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Understanding Genetics: Punnett Squares
• How do Punnett Squares help us
understand dominant and recessive
traits and the probability of specific
genetic outcomes?
• How does the knowledge of
genetics come into play in
selective breeding of food crops?
Science & Math Connections
Can We Accurately Predict Inherited
Genetic Traits? Can They Be Controlled?
Humans have been selectively breeding various plants and animals for centuries. The goal is to maximize desired traits and minimize or eliminate undesirable traits. But how does selective breeding work? Can scientists and breeders really predict genetic outcomes or are they simply making a lucky guess? Let’s learn some of the basics of genetics, including important vocabulary and how to use a Punnett Square. What connections can we make to the selective breeding of plants to generate hybrid seeds for food crops? Common Core Standards CCSS.Math.Content.7.SP.C.8 Find probabilities of compound events using organized lists, tables, tree diagrams, and simulation. CCSS.Math.Content.7.SP.C.8c Design and use a simulation to generate frequencies for compound events. Next Generation Science Standards Understanding about the Nature of Science Science assumes that objects and events in natural systems occur in consistent patterns that are understandable through measurement and observation. Science carefully considers and evaluates anomalies in data and evidence. Biological Evolution: Unity and Diversity MS-LS4- 6. Use mathematical representations to support explanations of how natural selection may lead to increases and decreases of specific traits in populations over time. Life Science LS3.A Inheritance of traits LS3.B Variation of traits In sexual reproduction, each parent contributes half of the genes acquired by the offspring resulting in variation between parent and offspring. Watch the following animated lesson from TED-Ed to get a head start: http://ed.ted.com/lessons/how-mendel-s-pea-plants- helped-us-understand-genetics-hortensia-jimenez- diaz#watch
Gregor Johann Mendel (July 20, 1822 – January 6, 1884) was a friar who gained posthumous fame as the founder of the science of genetics. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance. The profound significance of Mendel's work was not A Short History of Mendelian Genetics recognized until the turn of the 20th century when the independent rediscovery of these laws initiated the modern science of genetics. In the 1850s and 60s, in a monastery garden, Mendel was cultivating peas. He began separating the wrinkly peas from the shiny peas and studying and recording which characteristics were passed on when the next crop of peas were grown. In this slow and systematic way, Gregor Mendel worked out the basic law of heredity and stumbled upon what was later to be described as the fundamental unit of life itself…the gene. The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach to determine the probability of an offspring's having a particular genotype (combination of alleles). Dominance in genetics is a relationship between alleles of a gene, in which one allele masks the expression (phenotype) of another allele. A recessive gene is an allele that causes a visible or detectable characteristic that is only seen in a homozygous genotype (when an organism that has two copies of the same allele) and never in a heterozygous genotype (when an organism that has two different alleles – one dominant and one recessive.) An organism will always express the phenotype of the dominant allele. The only way a recessive trait is expressed is if both alleles in the gene are recessive. A heterozygous genotype can pass on a recessive allele to its offspring, even though it only displays the dominant phenotype. In other words, it can have both dominant and recessive genes, but not necessarily look like it does from the outside! Mendel made careful observations and kept systematic records… Images: Jennifer Sheffield
AA (^) Aa A a Recessive allele Dominant allele Parent 1 Parent 1 Parent 2 A Parent 2
Parent 1: Aa genotype
Parent 1: Aa genotype
Parent 2: aa genotype
Parent 1 carries one dominant and one recessive gene. Parent 2 carries two recessive genes. Parent 1 will exhibit the dominant trait, or phenotype. Parent 2 will exhibit the recessive. There are two (2) possible outcomes of genotype combinations for the offspring: 50% - one gene is dominant, one is recessive 50% - both genes are recessive Both parents carry one dominant and one recessive gene. Both parents will exhibit the dominant trait, or phenotype. There are three (3) possible outcomes of genotype combinations for the offspring: 25% - both genes are dominant 50% - one gene is dominant, one is recessive 25% - both genes are recessive A Punnett Square is used to represent all of the possible combinations of genes that could be inherited by the offspring of two parents. Each parent contributes one gene to the genotype, or gene combination, of the offspring. If a genotype contains two of the dominant alleles, or single genes, the organism will exhibit the dominant trait. If both alleles in the genotype are recessive, the organism will display the recessive trait. If both a dominant and recessive allele are present, the exhibited trait, or phenotype, will present as the dominant trait.
Punnett Squares (^) Name: ___________________________________________ Fill in the following Punnett Squares with the genotype information given for both parents. (Either parent’s information can go on the top or the left side.) Dominant genes are always capital letters and are listed first. Recessive genes are always lowercase letters. Can you correctly fill out the array for gene combinations, or genotypes, for their offspring? What is the probability of each outcome? What phenotype, or evidence of a dominant or recessive trait, will each combination have? Parent 1: Freckles dominant FF Parent 2: Freckles dominant Ff Possible genotypes of outcomes and percentage of probability: Which genotype(s) will display the dominant trait? Which genotype(s) will display the recessive trait? Parent 1: Freckles dominant Ff Parent 2: Freckles recessive ff Possible genotypes of outcomes and percentage of probability: Which genotype(s) will display the dominant trait? Which genotype(s) will display the recessive trait?
- List out all of the different genotypes possible from the combination of parents above, and how many times each combination occurs in your Punnett Square.
- What are the probabilities of each genotype occurring?
- Draw another 16-square Punnett Square on a blank sheet of paper. Create a simple visual representation of each of the phenotypes that correspond with the genotypes for each square from your Punnett Square above.
GEMS Academy
Genetics Glog
http://gemsacademy1.edu.glo gster.com/genetics/ This link will take you to a Glog we created for use at GEMS Academy. It contains active links that students can explore to genetics interactives, interviews with experts about the pros/cons of GMO’s, and BrainPOP videos (you will need your own BrainPOP login to watch the videos on genetics and DNA.)
Education Development
Center: Punnett Square
Interactive Tutorial
http://www2.edc.org/weblabs/P unnett/punnettsquares.html Students can actively participate in this fun and informative animated interactive on the use of Punnett Squares to predict outcomes when considering dominant and recessive gene combinations from two parents. Includes an interactive tutorial and a student task that can be repeated without penalty until the desired results are obtained. Technology Resources Punnett Square Challenge Extensions
1. Have students change the genotypes of one the parent rabbits to all dominant
genes (SSEE) and construct and complete a new Punnett Square matrix.
2. Have students extend the Punnett Square to include all of the possible
combinations for 2
nd
generation “grand-rabbits” from the parent pair from
Extension 1, using a set of new parents possible from the 1
st
generation of their
offspring, and analyze their results.
3. Can students detect a mathematical pattern to the Punnett Square system?
Can they express this pattern in an equation, or a series of equations?
4. Have students research albinism (a defect of melanin production that results
in little or no pigment in the skin, hair, and eyes) and create a product to present
their results.
The Khan Academy
Biology: Genetics and
Heredity
http://www.khanacademy.org/scie nce/biology/heredity-and-genetics A series of very detailed, higher level tutorials about various principles of genetics and heredity.
BrainPOP
http://www.brainpop.com/science /cellularlifeandgenetics/heredity/ A very basic animated introduction to principles of heredity. This particular video is free!
Materials and Equipment For each student:
- heavy duty Ziploc bag (freezer or storage bag)
- 1 strawberry
- DNA extraction buffer (900mL water, 50mL dishwashing detergent, 2 teaspoons salt)
- small plastic cup to hold extraction buffer
- cheesecloth to fit in small funnel (4” X 4” should be appropriate)
- small funnel
- 50mL vial / test tube
- glass rod or popsicle stick
- cold ethanol
- ice Assessment Lab report and/or discussion questions. Discuss questions as a class to assess the students understanding and ability to communicate scientific concepts. Discuss why each step was needed and how this relates to the organization of genetic material.
Name_________________________________________________ Date________________ DNA Extraction: Strawberry
Background:
The long, thick fibers of DNA store the information for the functioning of the chemistry of life. DNA is present in every cell of plants and animals. The DNA found in strawberry cells can be extracted using common, everyday materials. We will use an extraction buffer containing salt, to break up protein chains that bind around the nucleic acids, and dish soap to dissolve the lipid (fat) part of the strawberry cell wall and nuclear membrane. This extraction buffer will help provide us access to the DNA inside the cells.
Pre-lab questions:
- What do you think the DNA will look like?
- Where is DNA found?
Materials:
heavy duty ziploc bag 1 strawberry 10 mL DNA extraction buffer (soapy, salty water) cheesecloth funnel 50mL vial / test tube glass rod, inoculating loop, or popsicle stick 20 mL ethanol
Procedure:
- Place one strawberry in a Ziploc bag.
- Smash/grind up the strawberry using your fist and fingers for 2 minutes. Careful not to break the bag!!
- Add the provided 10mL of extraction buffer (salt and soap solution) to the bag.
- Knead/mush the strawberry in the bag again for 1 minute.
- Assemble your filtration apparatus as shown to the right.
- Pour the strawberry slurry into the filtration apparatus and let it drip directly into your test tube.
- Slowly pour cold ethanol into the tube. OBSERVE
- Dip the loop or glass rod into the tube where the strawberry extract and ethanol layers come into contact with each other. OBSERVE
REMEMBER: Students may place either parent’s genotype on the top or side rows of their Punnett Square. This key represents only one way that a properly completed matrix may look. Both variations should render the same percentages of genotypes for the offspring, however. Parent 1: ssEe Solid coat (ss) Stand up ears (Ee) Parent 2: Ssee Spotted coat (Ss) Lop ears (ee) se se se sE se sE
ssEe
Key
- List out all of the different genotypes possible from the combination of parents above, and how many times each combination occurs in your Punnett Square. SsEe - 4 ssEe - 4 Ssee - 4 ssee - 4
- What are the probabilities of each genotype occurring? 25% change for each genotype equally
- Draw another 16-square Punnett Square on a blank sheet of paper. Create a simple visual representation of each of the phenotypes that correspond with the genotypes for each square from your Punnett Square above. SsEe = Spotted coat, upright ears ssee = Solid coat, lop ears Ssee = Spotted coat, lop ears ssEe = solid coat, upright ears
