Download Lab #3: Review of Simple Mendelian Genetics

Lab #3: Review of Simple Mendelian Genetics
Objectives
In today’s lab you will explore some of the simpler principles of Mendelian genetics using a
computer program called CATLAB. By the end of this lab you should be able to demonstrate
and interpret the products of monohybrid and dihybrid crosses, design crosses to identify
dominant and recessive traits and design crosses to identify the genotype of individuals bearing a
dominant trait.
Introduction:
The genetic cross, or mating, is the bread-and-butter technique of genetic analysis. Mendel
derived his particulate theory of inheritance from many crosses involving pea plants. To this
day, in thousands of laboratories throughout the world, genetic crosses of experimental
organisms are still carried out on a daily basis to test hypotheses of gene inheritance and
function. In today’s lab you will use a computer program called CATLAB to simulate genetic
crosses of cats in a variety of experimental contexts. You will be approaching genetic analysis
the way professional geneticists do: you will only be able to see phenotypes but, through selected
crosses, you will be able to draw conclusions about genotypes.
Opening up CATLAB
1. Click on the Catlab icon.
2. Click “OK” on the title screen. This will display the simulation screen and the program’s
controls.
3. Note the tabs labeled Instructions, Introduction, Usage and Phenotype. Click on and read
each to familiarize yourself with CATLAB.
4. Note the menu bar across the top, left of the screen. It consists of File, Cat and Options.
You will be working primarily in the Cat menu.
The Cat menu includes:
Add - this allows you to create a cat with desired characteristics.
Set as Parent - this allows you to select a particular cat as a parent.
Set Litter Size - allows you to control litter size from 4 to 7.
Mate Cats - Produces a litter from the selected parents.
Display Phenotype - This shows you the phenotype of any selected.
Exercise #1: Single Gene Inheritance (monohybrid cross)
In cats, one gene controls the density (or intensity) of fur color. The different alleles of this gene
produce phenotypes in which the color is dense (black fur will be the example of dense coloring
used in this simulation) or dilute (gray fur will be the example of dilute coloring used in this
simulation). The purpose of this exercise is to explore the inheritance of this gene.
The alleles for color density are:
D for dense fur (black fur in this case)
d for dilute fur (gray fur in this case)
Based on these symbols, which is the dominant trait and which is the recessive trait?
The P1 cross:
Generate black and gray cats as follows: First click on add under the cat menu.
1. Black female (click on: female, has a tail, not all-white, no white areas, no tabby stripes,
black).
2. Gray male (click on: male, has a tail, not all-white, no white areas, no tabby stripes, gray).
Check the phenotype: First select a cat by clicking on the cat’s number. Then click on “Display
Phenotype” under the Cat menu.
What are the possible genotypes for the black cat?
What are the possible genotypes for the gray cat?
Make a prediction: What kinds of kittens do you expect to get from these parents?
Test Your Prediction: Select each cat as a parent. Set the litter size to a constant value of 5.
Test your prediction by mating cat #1 and cat #2. Obtain three litters (15 kittens total). This is
your F1 generation.
Record their colors in the space below:
Analyzing Your Results
Based on your results, what do you think the genotype of your black female is? Can you be
certain about this conclusion?
All gray kittens from this mating are genotype:
All black kittens from this mating are genotype:
The F1 x F1 cross:
Select a black female and a black male from the group of 15 kittens as your new parents. Re-set
the litter size to 6. DO NOT MATE THEM YET.
Record the genotype of the new mother and the new father.
The mother will produce eggs with allele: ________ or ________.
The father will produce sperm with allele: ________ or ________.
What is the biological process that results in the formation of gametes (sperm and egg cells)?
Make a prediction: Predict the possible genotypes and the phenotypic ratio among the offspring
of these two cats. Use a Punnett square to make your predictions. Write out all possible
genotypes and the corresponding phenotypes.
Test Your Prediction: Set the litter size to a constant value of 6.
Test your prediction by mating your two new parents. Obtain four litters (24 kittens total).
Record your results in the following table.
Color
Observed number
Black
Gray
Do your results fit your prediction? Why might your results differ from your prediction?
Exercise #2: Determining Genotype
In this exercise you will generate four black cats (follow the instructions for making black cats
provided above). The computer program will randomly assign genotypes to each cat. Since
black is dominant, the cats will be either DD or Dd. Your mission is to identify the genotype of
each cat. Before you start, decide on a strategy for solving this problem.
Cat
Genotype
1
2
3
4
Describe your strategy and present the results of an experiment that identified a cat with the DD
genotype and the results of an experiment that identified a cat with the Dd genotype.
Exercise #3: Identifying Dominant and Recessive Alleles
In cats, the tabby pattern (either blotched or mackerel) is controlled by a single gene. In this
exercise you will use CATLAB to determine which trait is dominant and which trait is recessive.
Instructions for generating blotched and mackerel tabby cats:
Select the following:
male or female (choose one)
Has a tail
Not all-white
No white areas
Blotched or Mackerel (choose one)
Orange
Once you generate your cats, check their phenotype. Note the difference in the striping pattern
between blotched and mackerel.
Formulate a hypothesis:
Design an experiment to test your hypothesis:
Results:
Do the results support your hypothesis? If not, state your new hypothesis.
Exercise #4: Dihybrid cross
Some breeding experiments require that the researcher follow more than one trait in a cross. A
dihybrid cross involves following the inheritance of two traits. In this exercise you will follow
the inheritance of the color density gene and the tabby striping pattern. These genes are located
on non-homologous chromosomes (i.e. they are unlinked) and will therefore obey Mendel’s law
of independent assortment.
The alleles for color density:
The alleles for tabby striping:
D for dense fur (orange fur in this case)
d for dilute fur (cream fur in this case)
T for mackerel striping
t for blotched striping
1. Generate an orange/mackerel cat and a cream/blotched cat. These cats will be your P1
generation.
What is the genotype of each cat?
2. Mate the two cats to get an F1 litter.
3. Select two orange/mackerel cats from the F1 generation as parents. DO NOT MATE THEM
YET. Answer the following questions.
What is the genotype of orange/mackerel cats in the F1 generation?
What is the genotype of the gametes produced by the F1 orange/mackerel kitten?
In the space overleaf, use the Punett square method to predict the F2 genotypes.
What is the predicted ratio of phenotypes among the F2?
If the F1 cats produced 50 kittens, how many of each phenotypic class do expect to get (round to
whole numbers)?
Punnett Square for Dihybrid cross:
4. Set the litter size to 7. Mate the F1 cats and produce 7 litters (a total of 49 kittens in the F2
generation).
Note the number of kittens with each phenotype. Do these numbers agree with your predictions?
orange/mackerel
orange/blotched
cream/mackerel
cream/blotched