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