Lab 3D- Sordaria
Lab 3D- Sordaria
BACKGROUND:
When homologous chromosomes pair
in prophase I of meiosis, they can exchange
parts, which is called crossing-over. When
chromosomes exchange parts, genetic
material is transferred from one chromosome
to another. This alters expected inheritance
patterns. For example, the fruit fly,
Drosophila, has four chromosomes.
Chromosome 2 carries a gene for eye color. The normal from of this gene (Pr, the “wild-type”)
produces flies with red eyes, but there is another form or “allele” of the gene (pr) that produces
purple eyes. On the same chromosome is another gene that affects wing form. The normal wildtype allele (Vg) produces normal wings while the alternate form (vg) produces vestigial wings,
which are smaller and useless for flight. Because these genes are located on the same
chromosome, they are considered “linked” and are inherited together. For example, if one
chromosome of a homologous pair carries the alleles for red eyes and normal wings and its
homologue carries the alleles for purple eyes and vestigial wings, we would expect half the
gametes to have the linked alleles for red eyes and normal wings and half the gametes to have
linked alleles for purple eyes and vestigial wings.
If crossover occurs between the locations (gene loci) of the genes for eye color and wing
type, there will be two new combinations of alleles in the gametes: red eyes with vestigial wings,
and purple eyes with normal wings.
The farther apart two gene loci are on a chromosome, the more likely it is that a crossover
will occur between them. By counting the frequency of crossover events between two gene loci,
geneticist can determine the relative distance between them. In this way, linkage maps have
been produced for many organisms, including Drosophila and even humans. In Activity D, you
will observe the results of crossing-over for a spore color gene. You will collect data on the
frequency of crossover for this gene and calculate the relative distance of the gene locus from its
centromere.
Lab 3D- Sordaria
Sordaria fimicola is a common species of ascomycete that grows on the dung of
herbivores. Eight spores (ascospores) are produced in an ascus. Many asci are grouped together
within a vase-shaped structure called a perithecium. Two nuclei within a developing ascus fuse
to produce a diploid (2n) nucleus. This diploid nucleus then undergoes meiosis, followed by a
mitotic division to produce eight ascospores in a linear series within the ascus.
To observe crossing over in Sordaria, one must make hybrids between wild type and
mutant strains of Sordaria. Wild type Sordaria have black ascospores (+). One mutant strain has
tan spore (tn). When mycelia of these two different strains come together and undergo meiosis,
the asci that develop will contain four black ascospores and four tan ascospores. The
arrangement of the spores directly reflects whether or not crossing-over has occurred. In Figure
5, no crossing-over has occurred. Figure 6 shows the results of crossing over between the
centromere of the chromosome and the gene for ascospores color.
Two homologous chromosomes line up at metaphase I of meiosis. The two chromatids
of one chromosome each carry the gene for tan spore color (tn) and the two chromatids of the
other chromosome carry the gene for wild type spore color (+).
The first meiotic division (MI) results in two cells, each containing just one type of spore
color gene (either tan or wild type). Therefore, segregation of these genes has occurred at the
first meiotic division (MI). Each cell is haploid at the end of meiosis I.
The second meiotic division (MII) results in four haploid cells, each with the haploid
number of chromosomes (1N). A mitotic division simply duplicates these cells, resulting in 8
spores. They are arranged in the 4:4 pattern.
In this example, crossing over has occurred in the region between the gene for spore color
and the centromere. The homologous chromosomes separate during meiosis I.
This time the MI results in two cells, each containing both genes (1 tan, 1wild type);
therefore, the genes for spore color have not yet segregated, although the cells are haploid.
Meiosis II (MII) results in segregation of the two types of genes for spore color. A mitotic
Lab 3D- Sordaria
division results in 8 spores arranged in the 2:2:2:2 or 2:4:2 pattern. Any one of these spore
arrangements would indicate that crossing over has occurred between the gene for spore coat
color and the centromere.
PROCEDURES:
1. Get 5 bags of ascospores (there should be at least 10 Asci in each bag)
2. Count the number of 4:4 Asci and the number of Asci showing crossover and record in
Table 1.
3. For each bag, calculate the total asci, % asci showing crossover, and gene to centromere
distance using the formulas in Table 1.
4. Add data from all 5 bags to get group data for number of Asci, then fill out the rest of the
row using formulas.
5. Obtain data from the entire class.
DATA: Table 1
Bag Number
Totals
Number of
4:4 Asci
Number of
Asci showing
crossover(2:4:2
or 2:2:2:2)
Total Asci
% Asci
showing
crossover (Asci
showing
crossover /
Total)
Gene to
Centromere
Distance (%
Asci showing
crossover /2)
Group Data
Class Data
ANALYSIS:
1. Explain the difference between how the 4:4 asci and the asci showing crossovers are
produced.
2. Why are there 8 spores in each acsi when normally meiosis produces 4 gametes?
3. Does crossing over increase or decrease genetic variation? Explain.
4. What are linked genes? Use an example from Drosophila to explain.
5. How do scientists discover distances between linked genes?
Lab 3D- Sordaria
6. A city creates a new lake for its water supply system. The lake is colonized by two water
plants, species A and species B. Species A reproduces exclusively by means of buds that
grow from rhizomes (runners). Species B reproduces by budding but also reproduces by
seeds, which involves sexual reproduction. Given that for both species n = 7, would you
expect to find more genetic variation in the population of species A or species B? Explain
your answer.
7. Suppose your class data show 397 MI asci and 0 MII asci. What would you conclude
from this?
EXTENSION:
The frequency of crossing over appears to be governed largely by the distance between
genes, or in this case between the gene for spore coat color and the centromere. The probability
of a crossover occurring between 2 particular genes on the same chromosome (linked genes)
increases as the distance between those genes becomes larger. The frequency of crossover,
therefore, appears to be directly proportional to the distance between genes.
A map unit is an arbitrary unit of measure used to describe relative distances between linked
genes. The number of map units between two genes or between a gene and the centromere is
equal to the percentage of recombinants. Customary units cannot be used because we cannot
directly visualize genes with the light microscope, so we use the map unit (mu).
Below are hypothetical results of test crossing GgNn (gray, normal) with ggnn (black,
vestigial) flies. Match the results below (reread the background information for assistance):
a. random assortment with no linkage
b. complete linkage of G with N and g with n
c. linkage as in b, but with some crossing over
_________ 217 gray normal, 196 black vestigial
_________ 138 gray normal, 22 gray vestigial, 47 black normal, 140 black vestigial
_________ 138 gray normal, 122 gray vestigial, 147 black normal, 140 black vestigial
For the set you determined involved crossing over, figure the cross over frequency. __________
Construct a gene map from the following crossover frequencies in tomatoes. Label the
chromosome with the name of the genes and the map units between genes. Remember: the
larger the %, the farther apart on the chromosome the genes are).
Example:
29% crossover between inflorescence gene and stem color gene
56% crossover between nipple tip gene and leafy branch gene
49% crossover between hairy branch gene and inflorescence gene
36% crossover between stem color gene and leafy branch gene
92% crossover between stem color gene and nipple tip gene
72% crossover between hairy branch gene and nipple tip gene
12 mu
15.5 mu
2 mu
1.5 mu