Complete Dominance
Student Activity
Name____________________________________
Class_____________________________________
Open the TI-Nspire document: complete_dominance.tns
1.2. An allele is a form of a gene. Alleles determine traits,
such as eye color, skin color, and risk for certain disorders and
diseases. Most offspring inherit pairs of alleles—one allele
from each parent. The specific alleles that make up the pair
are important to what trait is expressed as well as what can be
passed to the next generation.
1.3. Although the combinations of alleles can interact with
one another in different ways to express a trait, one of the
most fundamental interactions is known as complete dominance. With complete dominance, a dominant
allele is always expressed whenever present. Recessive alleles will not be expressed unless it is the only type
of allele present in the pair.
1.4-1.5. A genotype describes the specific pair of alleles an organism has in its genetic information. It is
usually described in one of three ways:



homozygous dominant – both alleles in the pair are dominant.
homozygous recessive – both alleles in the pair are recessive.
heterozygous – one of each allele is present (one dominant and one recessive).
Note: to help you remember the terms here’s what the parts of the words above mean:



“homo-” – same
“hetero-” – different
“-zygous” – form
Example: homozygous dominant
("same" - "form" - "dominant") means the
alleles are the same and both are dominant
1.6. A phenotype is the trait that is outwardly expressed in the organism. With complete dominance, a
dominant allele is always expressed whenever present. So, a dominant trait would be expressed as the
organism’s phenotype if it had a homozygous dominant or heterozygous genotype. The organism’s
phenotype would express the recessive trait if it had a homozygous recessive genotype.
1.7. Overview of this Activity. During this activity you will learn the following:


To predict possible outcomes of various monohybrid crosses.
To understand genotype, and how it relates to the expression of a phenotype.
From: This Session Contains MSG: Math, Science, and Genetics by Shawn Schlueter.
complete_dominance.tns: Page 1
Complete Dominance
Student Activity
Complete Dominance
2.1. Gregor Mendel described how different traits in pea plants could be bred in and out of offspring, and the
probabilities those traits would be expressed, through a
monohybrid cross.
Mono - "one" - a single trait
hybrid - "to mix" “to have multiple”
2.2. Use the sliders on the page to select a set of genotypes to
cross. A cross is a mating between two organisms. As you
change the sliders, look at the Punnett square and see how
different crosses result in different possible genotypes for the
offspring.
Questions:
2.3. Of the following, which cross produced the
greatest number of variations in genotypes?
o
o
o
o
BB
BB
Bb
Bb
x
x
x
x
BB
bb
Bb
BB
2.4. Which of the following crosses produce
offspring where all possible genotypes
match the parents? Circle all that apply.
o
o
o
o
homozygous recessive x homozygous
recessive
homozygous dominant x homozygous
dominant
heterozygous x heterozygous
homozygous dominant x homozygous
recessive
2.5. Complete the Punnett square for the cross:
Bb x bb
Bb
bb
Bb
bb
2.6. Complete a Punnett square for the cross of an
organism that is homozygous recessive, with one
that is heterozygous for the trait.
Bb
bb
Bb
bb
From: This Session Contains MSG: Math, Science, and Genetics by Shawn Schlueter.
complete_dominance.tns: Page 2
Complete Dominance
Student Activity
3.1. Let’s explore how different alleles affect the genotypes, and phenotypes of offspring in pea plants similar
to those studied by Gregor Mendel.
3.2. The dominant allele codes for a yellow seed color, and will be represented with an uppercase Y. The
recessive allele codes for a green seed color, and will be represented with a lowercase y. The presence of a
dominant allele will result in a yellow seed. The absence of a dominant allele results in a green seed color.
3.3. Activity. Select a specific cross to study. On the next page, use the arrows to select the genotypes to
cross, and then answer the questions using the data from the cross.
3.4. Use the sliders on the page to select your set of genotypes to cross. Record the cross below:
Cross: __________ x ___________ (student responses will vary)
3.5. Select a number of offspring. The handheld will randomly assign the cross’s possible genotypes to the
offspring. Review the graph of the genotypes’ distribution across the offspring. What happens when you
change the number of offspring?
The numbers of offspring with each genotype will change. Unless the number of offspring is very small, the
distributions of each genotype will remain similar in relation to one another.
3.6. Study the Punnett square with the expected percentages of each genotype. You can hover your cursor
over the pie chart to see the actual percentages of offspring with each genotype.
Questions
3.7. Is there a difference between your simulation and the number predicted by the Punnett's square? Is
this significant? Why might this be the case? There is a difference, as the percentages of genotypes are not
the same. The numbers from the simulation are determined at each individual cross, or mating, while the
predicted percentages are based upon the probability across an entire population.
3.8. Does increasing the number of offspring cause affect the difference between actual and expected
results and their similarity? As you increase the number of offspring the difference between the
experimental results and theoretical results become smaller.
3.9. Think-Pair-Share. Share your explanations with a nearby student. Is there anything you would change
about your explanation after sharing with another student? (Student answers will vary. Students will
probably notice where actual and expected results vary from student to student with each different cross.)
From: This Session Contains MSG: Math, Science, and Genetics by Shawn Schlueter.
complete_dominance.tns: Page 3
Complete Dominance
Student Activity
4.1. Genotype vs. Phenotype. At the beginning of this document we mentioned that a phenotype is the trait
that is outwardly expressed in the organism. However multiple genotypes can sometimes result in the same
phenotype.
4.2. Activity. Use the Punnett's Square on page 4.3 to look at various crosses. The pie graphs on 4.4 compare
the genotypes of the cross you selected to the phenotypes expressed in a random set of 50 offspring. Use 4.3
and 4.4 to answer the questions that follow.
Questions
4.5. In humans the photo sneeze reflex, when
suddenly exposed to light you sneeze, is a
dominant trait. After leaving a dark movie
theater, a family walks outside. The mother and
father sneeze, their son sneezes, but their
daughter doesn't. What are the possible
genotypes of the parents. Choose all that apply.
o
o
o
o
both heterozygous
one heterozygous
one homozygous dominant
one heterozygous, one homozygous
recessive
o both homozygous recessive
4.6. Dimples are a dominant phenotype and are
easiest to see when smiling. A mother smiles at
her son and you do not see dimples on her face.
The son smiles at her and you see his dimples.
Which of the following could be the father's
genotype? Choose all that apply.
o homozygous recessive
o homozygous dominant
o heterozygous
4.7. The dominant trait is for ear lobes to hang
free, a bit of lobe hanging down prior to the point
where the bottom of the ear attaches to the head.
With the recessive phenotype, the lobes are
attached directly to the head. In a family photo,
the husband has attached ear lobes while the
wife's lobes hang free. If they have children,
which of the following could be the probabilities
their children are born with attached ear lobes.
Choose all that apply.
o
o
o
o
0%
25%
50%
75%
4.8. Some people have hair on the second
(middle) joint of one or more of their fingers,
while others don't. Having hair at all means that
you have the dominant phenotype. Complete
absence of hair is recessive. A family has 3
children. 2 have hair on their fingers, 1 child does
not. What is the most probable genotypes for the
parents?
o
o
o
o
both homozygous dominant
both homozygous recessive
both heterozygous
one heterozygous, one homozygous
recessive
From: This Session Contains MSG: Math, Science, and Genetics by Shawn Schlueter.
complete_dominance.tns: Page 4