Solving Genetic Problems
Using Punnett Squares
SBI3U: Genetic Processes
Nushaye Henry & Jenny Kellar
July 16, 2012
Why Study Genetics?



To predict the likelihood of inheriting
particular traits.
To help plant and animal breeders in
developing varieties that have more desirable
qualities.
To help people explain patterns of inheritance
in family lines.
Preparing to Learn About
Punnet Squares

Students should have knowledge of Mendel’s
experiments, including an understanding of:






a monohybrid cross between true-breeding parents
the P (parent) generation
first filial (F1) generation
second filial (F2) generation
the Law of Segregation
the Law of Independent assortment
Preparing to Learn About
Punnett Squares

Students should also have an understanding
of the following vocabulary:








Gene
Allele
Genotype
Phenotype
Dominant
Recessive
Homozygous
Heterozygous
The Punnett Square




Technique invented by an early 20th century English
geneticist named Reginald Punnett.
One of the easiest ways to calculate the
mathematical probability of inheriting a specific trait.
A graphical way of determining the potential
genotypes of offspring, given the genotypes of their
parents.
It also shows us the odds of each of the
offspring genotypes occurring.
Using the Punnett Square


Setting up and using a Punnett square is
quite simple once you understand how it
works.
Begin by drawing a grid of perpendicular
lines:



Place the genotype of one parent across
the top and that of the other parent down
the left side.
Note that only one letter goes in each box for the
parents.
Example: if parent pea plant genotypes were YY
and yy respectively, the setup would be:


Fill in the boxes by copying the row and
column-head letters across or down into
the empty squares.
This gives us the predicted frequency of all of
the potential genotypes among the offspring
each time reproduction occurs.
Interpretation

In the previous example, 100% of the offspring
will be heterozygous (Yy).

Since the Y (yellow) allele is dominant over the y
(green) allele for pea plants, 100% of the Yy
offspring will have a yellow phenotype, as
Mendel observed in his breeding experiments.
Curriculum Expectations
By the end of this lesson sequence, students will be able to:

D2.1 Use appropriate terminology related to genetic processes,
including, but not limited to: haploid, diploid, spindle, synapsis,
gamete, zygote, heterozygous, homozygous, allele, plasmid,
trisomy, non-disjunction, and somatic cell

D2.3 Use the Punnett square method to solve basic genetics
problems involving monohybrid crosses, incomplete dominance,
co-dominance, dihybrid crosses and sex- linked genes

D2.4 Investigate through, lab inquiry or computer simulation,
monohybrid and dihybrid crosses and use Punnett square
method and probability rules to analyze the qualitative and
quantitative data and determine the parent genotype.
New Vocabulary
Throughout this lesson sequence, students will
learn and use the following:





Monohybrid cross
Dihybrid cross
Codominance
Incomplete Dominance
Sex Linkage

These definitions can be found in the accompanying
Presentation Summary
Lesson Sequence
Lesson1: Introduction to Punnett Squares
 Review phenotype, genotype, dominant and recessive alleles
 Use of “Get to Know Yourself” Power Point
 Use of the Mouse Genetics (One Trait) Gizmo
Lesson 2: Monohybrid Crosses continued/Introduction to
Dihybrid
 Review and Practice
 What happens if we want to examine more than one trait?
Lesson 3: Solving Dihybrid crosses with Punnett Squares
 Use of Online Mix Those Genes game from The GEEE! In
Genome website
Lesson Sequence (cont’d)
Lesson 4: Complex Patterns of Inheritance
 Co-dominance, Incomplete Dominance and Multiple
Alleles
 Genetic Traits in Harry Potter
 Additional focus on blood types
Lesson 5: Sex-Linked Traits
 Monster Genetics Lab
 Sex-linked Traits
 Genetic Disorders
Teaching Strategy 1: Inquiry


Use of Power Point presentation and “Get to Know
Yourself” worksheet
Students determine whether they display the dominant
or recessive phenotype for a number of traits (hair
colour, tongue rolling, ear lobes, freckles) and their
possible genotypes
Teaching Strategy 2: Gizmo

Students complete the Gizmos Activity Mouse Genetics
(One Trait)
http://www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=449&ClassID=219230

Other Gizmos applicable to this unit are Mouse Genetics
(Two Traits) and Chicken Genetics (Codominance)
Teaching Strategy 3:
Problem Solving Practice

Students complete a variety of genetic
problems that require them to draw Punnett
squares using good old fashioned….pencil
and paper!
Teaching Strategy 4:
Online Game

Students learn about dihybrid crosses using a
simulation about eyecolour on The GEEE! in
Genome website
http://nature.ca/genome/04/041/041_e.cfm
Teaching Strategy 5: Monster
Genetics Lab

Students flip a coin to determine the
genotype of their monsters. They then use
them to solve problems involving complex
patterns of inheritance.
http://www.nlm.nih.gov/exhibition/harrypottersworld/pdf/monstergeneticslab.pdf
Practical Applications

Breeds of dogs, varieties of vegetables, domestication of
animals. A recent National Geographic article describes a long
running experiment in which wild foxes were strategically bred
until they became domesticated like dogs.
http://ngm.nationalgeographic.com/2011/03/taming-wild-animals/ratliff-text/1paid

Ethical issues related to genomics, genetic testing or genetically
modified organisms – these can be brought to life with films (ex:
Gattaca or My Sister’s Keeper) or Case Studies.
http://sciencecases.lib.buffalo.edu/cs/

Examination of patterns of inheritance through
families.

Solving questions of paternity or maternity.
Potential Student Difficulties

The use of uppercase and lowercase letters to represent
dominant and recessive alleles rather than two different letters
(for example T to represent tall, and t to represent short, rather
than using T and S)
Solutions: Proper modeling and continued practice

The use of superscript notation to solving problems that involve
complex patterns of inheritance
Solutions: Proper modeling by the teacher, continued practice,
ensure that the resources students are given to help them use
proper notation, as not all sources do.

Distinguishing between the concepts of codominance and
incomplete dominance
Solution: Use real-life examples
Accounting for Different Types
of Learners




Use of a variety of strategies including Power Point
presentations, diagrams, computer simulations,
pencil and paper tasks, and real-life applications
ELL students will be paired with students who are
proficient English speakers and speak their first
language, a variety of visual resources will be
available for their use
Flexible groupings will be used to ensure students
work with a wide variety of peers
Additional support from teacher in a small group
setting to reinforce concepts, when necessary
Differentiated Assessment




Quizzes, lab work, online activities, problem sets
and in-class discussion will be used as formative
assessment throughout the unit
Students will keep journals in which they will log
their learning through their choice of method
A unit test will evaluate student understanding
Students may choose to present their understanding
through a variety of products for the culminating task
(poster, song, rap, video, brochure, game)
Resources

An extensive list of annotated resources can
be found in the accompanying Presentation
Summary