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A tetrad:

BIO341F 2014 Figures for: Topic Two “ Tetrad Analysis “ Several figures are derived from Chapters 3 & 4, Griffiths et al. (8th Edition). It is best to view this this file in Power Point “ presentation mode ” so that the animations will work! James B. Anderson. A tetrad:.

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A tetrad:

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  1. BIO341F2014Figures for:Topic Two“Tetrad Analysis “Several figures are derived from Chapters 3 & 4, Griffiths et al. (8th Edition).It is best to view this this file in Power Point “presentation mode” so that the animations will work!James B. Anderson

  2. A tetrad: • contains all four meiotic products, which can be physically separated from one another and genotyped.

  3. What questions can be answered by examining the genotypes of all four products of each of several meioses? • Is allele segregation 1:1 with each meiosis? • Is each recombination event reciprocal? (In the cross AB X ab, is the aB recombinant ALWAYS accompanied by the Ab recombinant?) • Which parts of which chromatid strands were involved in crossing over?

  4. There are two kinds of tetrads: • 1.Unordered

  5. AND: • 2. Ordered

  6. For ordered, let's consider the case of no crossover between a gene locus and it's centromere. In this case, segregation of alleles occurs at meiosis I. We will call this pattern "MI segregation".

  7. Let's look at another case in which there is a crossover between a heterozygous gene locus and its centromere. In this case, segregation of alleles occurs at meiosis II. We will call this pattern "MII segregation".

  8. A - MII segregation MII segregation pattern extends distal from the point of exchange. B - MI segregation

  9. a+ +b ++ = ab ab ++ +b a+ a+ ++ ++ a+ = ab +b +b ab Tetrad analysis • Step one: Determine individual genotypes within tetrads. • Step two: Pool tetrad types. Lump reversals of halves and, within halves, between spore pairs.

  10. Step three: List according to abundance. • Step four: Set up a table. • Register each pair of markers as N, P, or T. • Score each allele pair as 1st or 2nd division segregation. 1. How many genotypes are present? P N T ab a+ ab ab a+ a+ ++ +b +b ++ +b ++ all all all parental recomb- four possible geno- inant types present 2. Are these genotypes parental or recombinant?

  11. Consider the following cross of Neurospora crassa: me + a X + ad A

  12. me/ ad me/ A ad/ A ad A me P P P 1 1 1 Step five: Calculate recombination frequencies. gene-to-gene: RF = N + 0.5T/Total gene-to-centromere: RF = 0.5 MII/Total T T P 2 1 1 P T T 1 1 2 P T T 2 2 1 T P T 2 1 2 T T T 2 1 2

  13. 0.07 0.07 ad 0.02 0.06 me A 0.09 0.11 Step six: Construct the map Note that recombination frequencies are not additive!! • me/ad, RF = 0.5 X 9/61 = 0.07 • me/A, RF = 0.5 X 13/61 = 0.11 • ad/A, RF = 0.5 X 9/61 = 0.07 • me/cent, RF =0.5 X11/61 = 0.09 • ad/cent, RF = 0.5 X2/61 = 0.02 • A/cent, RF = 0.5 X7/61 = 0.06

  14. Step seven: Locate the crossovers (Note: ALWAYS used the solved map order) Parental (solved map order) Tetrad no. 1 me + 1 a me + a 2 + ad 3 A + ad A 4 me + a me + a + ad A + ad A NO CROSSOVERS

  15. Step seven (cont.) Locate the crossovers. Parental (solved map order) Tetrad no. 2 me + 1 a me + a 2 + ad 3 A + ad A 4 me + a + + a me ad A + ad A

  16. Step seven (cont.) Locate the crossovers. Parental (solved map order) Tetrad no. 3 me + 1 a me + a 2 + ad 3 A + ad A 4 me + a me + A + ad a + ad A

  17. Step seven (cont.) Locate the crossovers. Parental (solved map order) Tetrad no. 4 me + 1 a me + a 2 + ad 3 A + ad A 4 me + a + ad a me + A + ad A

  18. Step seven (cont.) Locate the crossovers. Parental (solved map order) Tetrad no. 5 me + 1 a me + a 2 + ad 3 A + ad A 4 me + a + + A me ad a + ad A

  19. Step seven (cont.) Locate the crossovers. Parental (solved map order) Tetrad no. 6 me + 1 a me + a 2 + ad 3 A + ad A 4 me + a + + A me ad A + ad a

  20. What is the relationship between the frequency of crossing over and the frequency of recombination? • Distances based on recombination frequencies are not additive. Why? • Some meioses with more than one crossover produce only parental genotypes. • “True map distance” is the mean number of crossovers per meiotic product within an interval X 100. This is NOT the same as recombination frequency, especially when the interval is large. • True map distance is expressed in centimorgans (cM)

  21. Mapping: relating recombination frequency to crossover frequency

  22. P T T N Meioses with one or more crossovers yield, on average 50% recombination

  23. The Poisson distribution: f(i) = (e-m mi )/i! i=0 i=1 i=2 i=n e-m e-mm (e-mm2)/2 (e-m mn)/n! Ave. 50% recombination, whenever n is one or more 0% recombination Ave. 50% recombination Ave. 50% recombination where i = the number crossovers per meiosis and m = the average number of crossovers per meiosis 1

  24. Let’s first worry about f(i) where i=0 • This is e-m • f(i) where i > 1 is: 1- e-m • Half of the meiotic products in these meioses are recombinant. • Therefore, RF = 0.5 (1- e-m ) • This assumes that crossovers are distributed randomly - i.e., no interference

  25. Crossover interference (cross abc X ABC) RF = 0.10 = (aB +Ab)/ total RF = 0.30 = (Bc +bC)/ total C A B What frequency of double crossovers would be expected? Answer 0.10 X 0.30 = 0.03 = (AbC+aBc)/total Example of NO interference: in 1000 random meiotic progeny, there were about 100 recombinants for AB, about 300 for BC. Of those, about 30 were double crossovers (AbC+aBc)

  26. Remember, this assumes NO INTERFERENCE. (What if there were TOTAL interference?) 1.0 -m R R = 0.5 (1 - e ) E C O M B I 0.5 N A T I O N 1.0 2.0 MEAN NO. CROSSOVERS/MEIOSIS (m) 0.5 1.0 MEAN NO. CROSSOVERS/MEIOTIC PRODUCT (x) 100 50 "TRUE" MAP DISTANCE (cM)

  27. Correction for double crossovers • Map distance = 0.5(SCO + 2DCO) • SCO = T-2N • DCO = 4N • Map distance = 0.5(T-2N+8N) • Map distance = 0.5 (T+6N)

  28. Summary of Tetrad Analysis and Mapping • Alleles segregate 1:1 and recombination is reciprocal within individual meioses. • We can use tetrad types for mapping. • For individual tetrads, we can locate the crossovers that occurred in a meiosis. • We can use tetrad analysis to relate crossover frequency to recombination frequency.

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