Assortment (linkage): AB x ab •Tetradspeak: Are PD = NPD Yes No Two genes are unlinked Two genes are linked TETRATYPE? Yes NO 100% linked Map Distance = 1/2 TTcentiMorgans TOTALHow do you determine linkage? You cross AB x ab and find in 100 tetrads: 90 PD 10 TT Are they linked? 90 PD = 90 X4 = 360 Non-Recombinant Progeny 10 TT = 10 x 2 = 20 Recombinant Progeny MD in centimorgans = Recombinants X 100 Total tetrads MD = 20/400= 5 centiMorgans Map Distance centiMorgans = 1/2 TT TOTALLinkage AB x ab Are PD = NPD? Yes No Two genes are unlinked Two genes are linked TETRATYPE? Yes NO 100% linked Map Distance = 1/2 TT TOTAL WHAT ABOUT DOUBLE CROSSOVERS?DOUBLE CROSSOVERS A B A BAB AB AB AB aA B Ab TTPD a b ab a b abXX X ab ab b aB A B AaA B Ab Ab AB AB B Ab NPDX aB TT aX b aBa b ab b aB ab 1PD : 1NPD :2TTCROSSING OVER OCCURS AT THE 4 STRAND STAGE A B A B a b a b XX A B A B a b a b X BAB AB BAB Ab TTPD ab b ab AAaa X ab b aB A B Ab Ab Aa BAB Ab NPD aB TT aX b aB ab b aBLinkage AB x ab Are PD = NPD? Yes No Two genes are linked TETRATYPE? Two genes are unlinked Yes NO Map Distance = 1/2 TT 100% linked TOTAL NPD? NO YES SINGLE DOUBLE MD = 1/2 TT + 3NPD TOTALESTIMATION OF DOUBLE CROSSOVERS Singles = TT Doubles = 1PD :2TT: 1NPD Single 1/2 (TT-2NPD) Doubles 4NPD MD = 1/2(TT-2NPD) + 4NPD Total SINGLE DOUBLE MD = 1/2 TT + 3NPD TOTALHow do you determine linkage? You cross AB x ab and find in 100 tetrads: 70 PD 20TT 10NPD Count crossover gametes = 40 + 4 (10) X 100 = 20 cM 400 Analyze tetrads = 10 + 30 X 100 = 40 cM 100 If we just count crossover gametes we would underestimate the distance.THE THREE TYPES OF TETRADS Parental Non-Parental Tetratype A B A bA B A B A bA b a b a Ba B a b a Ba b NO ONE TETRAD TYPE IS SUFFICIENT. IT IS THE RELATIONSHIP THAT TELLS
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