Download introduction to mendelian genetics

INTRODUCTION TO GENETICS USING TOBACCO
(Nicotiana tabacum) SEEDLINGS
By Dr. Susan Petro
Based on a lab by Dr. Elaine Winshell
Objectives
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To apply Mendel’s Law of Segregation
To use Punnett squares to predict the genotypic and phenotypic
outcomes of a cross
To review simple dominance, incomplete dominance and
polygenic inheritance
To determine the method of inheritance for chlorophyll
production in two strains of tobacco
To learn about null and alternate hypotheses
To use the Chi-square (X2) test to evaluate a hypothesis
Nicotiana tabacum
Introduction
The genes of most organisms exist in pairs with one gene occupying each of the
homologous chromosomes (except for the genes on sex chromosomes that are not
homologous). The genes may be identical or not. In fact, there may be many forms of a
gene in a population. These forms are called alleles. If the organism has identical alleles
of a gene on each of its homologous chromosomes, it is called homozygous; if the alleles
are different it is called heterozygous. When an organism produces its gametes by
meiosis, the alleles separate. This is the explanation for Mendel’s First Law - the Law of
Segregation. If the organism is heterozygotic, it produces two kinds of gametes; if
homozygotic, it produces only one kind. The male and female gametes combine at
fertilization. A random process unites the gametes in different combinations.
If one gene with two alleles affects a trait, you can determine the F2 generation of
a cross between two F1 heterozygotes by use of the Punnett square. For example, suppose
you have a heterozygote with the genotype Aa and you mate it with a second heterozygote
also Aa. (This is called a monohybrid cross.) The Punnett square that shows this cross is
diagrammed below:
male gametes
A
a
female gametes
A
a
AA
Aa
Aa
aa
Genotype Frequencies:
AA = 0.25
Aa = 0.50
aa = 0.25
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If you had two genes A and B each with two alleles affecting one trait (polygenic
inheritance) you would use a Punnett square to do a dihybrid cross to determine the F2
generation. See below.
Male
gametes
AB
Ab
aB
ab
Female gametes
Ab
aB
AABb
AaBB
AAbb
AaBb
AaBb
aaBB
Aabb
aaBb
AB
AABB
AABb
AaBB
AaBb
ab
AaBb
Aabb
aaBb
aabb
If you had three genes each with two alleles affecting a trait (also polygenic
inheritance) then you would do a Punnett square for a trihybrid cross with A, B and C
genes.
Purpose
In this experiment, you will try to determine the inheritance pattern for
chlorophyll production in two strains of tobacco. You have learned about a number of
types of inheritance in lecture – simple dominance, incomplete dominance, and polygenic
inheritance. (Codominance will be investigated using human blood types in this lab
period so it will not be a possibility for the tobacco inheritance.) Your task is to decide
which, if any, of these types of inheritance fits your data for Strain1 and Strain 2. You
will do this by determining the expected phenotypic and genotypic ratios of the F2
generation for the possible methods and comparing them to your data using the Chi
square test.
Read Comparing Count Data: Dealing with Variability in your lab manual in
Appendix C beginning at the bottom of the second column on page 449 and ending
on page 452. This will explain the null and alternate hypotheses and the chi square
test.
Procedure
One week prior to the laboratory each table will plant a packet of seeds of the F2
generation of Strain1 on a black agar plate. The seed packets are labeled Strain1.
Sprinkle the seeds carefully so you get an even distribution over the entire plate. Each
table will also plant on another black agar plate a packet of seeds of the F2 generation of
Strain 2. These seed packets are labeled Strain 2. Label your plates with your lab class
and table number (e.g. Tuesday PM Table 3) and which seeds are in each plate –Strain1
cross or Strain 2 cross.
The laboratory staff will germinate them for a week in the light at a temperature
between 15° - 22°C. If the week preceding this lab is spring break then the
laboratory staff will plant the tobacco seeds for you.
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The answers to the questions below are to be typed including the Punnett squares
and all tables. Handwritten work will not be accepted.
Question 1: What would your null and alternate hypotheses be for each of the three
possible types of inheritance for the two strains of tobacco?
Question 2: In order to make predictions based on your hypotheses you must determine
the expected phenotypic ratios of offspring in the F2 generation. In order to do this begin
by doing a Punnett square for each of the three types of inheritance. You may choose
whatever letters of the alphabet you like for the alleles. See lecture notes for conventional
way to write these i.e. when does one use capital letters and when does one use lower
case letters.
Question 3: After you have completed the three F2 Punnett squares above use the
examples given in lecture to help you determine what might be expected (e.g. how many
possible colors for polygenic inheritance involving two genes each with two alleles like
the wheat grain color example or how many possible colors in a case of incomplete
dominance (one gene with two alleles) like the four o’clock (Mirabilis jalapa) example or
how many colors in a case of simple dominance (one gene with two alleles) such as with
Mendel’s pea blossoms.
Number of colors (not the names of the colors) expected in:
Simple dominance
Incomplete dominance
Polygenic inheritance
Question 4: Now what would the expected ratios (again the numbers not the names) of
the colors be in the F2 generation of these types of inheritance?
Simple dominance
Incomplete dominance
Polygenic inheritance
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Step One After making your null and alternate hypotheses and predictions for F2
generations for each of the three types of inheritance above you will use the dissecting
microscope to classify the seedlings by color.
Question 5 - How many colors of seedlings do you see in Strain 1? How many colors in
Strain 2?
Question 6 - What terms would you use to describe the colors that you see e.g. dark
green, yellow, etc? In Strain #1? In Strain #2?
Step Two Once you have reached a consensus at your table list your colors in the table
below and then remove each seedling with a forceps as it is counted to prevent counting
the same seedling twice. Enter the experimental data in the tables below.
Results
There are five rows in the tables below, but you may not have observed five different
colors, you may find you have observed only two or three. You will definitely not have
observed more than five colors.
Table 1 Experimental data for F2 offspring of Strain 1 cross
Phenotype
Number observed
Total
Table 2 Experimental data for F2 offspring of Strain 2 cross
Phenotype
Number observed
Total
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Question 7: Based on the number of colors you found can you eliminate any of the three
possible types of inheritance for Strain 1? Which type(s) were you able to eliminate?
What about Strain 2?
To what type(s) of inheritance have you narrowed down your choice(s) for Strain 1?
Strain 2?
Write the null hypothesis (hypotheses) for your choice(s) for each strain
Null hypothesis (hypotheses) for Strain 1
Null hypothesis (hypotheses) Strain 2
Question 8: For the null hypothesis (hypotheses) you just made for Strain 1 and Strain 2
what number of each phenotype should you have expected to get based on the total
number of seedlings you counted using the ratios you determined with your Punnett
squares?
Strain 1
Strain 2
Use these answers to go ahead with the calculation of the Chi-square test of your
results.
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CALCULATION OF CHI-SQUARE (χ2)
Strain 1 Cross - Table data
Phenotype Observed (o) Expected (e)
Degrees of Freedom -
(o-e)
Degrees of Freedom -
(o-e)
Confidence Level -
(o-e)2
e
Total = χ2
Critical Value -
Confidence level -
Strain 2 Cross - Table data
Phenotype Observed (o) Expected (e)
(o-e)2
(o-e)2
(o-e)2
e
Total = χ2
Critical Value -
Question 9: Based on your Chi square data above should your final null hypothesis be
rejected or accepted for the method of inheritance of Strain 1? Why?
Question 10: Based on your Chi square data above should your final null hypothesis be
rejected or accepted for the method of inheritance of Strain 2? Why?
Last revised April 8, 2013
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