Lab Mendelian Genetics-Exploring Genetic Probability
-Revisiting Mendel’s Observations
CLASS COPY
Purpose:
Students will
1. Learn that probability is strongly related to genetic outcomes.
2. Determine whether probability supports or does not support the 19th century data and conclusions of
Gregor Mendel.
3. Be introduced to the probability of crossing two traits at a time, known as dihybrid crosses.
Vocabulary:
Gene
Allele
Trait
Gamete
Sperm
Egg
Heterozygous
Homozygous
Probability
Law of Segregation
Law of Independent
Assortment
Monohybrid
Dihybrid
Zygote
Fertilization
Punnett Square
Background:
When a coin is tossed, it can only one land of two ways-either heads up or tails up. This is similar to when an
organism makes gametes. Since an organism receives 2 alleles for a gene , one from the mother of the
gamete maker and one from the father of
the gamete maker, only one of these
alleles will be transferred to a single
gamete (sperm or egg)-not both.
If the gamete-making organism is a
heterozygous (ex. Aa), the resulting
gametes will either have the A
(dominant) or the a (recessive) allele.
Mathematically, this situation is
described as that the probability of
getting one of the alleles (or side of the
coin) over the other is 50%, 0.5, ½ or a
1:1 ratio. Mendal summarized this
finding in his “Law of Segregation.”
Part 1: Monohybrid Crosses
In this Laboratory Activity, each side of a coin will represent an allele for a trait. Two coins will be tossed, one
representing the allele in the sperm and one representing the allele in the egg. The results of flipping both
coins will represent the resulting genotype of the zygote created by the union of the egg and sperm
(fertilization). Remember that the zygote is the first somatic cell of a new organism. From this genotype, the
phenotype can be determined.
The results of flipping these two coins will be used to mimic the genotypic and phenotypic outcomes recorded
by Mendel for a single trait in a hybrid cross, specifically the mating of two heterozygous parents.
In pea-plant flowers, the color purple (P) is dominant over white (p). A heads-heads combination will
represent a homozygous dominant outcome (purple flowers). A heads-tails combination will stand for a
heterozygous outcome (purple flowers). A tails-tails combination will represent a homozygous-recessive
outcome (white flower).
Materials:
2 Pennies
2 Nickels
Investigative Questions:
What is the effect of crossing 2 purple-flowered heterozygous parents on the resulting genotypes of the
offspring?
Hypothesis:
Write a hypothesis for the investigative question and use a Punnett Square and the genotypic ratios for this
cross as the “because” part of the hypothesis. Mendel observed this outcome many times during his testing.
The results were repeated with great precision.
Procedure:
1. Copy the following data table onto your piece of paper.
2. Label the top box in the left-most column of your data table with the phenotype that results from a PP
genotype
3. Label the second box in the left-most column of your data table with the phenotype that results from
a Pp genotype.
4. Label the third box in the left-most column of your data table with the phenotype that results from a
pp genotype.
5. Record the “expected probability” in the appropriate column of the table.
6. Using 2 pennies, complete 100 simultaneous flips. Heads represents P and tails respresnts p
7. Record your results as appropriate tally marks in the appropriate boxes under “Tally.”
8. When you finish, divide the results of each genotype (under “Tally”) by the total of tosses (100) to
obtain the “Experimental Outcome.”
Data Table A-Monohybrid Cross
Phenotype
Coin Combination
PENNY
PENNY
HEADS
HEADS
GENOTYPE(S)
PENNY
PENNY TAILS
HEADS
GENOTYPE(S)
PENNY TAILS
PENNY TAILS
GENOTYPE(S)
Tally
Expected
Probability
Experimental
Outcome
Lab Mendelian Genetics-Exploring Genetic Probability
-Revisiting Mendel’s Observations
Part 2: Dihybrid Crosses
CLASS
COPY
We will use four coins (two pennies and two nickels) to
test the genotype and phenotype ratio which would result
from a mating between parents who are heterozygous for
each of two different traits. We call this a dihybrid cross.
We will repeat the experiment in Part 1, this time
completing 200 coin flips. Because we are looking at 2
traits, we will need different coins for each parent. One
type of coin (penny) will represent one of the 2 traits
contributed by the parent. The other type of coin (nickel)
will represent the second trait contributed by that parent.
There are 2 sides to each coin and 2 possible outcomes for
each trait since each heterozygous parent carries 2
different alleles for each trait.
Do the results on the penny affect the results on the
nickel? No, they flip independently from each other. In
the same way, the genes for the two traits assort
independently from each other into gametes. Mendel called this the “Law of Independent Assortment. “
Any 1 of 4 possible combinations may therefore turn up in any single gamete (sperm cell or egg cell). There
are, therefore, 16 possible outcomes when the egg and sperm eventually meet.
Prelab Questions: As a class, let’s work on the pre-lab questions
1. Use your notes to explain the Law of Independent Assortment in your own words.
2. In your booklet write down the possible gamete combinations for both the mother and father if both
parents are heterozygous for brown eyes and heterozygous for the recessive trait for spinal muscular
atrophy (SMA-a rare genetic disorder).
 Copy the Punnett Square below into your journal and fill it out.
Father (BbNn)
BN
Mother (BbNn)
Bn
bN
B = brown eyes
b = blue eyes
bn
BN
N = normal Central Nervous System
n = Spinal Muscular Atrophy
Bn
a. What is the chance of having a
child with blue eyes?
bN
b. What are the odds of having a
child with blue eyes and SMA?
bn
c. What are the odds of having a
child with brown eyes and SMA?
Investigation:
In Pea Plants,
 Rough seed shape (R) is dominant over smooth seed shape (r).
 Yellow seeds (Y) are dominant over white seeds (y).
Investigative Questions:
What is the effect of crossing 2 rough seed shape, yellow seeded heterozygous parents on the resulting
genotypes of the offspring?
Hypothesis:
Write a hypothesis for the investigative question and use a Punnett Square and the genotypic ratios for this
cross as the “because” part of the hypothesis. Mendel observed this outcome many times during his testing.
The results were repeated with great precision.
Procedure:
1. Choose one person to be the Director and one person to be the data collector.
2. Determine the 4 possible gametes produced by either parent.
3. Create a Punnett square to show the possible offspring from such a cross.
4. Determine the possible seed shape and color of all offspring whose parents are each heterozygous for
the two traits.
5. Copy the following data table into your lab notebook.
6. Label the heads side of both pennies “R” and the tails “r.”
7. Label the heads side of both nickels “Y” and the tails “y.”
8. Label the second column of the table (below coin combination) with the correct genotypes.
9. Label each box in the first column of the data table “Phenotype” with the appropriate phenotype.
10. From the outcomes in your Punnett Square, enter the “expected probability” for each genotype. This
is done by counting up the boxes in the Punnett Square that match each genotype. Remember that
some of the alleles in your data table are produced in more than one way (ex. Heads-tails is the same
as tails-heads). Then take the number of Punnett square boxes that match one genotype and divide by
16 to get the probability of that outcome. (Number of Squares for a Genotype/16 = Probability)
11. Toss both coins 200 times.
12. Record your results as appropriate hash-marks in the appropriate boxes under “Tally.”
13. When finished, divide the results of each genotype (under “Tally”) by the total of tosses (200) to obtain
the “Experimental Genotypic Outcome” and figure out the “Experimental Phenotypic Outcome.”
Conclusion: In a paragraph, answer the following questions. Use your Conclusion Rubric to help you!
1. What conclusion do you draw from this experiment?
 Answer the Investigative Question
 Or, state whether the hypothesis was supporter/not supported, and restate the hypothesis.
2. What data supports this conclusion?
 Use numbers (either percentage or fractions) for each genotype
3. How do those numbers prove your conclusion in #1?
 Link the percentages you found to the ones you predicted!
4. What is a scientific explanation for your findings?
Data Table B-Dihybrid Cross
Phenotype
Coin Combination
Rough,
yellow seeds
PENNY
PENNY
HEADS
HEADS
GENOTYPE(S)
RR only
PENNY
PENNY
HEADS
HEADS
GENOTYPE(S)
RR only
NICKEL
NICKEL
HEADS
HEADS
GENOTYPE(S)
YY only
NICKEL
NICKEL
HEADS
TAILS
GENOTYPE(S)
Yy or yY
PENNY
PENNY
HEADS
TAILS
GENOTYPE(S)
NICKEL
NICKEL
HEADS
TAILS
GENOTYPE(S)
PENNY
PENNY
HEADS
TAILS
GENOTYPE(S)
NICKEL
NICKEL
HEADS
HEADS
GENOTYPE(S)
PENNY
PENNY
HEADS
HEADS
GENOTYPE(S)
NICKEL
NICKEL
TAILS
TAILS
GENOTYPE(S)
PENNY
PENNY
HEADS
TAILS
GENOTYPE(S)
NICKEL
NICKEL
TAILS
TAILS
GENOTYPE(S)
PENNY
PENNY
TAILS
TAILS
GENOTYPE(S)
NICKEL
NICKEL
HEADS
HEADS
GENOTYPE(S)
PENNY
PENNY
TAILS
TAILS
GENOTYPE(S)
NICKEL
NICKEL
HEADS
TAILS
GENOTYPE(S)
PENNY
PENNY
TAILS
TAILS
GENOTYPE(S)
NICKEL
NICKEL
TAILS
TAILS
GENOTYPE(S)
Tally
Expected
Genotypic
Probability
1 ÷ 16 =
0.0625
Experimental
Genotypic
Probability
(outcome)
(tally) ÷ 200 =
Part 1:
Exploring Genetic Probability Conclusion: In a paragraph, answer the following questions. Use your
Conclusion Rubric to help you!
5. What conclusion do you draw from this experiment?
 Answer the Investigative Question
 Or, state whether the hypothesis was supporter/not supported, and restate the hypothesis.
6. What data supports this conclusion?
 Use numbers (either percentage or fractions) for each genotype
7. How do those numbers prove your conclusion in #1?
 Link the percentages you found to the ones you predicted!
8. What is a scientific explanation for your findings?