Lab 7: Mendelian Genetics Exercise

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Name(s):____________________________
Lab 7: Mendelian Genetics Exercise
Due Friday, March 21 by 5 pm
For this exercise you will replicate Gregor Mendel’s (1822–1884) experiments on
plant hybridization by crossing different varieties of the garden pea (Pisum sativum).
Mendel did not know the processes behind inherited behavior, such as DNA, but he
determined the rules by which characteristics are passed from generation to generation.
Objectives:
•
•
•
Learn how to determine the genotype of an organism.
Learn and apply Mendel’s Laws of Inheritance.
Design and execute genetic experiments by doing test crosses.
TERMS YOU MUST KNOW
Sexual reproduction
Heredity
Genetics
Genetic recombination
Diploid
Haploid
Chromosomes
Meiosis
Mitosis
Allele
Locus
Dominant
Recessive
F1 generation
F2 generation
Hybrid
Monohybrid
Dihybrid
Pure-breeding
Genotype
Strain
Zygosity
Homozygous
Heterozygous
Gene
Phenotype
Genotype
Test cross
Punnett square
Principle of
Segregation
Principle of
Independent
Assortment
First read Chapter 8 (Sexual Reproduction and Heredity) of Biology of Plants, these
genetic experiments pertain particularly to pages 159–163 in the 8th edition [7th ed. pp.
148–152]. You must also download and install the program StarGenetics, which is freely
available from: http://web.mit.edu/star/genetics/. You will also have to download and
unzip the file “Peas2014.sgz.zip” from the BIOL 153L website. Unzipping this file
should take less than five seconds, so if you are struggling with that please ask for help
before spending too much time on it.
There is a bug in the StarGenetics program, occasionally if you breed two parents enough
so as to produce about 200 offspring you will get the message “Parents can not mate!”
and you will not be able to add any more offspring, and even if you discard those and try
other matings you will often get the same error message. If this happens, just close the
program and restart it. It is usually not necessary to produce more than 200 offspring.
1. Start the program StarGenetics and open the file “Peas2014.sgz.” This file contains six
strains of pea plants: Green pea plant 1, Green pea plant 2, White flowered plant,
Unknown 1, Unknown 2, and Unknown 3. In the “Strains” window you can see a
cartoon of each of these four varieties and in the “Properties” window you can see the
description of seven different phenotypic characters for each: Plant height, Flower
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color, Flower Pod position; Pod color, Pod shape, Pea color, and Pea shape.
a. What phenotypic differences are there between Green pea plant 1 and Green pea plant
2?
b. What phenotypic differences are there between Green pea plant 1 and White flowered
plant?
2. Mendel spent years observing numerous generations of his strains of pea to make sure
that they were pure-breeding (also called true-breeding), meaning that subsequent
offspring of any crossing between two plants of a given strain always look the same.
Determine which, if any, of the six strains of pea available to you in the file
“Peas2014.sgz” are pure-breeding by crossing a strain with itself several times. To
do so, drag the image of a given strain from the “Strains” box to the “Mating site”
twice, once to the “ovum site” and again to the “sperm site.” Mate them several times
by repeatedly clicking the “Mate” button. Then click “Summary” to see what
proportion of the offspring is of each phenotype. All of the offspring you have
produced are of the first generation, also called the F1 generation.
a. Which strains are pure-breeding?
b. Which of the non-pure-breeding strains produces the fewest number of different
phenotypes, and how many different phenotypes is that?
3. Crosses that produce offspring that differ in only a single trait, such as flower color,
indicate that the parent plants differ by having different alleles of that gene. Crosses
between individuals that differ in only a single trait are called monohybrid crosses.
a. Which of the provided strains, when selfed, produce offspring that differ by only a
single trait?
b. By what trait do the offspring differ?
4. Diploid organisms have two sets of chromosomes (2n), one from each parent. In a
somewhat overly simplistic way, a gene for a given trait is located at a given site, also
called a locus (plural loci), on a chromosome. Because diploid organisms have two
sets of chromosomes, they can have either two identical forms of a given gene, or two
alternative forms. These forms are called alleles. In the case of these classical
Mendelian traits, for each of them one allele is dominant and the other is recessive.
Organisms with identical alleles of a given gene are said to be homozygous for that
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gene, and ones that have differing alleles are said to be heterozygous for that gene.
This is the basis for Mendel’s Principle of Segregation, which can be summarized:
•
There are alternative forms of genes, the units determining heritable
characteristics. These are known as alleles.
•
An organism inherits one allele from each parent.
•
A sperm or an egg carries only one allele for each characteristic.
•
A sperm and an egg fuse during fertilization/syngamy, and each brings its allele
for each character. When the alleles are different, one is fully expressed
(dominant) and the other is masked (recessive), at least when there is not
incomplete dominance.
Locate on Table 1 below the variable trait that you identified in the monohybrid cross
in number 3 above.
a. Mate a strain that results in a monohybrid cross so that you have between 100 and 200
F1 offspring. What percentages of these F1 offspring are of each of the varieties?
b. What genotype for this allele, crossed with itself, would produce this result?
c. Which allele for this trait is the dominant one and which is the recessive one?
d. What is the difference between pure-breeding and dominant?
e. What is the difference between zygosity (whether an individual is heterozygous or
homozygous) and pure-breeding?
f. Based solely on the Principle of Segregation, is it possible for an offspring to be
genetically identical to one or the other of its parents if they are genetically different
from each other?
5. Design and execute an experiment to determine the dominant and recessive conditions
for each of the seven traits present in the six plants available to you. Use a brief 1–3
word description for each condition (for example “purple”).
Trait
Plant height
Flower color
Flower Pod position
Dominant
Recessive
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Pod color
Pod shape
Pea color
Pea shape
6. The Punnett square (Table 2 below) is a diagram that is used to predict the outcome of
a particular cross or breeding experiment. It is named after Reginald Punnett (1875–
1967), a geneticist who devised it to determine the probability of a given offspring
having a particular genotype. StarGenetics provides a helpful Punnett square tool in
which you can select different parental genotypes to predict the genotype frequency in
offspring.
Fill in the following Punnett square for the trait that you identified in the monohybrid
cross in number 3 above.
Paternal (sperm donor) Maternal
(ovum
donor) ___
___
___
_____
_____
___
_____
_____
7. Cross the plant “Unknown 1” with itself and produce 100–200 F1 offspring.
a. How many different phenotypes do you recover?
b. How is it possible that you recover F1 offspring that have a different phenotype as
“Unknown 1”?
c. Combined with your knowledge of what the dominant and recessive conditions are,
what is the genotype of “Unknown 1”? Use the letter codes for each trait from Table 1
below.
In this example there is more than one variable trait, and the result of getting offspring in
which these variable traits vary independently illustrates Mendel’s Principle of
Independent Assortment.
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8. Design and execute an experiment to determine the full 7-gene genotype of each of the
six plants. Use the letter codes for each trait from Table 1 to fill in the table below.
Trait
Plant
height
Flower
color
Flower
Pod
position
Pod
color
Pod
shape
Pea
color
Pea
shape
Purple
Purple
flowered flowered White
plant 1
plant 2
flowered plant
Trait
Plant height
Flower color
Flower Pod position
Pod color
Pod shape
Pea color
Pea shape
Maternal
(ovum
donor) Unknown 1
Unknown 2
Unknown 3
Determinants
S, s
W, w
T, t
Y, y
P, p
U, u
R, r
Table 1. The seven variable phenotypic
traits in Mendel’s peas. The dominant
condition of a given trait is indicated
by an upper case letter and the
recessive condition by a lower case
letter. The letters are arbitrary, but
usually refer in some way to the trait,
for example “s” is for short and “S” is
for tall.
Paternal (sperm donor) Table 2. A sample Punnett square
depicting a cross between two parents
with genotype Aa. Approximately one
quarter of their offspring will have
genotype AA, ½ will have genotype
Aa, and ¼ will have genotype aa.
Given that allele A is dominant and
allele a is recessive, ¾ of the
offspring will have the dominant
phenotype and ¼ will have the
recessive phenotype for this trait.
A
a
A
AA
Aa
a
Aa
aa

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