Lesson Plan Industry Sector
Agriculture & Natural Resources

## Punnett Square Discovery

### Lesson Plan Overview / Details

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett Square! After an introduction to genetics and a vocabulary overview this lesson can be used to build and practice Punnett squares with monohybrid and dihybrid crosses using animal science examples.

### Lesson Time

Monohybrid Punnett Squares
2 Class Periods
Dihybrid Punnett Squares
1 Class Period

### Objectives and Goals

• Use practical, everyday animal and plant examples to practice and show how to use a Punnett Square to determine offspring results.
• Understand how to predict phenotypic and genotypic results of a dominant and recessive gene pair.
• Using the practice and after reviewing how to make a Punnett Square to predict offspring geno and phenotypic results, students will be asked to determine a ranch and pet store budget for expected progeny.
• Apply appropriate problem-solving strategies and critical thinking skills to work-related issues and tasks.

### Activities in this Lesson

• As each student enters the room they will flip a coin and heads sit in the front and tails sit in the back. Once all the students are seated you have them look around and see, did the flipping of coins, which on paper should result in 50/50 probability for each student to be sitting in the front or back of the room, really work? Discuss how the more times we toss our coin, the more likely we are to get closer and closer to our expected probability of 50% “heads” and 50% “tails.” Based upon what we know about coins, we form an hypothesis that the probability of getting a “heads” when we toss the coin is ½ or 50%. Test the hypothesis based on the visual results in the class.

• The basic naked Punnet-square looks like a window pane with four squares:

When given enough info about two parent organisms, we can use this window pane to predict the genotypes & phenotypes of their offspring.

Please capture the vocabulary review words, if you know them, just write the word, if you are unsure, write the definition.

Vocabulary Review:

1· genotype = the genes of an organism; for one specific trait we use two letters to represent the genotype. A capital letter (H) represents the dominant form of a gene (allele), and a lowercase letter (h) is the abbreviation for the recessive form of the gene (allele).

2· phenotype = the physical appearance of a trait in an organism

3. dominant = trait that is seen

4. recessive = trait that is only seen if there is no dominant gene

5. homozygous = both genes (alleles are the same) BB or bb

6. heterozygous or hybrid= a mix of genes that are both dominant and recessive, Bb

1. For example, let's say that for hamsters, a brown hamster color is dominant over cream colored hamsters. B will represent the dominant brown color and b will represent the recessive cream color.

2. Since the "brown code" and the" cream code" are alleles (two forms of the same gene), we abbreviate them with two forms of the same letter. So we use "B" for the dominant allele/trait and "b" for the recessive allele/trait.

3. Our possible genotypes & phenotypes would be like so:

Symbol Genotype Name Phenotype
BB homozygous (pure) Dominant Brown Hamster
Bb heterozygous (hybrid) Brown Hamster
bb homozygous (pure) Recessive Cream Hamster

Note: Remember, we don't use "B" for brown & "C" for cream because that would make it two different genes which would code for two different traits, and the hamster color is one trait. What the genotype contains are two codes for the same trait, so we use two forms of the same letter (capital & lowercase). {Problems dealing with incomplete dominance & codominance are an exception to this "Note".}

One more note: A very very helpful thing to memorize is that the ONLY way for a recessive trait to show up in an organism is if that organism's genotype is homozygous recessive (two little letters, like "rr").

Let’s practice:

Cross a dominant brown colored hamster and a cream colored hamster (remember that is recessive) ___ ___ x ___ ____

Set up a Punnet square to illustrate the resulting offspring.

If every brown hamster was worth \$1 and every cream hamster was worth \$5, how much money did you get from your four hamster's you sold?

• Let's practice how to read a scenario to help understand how to set up a simple cross with a Punnett square.

Step #1: Determine the genotypes of the parent organisms.

· Sometimes this is already done in the question for you. If the question says "Cross two organisms with the following genotype: Tt & tt", it's all right there in the question already. But you will not learn how to interrupt the information.

· More likely is a question like this: "Cross a black bull with a red cow". Here, you have to use your understanding of the vocab to figure out what letters to use in the genotypes of the parents. Heterozygous always means one of each letter, so we'd use "Bb" (where "B" = black, & "b" = red). The only way for a calf to be red is when it has 2 lowercase "b's", so that a red parent is "bb".

· Now, we (us mean teachers) can make things just a little more tricky. Let's use the cattle example. Black is dominant (B), and red is recessive (b). What if a question read like this: "Predict the offspring from the cross of a red bull and a black cow if the black cow's mother was red". Oooooh, is this a toughy? First things first: the only way for the red bull to be red (the recessive trait) is if it's genotype is homozygous recessive (2 little letters), so the red bull is "bb". Now, the black cow's genotype could be either "BB" or "Bb". If its mommy was red (bb), then this black cow MUST have inherited a little "b" from its mommy. So the black one in our cross is "Bb" (not "BB"), and our cattle cross is: Bb x bb.

Step #2: Write down the genetic cross

Step #3: Make a P-square to determine the color of the offspring.

Step #4: Split the alleles and place them on the outside of the P-square

Step #5: Do the "math!"

Step #6: Have the first person done come up to the board and illustrate the answer they got and talk about genotypic/phenotypic ratios and what color the calves will be.

More practice: and be sure and determine the phenotypic and genotypic ratio

1. Cross a homozygous black bull and a heterozygous black cow.

2. Cross a heterozygous black bull and a heterozygous black cow.

3. Cross a red bull and a heterozygous black cow

4. You have 300 calves to market and 230 of them are black and the rest are red; what was the genotype of the bulls and cows and what color were they?

• Let's practice with sheep. Unfortunately sheep have a condition called "spider lamb" due to inbreeding in the Suffolk breed. Let's see if we can determine how many lambs will be spiders based on the genetics of the parents.

1. Normal body type is dominant over the spider condition (thank goodness!)

2. N = normal n = spider

3. If a ram is "clean" or in other words has a clean pedigree, and is bred to carrier (heterzygous) ewes, how many of the lambs will be spider lambs?

4. If a ram is heterozygous for the spider gene and is bred to heterozygous ewes, how many lambs will be spider lambs?

5. If you have a flock of 100 ewes that are all heterozygous bred to a heterozygous ram, and every lamb that is born that is free of the spider gene is worth \$500, a lamb that is normal appearing but a carrier for the gene is worth \$100, and a spider lamb, well, is worth \$0 because it is going to die, what will be your income from your flock?

Show all your work for full credit!

6. Now, it is time to complete the lab about probability and monohybrid crosses.

• punnett square lab.doc [ Download ] This lab should be completed once the students understand how the Punnett square helps to predict the offspring.
• Dihybrid Crosses - Demo / Modeling

Now that you all understand how to do a simple Punnett square ... get ready, we are going to add a trait to your cattle and see what kind of calves you end up having to market. Ready? You will need to capture the details of how to set up a bigger Punnett square that just four squares...it is now going to be 16!

1. You remember the black cattle and red cattle scenario, well, now we are going to add polled vs horned trait to your cattle.

2. Polled is dominant to horned (the easist way to dehorn cattle is to use a polled bull, no blood involved!)

3. B = black, b = red, H = polled, h = horned

4. What two traits are dominant? What two traits are recessive?

5. Lets set it up: you have a homozygous black polled bull bred to a red, horned cows. What do the offspring look like?

Bull = BBHH x Cows = bbhh

to illustrate the cross show the class how to align the alleles

BH BH BH BH x bh bh bh bh

Results are: all the calves will be BbHh or heterzygous black and polled

• Go over the assessment problems in class with all the students and then determine who in the class made the most money with their cows for the #2 problem.
Discuss in detail what genetic cross was the most profitable.
Talk with the class about being a real rancher and how important color is in the industry, especially in defining a specific market for your calf crop.

### Assessment

Assessment Types:
Demonstrations,

A problem set of three cow/calf operation problems are given to the students to show how they can set up a budget for the ranch based on the genetic make up of their cattle.  By monitoring the students you can determine how long it will take them to complete the problems.  You may even need to help them set up the parent genotype to get them started.

• punnett square bull lab.rtf [ Download ] Problem set to assess the unit