7 03 Fall 2006 Lecture 9 Meiosis in yeast is particularly easy to study All four meiotic products are packaged together as haploid spores The spores can be dissected apart with a needle grown up into colonies and their genotypes analyzed 4 spores tetrad The ability to look at the genotypes of all four gametes gives us extra information about the meiosis that is not obtainable in diploid organisms eg mice flies and peas where only one of the meiotic products is selected at random Consider two linked genes in a cross A B x a b Meiosis with no crossovers Meiosis with one crossover mate sporulate AB diploid Tetrad ab PD Parental ditype T Tetratype If the genes are close together multiple crossovers in this region will be very rare and only PD and T type tetrads will be seen 7 03 Fall 2006 The overall aim of tetrad analysis is to express distance as a function of tetrad types First we apply the formula for genetic distance Distance in cM 100 x crossover gametes total gametes There are two crossover gametes in each T tetrad number of tetrads Distance in cM 100 x 2T 4 100 x T 2 this holds true only for tightly linked genes ie no double crossovers For genes that are far apart association of A with B is random There are six equally likely arrangements of B alleles with A alleles A A a a B B b b PD b b B B B b B b b B b B b B B b B b b B T T T T NPD Nonparental ditype Thus for unlinked loci PD T 1 4 NPD 1 Now we will see how to use tetrad analysis to make a more accurate mapping function that will take the hidden double crossovers into account First crossover Second crossover Type 2 3 2 3 2 3 2 3 2 3 1 4 1 3 2 4 PD NPD T T NPD is unique designator of double crossovers that we can use to keep track of other double crossovers that look like single crossovers or no crossovers 7 03 Fall 2006 All four classes are equally likely therefore Total double crossovers 4 NPD T tetrads that are doubles not singles 2 NPD To make a better mapping function we will take into account both single and double crossovers number of tetrads double crossovers crossover gametes 4 NPD 4 By counting all of the spores in these tetrads as crossover gametes we have a more accurate mapping function single crossovers Distance in cM 100 x 100 x 100 x T 2 NPD 2 2 T 2NPD 4 4NPD 4 number of tetrads T 2NPD 8 NPD 2 T 6 NPD 2 Example 100 tetrads give 75 PD 20 T 5 NPD Applying the formula for linkage in tetrads we get 100 x 20 6 5 25 cM 200 40 4 5 If we were just to count crossover gametes 100 x 15 cM 400 A comparison of the mapping functions for tetrad analysis and random gametes looks something like this Tetrad analysis 80 Genetic distance Random gametes 50 cM Physical distance The mapping function for tetrad analysis is pretty accurate for distances 40 cM
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