Part B: Basic Genetics Experiments
A Genetic Test for Allelism
The experiment A Dihybrid Cross uses
mutants with different phenotypes to
demonstrate dominant and recessive gene
expression. In the experiment Two
Genes/One Trait, introduces two
refinements of the gene concept: 1) mutants
in different genes can have the same
phenotype, and there can be different forms
of the same gene (alleles.) This experiment
combines the two previous ones to explore
the interactions of the two red mutants,
ade1 and ade2 in a dihybrid cross.. The
four types of gametes produced in the F1
generation of a cross heterozygous for both
of the red adenine genes (ADE1/ade1
ADE2/ade2) would be ADE1 ADE2, ADE1
ade2, ade1 ADE2, and ade1 ade2. By
making all possible crosses of these strains
in both mating types, we get a graphic
demonstration of the dominant and
recessive relationships of the red color vs
cream color, and adenine-dependence vs
adenine independence.
This cross also illustrates a development
in genetics that was essential for
understanding the nature of the gene at the
molecular level. This is a genetic test for
allelism. It demonstrates that the two
mutations ade1 and ade2 are not allelic in
spite of having the same phenotype.
Experiment:
In this experiment you will use a set of
haploid strains to produce the second (F2 )
generation of a cross between two different
red mutants. This set of haploid strains
represents the eight different haploid
genotypes (spores or gametes) that are
expected in equal frequencies from a dihybrid
cross with independent assortment.
Geneticists get tired of writing the same
symbols repeatedly and use a simple
shorthand. The normal allele of a gene can
often be represented by just a "+" without any
confusion. That way the genotype ADE1
ADE2 is just + +, ADE1 ade2 is + ade2, etc.
The table of genotypes and phenotypes of
these strains becomes simply:
| Genotype | Phenotype on:
YED | MV |
| + + | cream-colored | grows |
| ade1 + | red | doesn't grow |
| + ade2 | red | doesn't grow |
| ade1 ade2 | red | doesn't grow |
|
Of course, we need to have each of these
strains in both mating types, so that makes a
total of eight strains.
The two different media, YED and MV,
allow you to distinguish the mutant phenotype
from the normal phenotype, but does not let
you distinguish among the different mutant
strains.
Time Line:
1st Day: 50 min. Make mating
grid and
subculture
haploid strains
2nd Day: 20 min. Make mating
mixtures
3rd Day: 10 min. Replica plate
to MV
4th Day: 50 min. Record and analyze results
Figure 1
Materials:
- Mating-type a strains:
- HA0, HA1, HA2, HA12
- Mating-type
strains:
- HB0, HB1, HB2, HB12
- Petri plate of YED
Petri plate of MV
Sterile toothpicks
Mating grid and tape Making a Prediction:
Use the information in the mating grid on
page 36 to figure the genotype of each cross,
and to predict the phenotype of each cross.
Do you expect to get a 9:3:3:1 phenotype
ratio?
Testing your Prediction:
1. 1st Day: Set up mating plate.
Tape the mating grid to the bottom of a YED plate.
Using a fresh sterile toothpick for each strain make
spots of the haploid strains (gametes) across the top
and down the side of the plate.
Use the grid as a guide.
Incubate the plate overnight.
2. 2nd Day: Make crosses.
Using sterile toothpicks transfer and mix the haploid
strains in all possible combinations at the
intersections of the grid.
Remember to use a new toothpick for each different
strain and mixture.
Incubate the plate overnight.
3. 3rd Day: Use toothpicks or replica
plating equipment to replica plate the
YED plate onto MV.
Incubate the plate overnight.
4. 4th Day: Read the colors of the mixtures
from the YED plate and the growth
pattern from the MV plate.
Record your results in a table and then compare you
results with your predictions.
5. How did your results compare with your
predictions?(
Teacher Tips)
Return to Yeast
Experiments
Last updated Wednesday, 04-Dec-2002 14:58:28 CST