9th Grade
Prior Knowledge: Students will have covered the preceding lessons on inheritable traits, chromosomes
and cell division, and sexual reproduction.
Lesson – Mendelian Genetics and Complex Inheritance Patterns
Guiding Question: How do we represent the possible inheritance outcomes of a single trait for a
genetic cross via sexual reproduction and predict the probability of each of those outcomes? Ask the
students: If you cross a purple flower with a white flower, what color flower will you get?
Ideal answer: Use a Punnett square to model the possible gamete combinations and calculate the
probability for each genotype and phenotype.
Primary Outcome: SWBAT develop a conceptual model that represents the possible gamete
combinations of inheritance for a genetic cross via sexual reproduction.
Secondary Outcome: SWBAT use that conceptual model to represent the probability of possible
genotypes and phenotypes for inheritance of a single trait resulting from a genetic cross.
Potential Tertiary Outcome: SWBAT differentiate between the different patterns of inheritance.
Scientific Practices – Developing and Using Models
Crosscutting Concepts – Systems and System Models
Core Ideas – Inheritance of Traits
High School Content Expectations – B4.1e – Determine the genotype and phenotype of monohybrid
crosses using a Punnett square.
B4.1c – Differentiate between dominant, recessive, codominant, polygenic, and sex-linked traits.
Objectives:
Cognitive –
SWBAT reproduce a hybrid cross of a single trait in a Punnett square format.
SWBAT interpret a phenotype from a genotype.
SWBAT demonstrate ability to produce hybrid crosses with new parent types.
SWBAT analyze the probabilities of each phenotype for the possible offspring.
Affective –
SWBAT engage in the modeling of various patterns of inheritance.
SWBAT compare the different phenotypes resulting from various parental crosses.
Psychomotor –
SWBAT master the use of the Punnett square as an abstract representation of a reproductive event.
SWBAT adapt the use of the Punnett square to new patterns of inheritance.
SWBAT organize their own way to write out the results of a hybrid cross and describe the possible
offspring.
Student Misconceptions:
1. A dominant trait is more ‘fit’ in terms of natural selection
2. A dominant trait is more prevalent in the population
( Allchin, 2000)
Preparation/Materials Needed:
A set of chromosomes needs to be prepared for each student.
Toothpick Chromosome Set (16 of each):
Pea Plant Flower Color
Human ABO Blood Type
Human Color Vision
Purple
White
Protein
Protein
No
Normal
Red-Green
Y
Flowers
Flowers
A
B
Protein
X
Colorblind X (half)
Each set contains 128 chromosomes with 8 different alleles marked, 16 of each kind.
The chromosomes are toothpicks (preferably flat), pre-colored with markers or paint of various colors
indicating the 8 different alleles (see above).
The sets can be stored in individual bags.
Each student will also need a genetic cross diagram and a Punnett square model (attached), preferably
laminated.
Process:
Engage – The class will begin with students being presented a dilemma. They are told that two pea
plants with purple flowers were crossed and some of the resulting offspring had white flowers. They
are asked to propose how two plants with purple flowers could produce a plant with white flowers.
Explore – Students are given a set of toothpicks and a genetic cross diagram and are told to gather the
purple and white colored toothpicks. It is then explained that the toothpicks represent chromosomes
and the colored spot represents the gene for flower color. Some chromosomes have the allele for
purple flowers and some have the allele for white flowers on the gene. Alleles are different forms of
the same gene – in this case the gene for flower color.
Since they know that each parent will have a homologous pair of each chromosome and will
pass only one chromosome on to their offspring, the students are to set up a cross using the diagram
and toothpicks for two true-breeding purple flowered plants. They are told that true-breeding means
that the plant has the same alleles on both chromosomes, so both parent plants will have two purple
alleles. They are then walked through the cross, which is rather simple given that all of the gametes
contain alleles for purple flowers, and see that all of the possible resulting offspring will be truebreeding for purple as well. They are then asked to do the same for true-breeding white, and verify that
all of the possible resulting offspring will also be true-breeding white.
Students are told that the two alleles each plant possesses is called its genotype and that the
actual result –purple or white flowers – is called the phenotype. So in our first cross the genotypes
were all two purple alleles and the phenotypes were all purple flowers; in our second cross the
genotypes were all two white alleles and the phenotypes were all white flowers. These genotypes are
called homozygous because they have the same two alleles.
Now students are asked to cross a true-breeding plant with purple flowers with a truebreeding plant with white flowers. When accurately crossed, students will find that the result is that
all of the genotypes have one purple allele and one white allele. They are told that this is called a
heterozygous genotype because the two alleles are different. Students are then asked what they think
the phenotype is for these flowers. After they offer their predictions, the students are told that Gregor
Mendel found that when he did this cross, all of the offspring had purple flowers.
Explain – Students are told that Gregor Mendel proposed that the allele for purple flowers actually
overpowered the allele for white flowers. He called the purple form of the gene the dominant allele
and the white form the recessive allele. So, when the genotype is heterozygous, the phenotype is
always that of the dominant allele – in this case, purple flowers. This type of dominant-recessive
inheritance pattern is called Mendelian.
Extend – Now students are asked to perform a cross of two heterozygous plants. They will find that
the resulting genotypes for the possible offspring are not all the same: one is homozygous for purple
flowers, one is homozygous for white flowers, and two are heterozygous. They are to see that in this
case the resulting phenotypes would then be 3 with purple flowers and 1 with white. It is explained
that these results are actually displayed as probabilities, so with this cross there is actually a 75%
chance that any offspring will be purple and a 25% chance that any offspring will be white. Students
are then led through the same crosses with their toothpick chromosomes on the Punnet Square diagram
so that they can learn to use the Punnett square. Once they have mastered this, students are shown how
to use the Punnett square with letters representing the alleles, rather than toothpicks. In this case, they
would use P for the dominant purple allele and p for the recessive white allele.
Evaluate – Students will be asked to do a final cross of a heterozygous plant with a homozygous plant
with white flowers. The cross is to be done on paper as a formative assessment and the final genotypes
and phenotypes are to be described in percentages.
The process can then be repeated to introduce the more complex codominance pattern of inheritance
and the concept of multiple alleles with the ABO blood type chromosomes. In this pattern, A and B
blood type alleles are both dominant over the O type (no protein), but when the genotype is
heterozygous for A and B, the result is a pattern called codominance, as both types are expressed and
the phenotype is called AB blood type.
The process can be repeated a third time to introduce the concept of X-linked genes with the color
vision alleles on the sex chromosomes. The gene for color vision is only located on the X
chromosome, so the Y chromosome does not contain an allele for this gene. The allele for
colorblindness is recessive to the allele for color vision, but since males only receive one X
chromosome, they will express the phenotype for whichever allele they receive. Since females receive
two X chromosomes, only those with the homozygous genotype for colorblindness will have the
colorblind phenotype. X-linked alleles can be represented as XC for the dominant color vision and Xc
for the recessive colorblindness.
When the students are comfortable making Punnett squares without the use of toothpicks, the concept
of incomplete dominance can be explained and students can be led through a cross of snapdragon
plants which are either homozygous for red colored flowers (RR), homozygous for white colored
flowers (WW), or heterozygous, which results in pink colored flowers (RW).
It should also be explained that some traits are polygenic, meaning that they are expressed through the
combination of multiple genes, so the pattern is not as easily traced.
(Adapted from Bryant, R. J. (2003). Toothpick chromosomes: Simple manipulatives to help students
understand genetics. Science Scope, 26(7), 10-15.)
Assessment:
Informal – Students will be informally assessed as they progress through their crosses through teacher
observation and discussion.
Formal – Students will be formally assessed based on completion and accuracy of their independent
formative assessment at the evaluate phase for each inheritance pattern.
Inquiry Activity:
This activity is considered a level 2 inquiry activity because it requires the instructor to model a
scientific phenomena while allowing the students to engage in scientifically-oriented questioning,
examine evidence, and formulate explanations (Eick, Meadows, & Balkcom, 2005, p. 51).
Differentiations/Modifications/Adaptions:
Differentiation can be made requiring above grade level students to to complete a dihybrid (4x4)
Punnett square crossing parents with two known genotypes simultaneously, and allowing below grade
level students a fill-in-the-blank assessment worksheet.
Modifications and adaptations can be made as needed for students with specific requirements, such as
written procedures, recorded directions, allowances of extra time to complete assessments, and
grouping for peer support.
Justification of Approach:
The engagement activity proposes a dilemma to capture the students’ interest in the lesson and gives
purpose to the class.
The explore phase of this approach allows the students to perform the crosses themselves and model
the link between Punnett squares and the biological process they represent. Students often use Punnett
squares to obtain correct answers to genetic problems but fail but to understand that a Punnett square
represents two biological processes-gamete formations and fertilization.
Allowing the students to perform the crossings and observe the results, rather than just have the
concept explained to them, provides them with evidence that they can utilize in the formation of their
explanations. Requiring students to formulate their own explanations increases the likelihood that
learning is taking place and gives the students ownership of that learning. Inaccurate explanations
provide the instructor with a better sense of the students' thinking and what further support they need
for understanding the concept.
Extension of the lesson to a cross between two heterozygous plants allows the student to see that the
resulting genotypes for the possible offspring are not all the same: one is homozygous for purple
flowers, one is homozygous for white flowers, and two are heterozygous. They can then begin to
understand that in this case the resulting phenotypes would then be 3 with purple flowers and 1 with
white. Results are actually displayed as probabilities, so with this cross there is actually a 75% chance
that any offspring will be purple and a 25% chance that any offspring will be white. This will help to
address several of the misconceptions students have with regard to expression of the dominant trait.
Safety and Welfare:
The materials for this activity pose little or no risk to safety and health. Students should take care when
handling toothpicks which have sharp edges and can pose a potential hazard by poking the student.
Handouts: Attached
Genetic Cross Diagram
Male Parent
Female Parent
Gamete
Formation:
Male
Gametes
Fertilization:
Possible
Offspring:
Female
Gametes
Punnett Square
Female Gametes:
Male
Gametes:
Possible Offspring
References:
Allchin, D. 2000, Mending mendelism. Am. Biology Teacher 62:632-639.
Bryant, R. J. (2003). Toothpick chromosomes: Simple manipulatives to help students understand
genetics. Science Scope, 26(7), 10-15.)
Eick, C., Meadows, L., & Balkcom, R. (2005). Breaking into inquiry: Scaffolding supports beginning
efforts to implement inquiry in the classroom. The Science Teacher, 72(7), 49-53.