Download Lab #4 - Mendelian Genetics

Genetics Lab #4: Review of 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 real 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 labled 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 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 three new 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,
should you choose to accept it, is to identify the genotype of each cat. Before you start,
decide on a strategy for solving this problem.
Cat
Genotype
1
2
3
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: Dihybrid cross
Some breeding experiments require that the experimenter follow more than one trait in a
cross. A dihybrid cross involves analyzing 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.
To make the cats, select the following:
Male or Female (choose one)
Has a tail
Not all-white
No white areas
Blotched or Mackerel (choose one)
Orange or Cream (choose appropriate color)
Once you generate your cats, check their phenotype. Note the difference in the striping
pattern between blotched and mackerel. What is the genotype of each cat?
2. Mate the two cats to get an F1 litter. Keep mating these original parents until you get
at least two orange/mackerel cats in the F1 generation.
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 below, use the Forkline 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)
Diagram the Forkline for the Dihybrid cross in the space below
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
Lab #4: Quiz (due February 28, 2008)
1. From which of the following crosses could offspring be produced that have a genotype
different from either parent?
Cross 1:
Cross 2:
Cross 3:
Cross 4:
aa x aa
AA x aa
Aa x aa
Aa x Aa
2. For exercise #2, 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.
3. The gametes produced by parent #1 all carried the F allele. The gametes produced by
parent #2 carried either the F allele or the f allele. What is the genotype of each parent?
4. Consider the cross of two heterozygous parents: Dd x Dd
i) What is the chance that the right parent will produce a gamete carrying the D allele?
ii) What is the chance that the left parent will produce a gamete carrying the d allele?
iii) What is the chance of an offspring of genotype dd?
iv) What is the chance of an offspring of genotype DD?
v) What is the chance of an offspring of genotype Dd?
5. Flower color in pea plants is controlled by a single gene with two alleles, designated
“P” and “p”. The “P” allele is dominant and determines the purple flower color
phenotype and the “p” allele is recessive and determines the white flower color
phenotype. Seed shape is controlled by a single gene with two alleles, designated “R” and
“r”. The “R” allele is dominant and determines the round seed shape phenotype and the
“r” allele is recessive and determines the wrinkled seed shape phenotype. Consider a
cross of two pea plants, one with purple flowers and round seeds and one with purple
flowers and wrinkled seeds. In genotypic terms the cross is: Pp/RR x Pp/rr. (show
Punnet squares to support your answer)
a) Out of 1000 progeny from this cross, how many plants do you expect to have white
flowers and wrinkled seeds?
b) Out of 1000 progeny from this cross, how many plants do you expect to have white
flowers and round seeds?
c) What is the chance of having an F2 plant with the genotype of pp/Rr
5. What was the genotype of your P1 orange/mackerel cat in exercise #3? What is your
evidence for assigning this genotype?