Bio 2970 Lab 6: Tetrad Analysis Sarah VanVickle-Chavez

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

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

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

Ordered tetrads – Sordaria fimicola

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.

1st vs 2nd Division Segregation

Calculating Map Units

Mitosis Meiosis II Meiosis I Gray x Tan

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

t + g t + g + t + t + t + t + g g + g g MII MI t t + g + g t g + t g + t t t t g + g g + g

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

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 + ------ #1 + t + t + 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

_________ Meiosis II Meiosis I _____ _____ Gray x Tan _________ g + ------ #2 + t _________ _____ _____ 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. _________

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

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

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

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

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