Sordaria Linkage
The Inheritance of Spore Color
Sordaria Life Cycle
I.
Spore color
• A. Natural spore color is black
• B. Produced by the action of two genes
• C. The alleles are represented as:
1. t+ for the wild type
2. t for the mutant (which by itself produces tan
3. g+ for the wild type
4. g for the mutant (which by itself produces gray.
5. if both mutants are present-the phenotype of the
ascus is clear
D. Possible genotypes and resultant phenotypes
•
•
•
•
1. recall that fungi normally exist in the haploid state
2. therefore no such thing as dominance and
recessiveness
3. g+t+ produces black
4. gt+ produces a gray spores
5. g+t produces tan spores
6. gt produces clear spores
7. you do not know if the genes collaborating for spore
color are on the same or different chromosomes
II.
Tan X Black A determination of cross over
frequency
• A. tg+ X t+g+ tan X black
• B. Since the second allele is the same in both
we can ignore the “g” allele for this exercise
• C. We will be trying to measure the percent
crossing over between the “t” gene and its
centromere
D.
Normally
• 1. notice parental types
are haploid
• 2. opposite mating hypha
would undergo
plasmogamy to produce
the dikaryotic stage
• 3. karyogamy follows
within the asci followed by
meiosis I and II
• 4. in this example
crossing over did not occur
• 5. no crossing over is
indicated by four spores of
one type followed by four
spores of the second
E.
If crossing over happens, spore arrangement
in the asci will be different than 4:4
• 1. Events are the
same initially as the
slide before
• 2. Now crossing over
occurs at prophase I
• 3. Sister chromatids
of the two
chromosomes are
now modified
• 4. Will result in a
different spore pattern
laid down in the asci
5.
Are any other spore patterns possible that would
indicate that crossing over has occurred?
• a. How would you get a “2:4:2” pattern?
6.
•
•
•
•
a.
b.
c.
d.
To calculate the percent crossing over
make a smash of the sordaria
count parental asci
recombinant asci
% crossover =
Typical Results for Tan X Black
Black X Tan
III. Linkage
• A. Recall that spore color
is due to two genes
• B. g+t+ gives you black
•
gt+ produces a gray
spores
g+t produces tan spores
gt produces clear
spores
C. Are the genes for spore
color on the same
chromosome or on different
chromosomes
D. The cross gray X tan
can give us the answer
E. Two possible alignments of the chromosomes at the
metaphase I plate if unlinked
•
•
•
•
•
•
1. What alleles will be
traveling together in the
products of meiosis in each
case?
2. What spore colors result?
3. What will be the
arrangement of spores in the
asci?
4. The arrangement at the left
will result in the parental
pattern.
5. The arrangement at the
right=recombinant
6. If the genes are unlinked
we will expect these two
classes of asci to be
produced in equal numbers
F. If the genes are linked on the same
chromosome, different results will occur
• 1. Notice that the genes
travel to the next
generation together
• 2. Most often you would
expect to find the tan and
gray ascospores in a 4:4
pattern
• 3. Recall that this is
called the parental
condition
• 4. How could you get the
CCCCBBBB pattern of
the previous slide?
F. Linked explanation continued
• 5. The t and g alleles must be
recombined on one chromosome
• 6. The t+ and g+ alleles must also
travel together
• 7. The only way for this to happen
is with a double cross-over event.
• 8. Recall that the recombinant
ascus has CCCCBBBB seen in the
ascus
• 9. Will this event be relatively rare?
• 10. What do you expect to find in
the counting of the asci from this
cross if the genes are linked?
IV. Tetratypes
• A. Other combinations of spore
phenotypes are possible in the
asci in the tan X gray cross
• B. They don’t shed any
evidence whether the genes
are linked or not.
• C. They do not provide
evidence of linkage and the
ability to do some thinking
about meiosis in Sordaria.
• D. Examine the case at the
right
• E. Just because the genes are
shown as being linked here,
does not mean that they are in
reality
F. Here is an example of
a tetratype without having
linkage come into play.
• 1. You should be able to explain
other spore patterns following the
type of reasoning seen at the
right.
• 2. Remember when you are
counting the tan X gray cross that
you are really interested in the
relative numbers of
• TTTTGGGG vs CCCCBBBB
• Parental vs. Recombinant
• The tetratypes are interesting but
they don’t answer the question of
linkage