Traits and Heredity

Paraphrase:

Mendel famously worked with purple and white blossom on pea plants, but for this simulation we will refer to flowers as purple and pink to better match the P & p letter representations. (P = PURPLE and p = pink)

Show Slide 1. Tell students that they are considering a flower that can be one of two colors: purple or pink. Explain that there are many species of plants that have flowers with much more complicated patterns, but our plant is very simple.

Show Slide 2 with three pairs or alleles and tell them the pair represents the color gene in three different flowers' DNA.

Ask students to predict what each flower will look like.

Show Slide 2 with the illustrations of flowers. Ask students to turn and talk in order to establish what the "rules" must be to determine color.

Intended answers:

Purple is the dominant allele, so if P is present in the pair the flower will be purple.

The combination PP has purple flowers

The combination Pp has purple flowers

The combination pp has pink flowers.

Note: The notation "pP" is not typically used to show pairs of alleles, but if it were, the flowers would be purple.

Paraphrase:

A parent organism with one dominant and one recessive allele for a given gene has an equal chance of contributing either allele to a particular offspring.

Mark 2 poker chips (or similar) P on the front and p on the back. Tossing the chips and seeing how they land simulates the way two parents generate pairs of alleles in their offspring. By doing a large enough number of simulations and noting the results, you can begin to see some very important statistical patterns.

Expected results:

About 25% of simulations will yield a pair of dominant alleles.

About 25% of simulations will yield a pair of recessive alleles.

About 50% of simulations will yield a mixed result: one dominant and one recessive allele.

Because dominant/dominant and dominant/recessive combinations of alleles both express dominant traits, about 75% of the simulations will yield dominant traits (PURPLE) and 25% should show recessive trait (pink).

The more trials students conduct, the more closely their results will match Mendel’s observed ratio, which is introduced later in this unit.

Ask:

How would you alter this experiment to predict the offspring from one plant carrying two recessive alleles of gene P and the other carrying one dominant allele and one recessive allele of gene P?

Have students note the two pairs of alleles they chose (pp & Pp). Then have them experiment again. Discuss results as a class and ask the class to look for patterns.

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Reginald C. Punnett devised a simple system to determine likely outcomes with mixed and matched allele pairs.

Introduce the first-generation TT x tt Punnett Square and describe how it works. Show the TT x tt example in the slide, with four Tt offspring.

Paraphrase:

Label the grid with one organism's alleles across, and other organism's alleles down.  Since each organism shares one allele with its offspring, the alleles are distributed accordingly.

Show the second-generation Tt  x Tt slide with the blank Punnett Square. Then ask students to Turn and Talk.

Ask:

Can you solve this one?

Have students make Punnett Squares for their toss-up experiments.

Have them compare the percentages of the real data with those of the theoretical models.

This activity shows that the same principles hold true for rabbits, which brings students back to the Science Scene: How Now Brown Bunnies. The letters (Bb vs. Tt) used to represent alleles are not important, but students should be aware that capital letters symbolize dominant alleles, and lowercase letters symbolize recessive alleles.

Paraphrase:

Now about those bunnies... How did we get a brown bunny in the third generation?

A helpful hint for students: It works best to start at the bottom of the family tree and work your way backwards. That is, start by figuring out what alleles cause the descendants’ visible traits, and then figure out where those alleles must have come from.

Answers:

BB Bb BB bb

Bb Bb

BB Bb bb