Punnett Squares

Are traits and variations equally shown?  Are all traits and their variations equally shown in organisms?  Why not?  If your mom has blue eyes and your dad has brown eyes, does the offspring have 1 blue eye and 1 brown eye?  How is it decided which trait you get?

Punnett Square  Has anyone ever heard of a Punnett Square?  What does it do?  A Punnett Square is a way to look at the traits of two parents and predict what trait the organism will have.  We are going to explore Punnett Squares with the trait of eye color

Symbols for traits  When scientists look at traits inside a Punnett Square, there is a specific way to write the trait.  A trait for is written with one letter, and the letter could be uppercase or lowercase. But if you look at a trait for an organism, the each trait always has 2 letter. For example, the trait for tasting PTC may be shown with a P. But when looking at a person, their trait would be Pp, PP, or pp.  Why do you think one trait has 2 letters?

 For this activity, the trait of eye color will be represented by a B. So organisms can have BB, Bb, or bb when representing their eye color trait  To set up a Punnett square, you write the 2 letters for Dad’s trait on top of each square  Then you write the 2 letters for Mom’s trait to the left of each square  Finally, you bring whatever letters are outside the square inside the square  Let’s do scenario one is the packet together. The dad is BB and the mom is bb.

 Now that you have the correct letters inside the Punnett square, look at the plastic bag you each have. There are brown and blue squares.  Each color of square has either a B or b on them. For each letter on your Punnett Square, I want you to place the colored square on top of the letter than matches the letter on the Punnett Square.

 After you have one colored square for each letter on your Punnett Square, I want you to place the squares on top on one another for each trait.  For example, the for two letters that go together for Dad’s trait, those 2 colored squares should be put on top of one another.  Are there any questions?

 Answer the questions below scenario 1 that we completed together.  Now try the next scenarios on your own.  Remember: To set up a Punnett square, you write the 2 letters for Dad’s trait on top of each square  Then you write the 2 letters for Mom’s trait to the left of each square  Finally, you bring whatever letters are outside the square inside the square  Answer the questions below each scenario

Making Sense  Now answer the Making Sense questions on your packet  What trait do you think is dominant?  What evidence do you have to support this?  What trait do you think is recessive?  What evidence do you have to support this?  What happened when you put the blue and brown squares on top of one another?

Making Sense  What do the four squares inside the Punnett Square represent?  So does this mean that every parent is going to have 4 offspring?  The four squares represent the chance, or probability of what trait their offspring will have.  If in a Punnett square, there are 3 brown squares, and 1 blue square, what is the chance of having a child with blue eyes?  Fill in the chart below the Making Sense questions

Homozygous and Heterozygous  Who knows what homozygous means?  Here is a hint. The root word “homo” means same in Greek.  So looking at scenario 1 on our worksheet, do you think either parent is homozygous for a trait?  What about heterozygous? The root word hetero in Greek means different. So now let’s look at scenario 2 in our worksheet. Are any of those parents heterozygous for their trait?

Homozygous and Heterozygous  Homozygous means having 2 of the same letters in the trait. It means that trait is homozygous.  Heterozygous means each letter is different for a trait  On the notes section of the worksheet, copy down the definition of homozygous and heterozygous

Recessive vs. Dominant  We said earlier that brown eyes was the dominant trait. If you had to define dominant trait, what would it be?  What about recessive trait?  So when writing dominant traits the letters are always capitalized. Recessive traits, the letters are always lowercase.  Write the definitions of dominance and recessive in your notes.

Bringing it All Together  Now let’s combine these terms.  On your packet there is a table. For each parent in each situation, you are going to write them in where they below.  So if the mom in scenario 1 is heterozygous dominant, under heterozygous dominant you will write scenario 1- mom.