Bio 2970 Lab 6: Tetrad Analysis

1. Bio 2970 Lab 6: Tetrad Analysis Sarah VanVickle-Chavez

2. Tetrad Analysis • In some species of fungi, each meiotic tetrad is contained in a sac-like structure, called an ascus • Each product of meiosis is an ascospore, and all of the ascospores formed from one meiotic cell remain together in the ascus • Several features of ascus-producing organisms are especially useful for genetic analysis: • They are haploid, so the genotype is expressed directly in the phenotype • They produce very large numbers of progeny • Their life cycles tend to be short

3. Tetrad Analysis • In tetrads when two pairs of alleles are segregating, three patterns of segregation are possible • parental ditype (PD) = two parental genotypes • nonparental ditype (NPD) = only recombinant combinations • tetratype (TT) = all four genotypes observed

4. Tetrad Analysis • When genes are unlinked, the parental ditype tetrads and the nonparental ditype tetrads are expected in equal frequencies: PD = NPD • Linkage is indicated when nonparental ditype tetrads appear with a much lower frequency than parental ditype tetrads: PD » NPD • Map distance between two genes that are sufficiently close that double and higher levels of crossing-over can be neglected, equals 1/2 x (Number TT / Total number of tetrads) x 100

7. Ordered tetrads – Sordaria fimicola

8. Two kinds of patterns appear among the loci on these chromosomes: • Patterns depend on whether there was a crossover between the locus and its centromere. • No crossover between the locus and its centromere – the allelic pattern is the same as the centromeric pattern (4:4), which is referred to as first-division segregation (FDS), because the alleles separate from each other at meiosis I. • If crossover has occurred between the locus and its centromere, patterns of a different type emerge (2:4:2 or 2:2:2:2), each of which is referred to as second-division segregation (SDS). • Because the spores are ordered, the centromeres always follow a first division segregation pattern. So, we can map the distance of a locus to its centromere.

9. 1st vs 2nd Division Segregation

10. Calculating Map Units

11. Mitosis Meiosis II Meiosis I Gray x Tan

12. Gray x Tan TAN Two haploid hyphae mate Nuclei fuse GRAY Dikaryon = 2n Zygote = 2n

13. t + t + t + t + t + t t + t g g + g g g + g + g + g MII MI t t + g + g t t t t g + g g + g

14. Meiosis I Meiosis II g t+ t + t + t t t + t + t + t + g g+ g + g + g + g g g g t+ t t + ------ #1 g + g g+ t t t t t g + g g + g g+ t

15. Example: When do alleles separate for #1 (Gray-Gray-Gray-Gray-Tan-Tan-Tan-Tan)? g t+ Meiosis II Meiosis I g t+ g t+ Gray x Tan g + g t+ g + g + + t + t ------ #1 + t g+ t g+ t g+ t **Look for when the g and g+ separate from each other, not when g + separates from t+ .** **Note g and t may separate during different times (e.g., g at MI and t at MII).** g+ t

16. _________ Meiosis II Meiosis I _____ _____ Gray x Tan _________ g + + t ------ #2 _________ _____ _____ How would you set up each of these crosses? Look where the alleles separate and draw arrows to indicate if it is Meiosis I or II. _________

17. _________ Meiosis II Meiosis I _____ _____ Gray x Tan _________ g + + t ------ #3 _________ _____ _____ _________

18. _________ Meiosis II Meiosis I _____ _____ Gray x Tan _________ g + + t ------ #4 _________ _____ _____ _________

19. _________ Meiosis II Meiosis I _____ _____ Gray x Tan _________ g + + t ------ #5 _________ _____ _____ _________

20. _________ Meiosis II Meiosis I _____ _____ Gray x Tan _________ g + + t ------ #6 _________ _____ _____ _________

21. _________ Meiosis II Meiosis I _____ _____ Gray x Tan _________ g + + t ------ #7 _________ _____ _____ _________