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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 - PowerPoint PPT Presentation
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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
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, equals1/2 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
Gray x Tan
Mitosis
Meiosis II
Meiosis I
Gray x Tan
TAN
GRAY
Dikaryon = 2n
Two haploid hyphae mate Nuclei fuse
Zygote = 2n
t
g +
t +
g
t
g +
t
g +
t
g +
t
g
t +
g
t +
g
t +
g +
t +
g
MI MII
t
g +
t
g
t +
g
t +
g +
t
g +
t
g +
t
g +
t
g
t +
g
t +
g
t +
g +
t +
g
Meiosis I Meiosis II
t
g +
t
g
t +
g
t +
g+
t
g +
t +
g
g t+
g t+
g+ t
g+ t
------ #1
g t+
g t+
g+ t
g+ t
------ #1
g t+
g t+
g+ t
g+ t
g +
g +
+ t
+ t
g +
+ t
Gray x Tan
Meiosis II Meiosis I
**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).**
Example: When do alleles separate for #1 (Gray-Gray-Gray-Gray-Tan-Tan-Tan-Tan)?
_____
_____
_____
_____
g +
+ t
Gray x Tan
Meiosis II Meiosis I_________
_________
------ #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.
_________
_________
g +
+ t
Gray x Tan
Meiosis II Meiosis I
------ #3
_____
_____
_____
_____
_________
_________
_________
_________
g +
+ t
Gray x Tan
Meiosis II Meiosis I
------ #4
_____
_____
_____
_____
_________
_________
_________
_________
------ #5
g +
+ t
Gray x Tan
Meiosis II Meiosis I
_____
_____
_____
_____
_________
_________
_________
_________
------ #6
g +
+ t
Gray x Tan
Meiosis II Meiosis I
_____
_____
_____
_____
_________
_________
_________
_________
------ #7
g +
+ t
Gray x Tan
Meiosis II Meiosis I
_____
_____
_____
_____
_________
_________
_________
_________
