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 willlearn 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 inheritanceSolutions: 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 dominanceSolution: 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