TETRAD ANALYSIS IN FUNGI Image removed to due to copyright considerations. THE PRODUCTS OF A SINGLE MEIOSIS ARE PACKAGED IN A SAC (ASCUS) A A a a THE 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 A 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. HOW DO WE KNOW WHETHER TWO GENES ARE LINKED ? b a X B A A A a a B B b b A B A B a b a b B B b b A 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 linked Independent Assortment (linkage): AB x ab ? Tetradspeak: Are PD = NPD Two genes are unlinked Two genes are linked Complete Linkage of Two Genes A B X a b AB AB ab ab AB A B ab a b Parental ditype AB AB ab ab The Products of a Single Crossover A B a b X A B AB Parental A B Ab Recombinant a b aB Recombinant a b ab Parental X TETRATYPE 2. 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 TT centiMorgans TOTAL How 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-Recombi nant 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 TOTAL Linkage 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 B AB AB A B AB a A B Ab TT PD a b ab a b ab X X X ab a b b aB A B Aa A B Ab Ab A B AB B Ab NPD X aB TT a X b aB a b ab b aB a b 1PD : 1NPD :2TT CROSSING OVER OCCURS AT THE 4 STRAND STAGE A B A B a b a b X X A B A B a b a b X B AB AB B AB Ab TT PD ab b ab A A a a X ab b aB A B Ab Ab Aa B AB Ab NPD aB TT a X b aB ab b aB Linkage 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 + 3NP D TOTAL ESTIMATION 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 TOTAL How 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 b A B A B A b A b a b a B a B a b a B a b NO ONE TETRAD TYPE IS SUFFICIENT. IT IS THE RELATIONSHIP THAT TELLS ALL