Itsy and Bitsy Flip Out

1994 Woodrow Wilson Collection

NOTE: Read the dialogue out loud and ACT out all stage directions. (The stage directions will be found in parentheses.)

(Itsy and Bitsy, meeting once again in the biology labs, can be seen gazing fondly at their growing populations of fruit flies in catcher-containers. See lab: "Collect Your Own Drosophila ."

Itsy: Well, Bitsy, I have to admit that your idea worked! We collected more flies than I ever thought we would.

Bitsy: (irritably as usual.--see dialogue: "Itsy and Bitsy Discuss Dinner") Now, look what you did...you made me lose count here. I'm trying to get an idea of how many wild type flies we have and you are always making my counts wrong!

Itsy: (offended) Maybe it's your math and not my interruptions! Why do you want to know the number of wild types we are raising, anyway?

Bitsy: I did a quick taste test the other evening and...

Itsy: (with a look of astonishment) You did a taste test without me??!

Bitsy: In the name of science, Itsy, only in the name of science. (burps loudly) What I found out is that wild types do taste significantly better than the other variations. Now we need to know how to predict whether these flies will give us wild type phenotypes and how to select for them! If this works (excitedly) we can franchise this business and we can retire to our Florida greenhouse!!!!

Itsy: Fortunately for you, Bitsy, I can probably help you out here. One of the "coolest" things that Mendel did was to apply the mathematical concept of probability to biology.

Bitsy: Probability?

Itsy: (unfortunately, a little smug) Yes, Bitsy, probability. I'm sure you remember that probability is the likelihood that a particular event will occur. It is determined by the following formula:

Bitsy: (slowly) OK...

Itsy: A great example of this is flipping a coin. When a coin is flipped, one of two possible events can occur: the coin can land heads up or it can land tails up. The probability of the coin coming up heads is 1 in 2 chances. In other words, you will probably get heads one time out of every two coin tosses. The larger the number of trials (or times you flip the coin) the closer you will get to the expected ratio of 1/2.

Bitsy: Oh, I remember this now....There is another important rule here: previous events do not affect future outcomes. Each flip of the coin is a separate, independent event!

Itsy: Good for you, Bitsy. Now, we can apply this thinking to our fly crosses...as I'm sure you remember from listening in on all (insert teacher's name here) ____________________________________'s lectures on Mendel's Laws of Segregation and Independent Assortment and his Law of Dominance and Recessiveness. The genetic makeup of an organism is its genotype. The external appearance of an organism is its phenotype. For example, the genotype of a pure (homozygous) wild type wing for Drosophila melanogaster is WW. Both alleles for the wing are the same and dominant.

Bitsy: Wait a minute, Itsy ! Allele...what's an allele?

Itsy: Simple, Bitsy. It's the contrasting forms of a gene. For example, a wing can look full or be undeveloped. Both types of wings are variations of wing shape. It's just that the shapes are different. Each different shape is controlled by an allele - different varieties of a gene!

Bitsy: (nods head thoughtfully) I see.

Itsy: As I was saying about homozygous, wild type wings for Drosophila melanogaster - both alleles for the wing are the same and dominant. The phenotype will be for a full wing. However, the fly could also be heterozygous for wings with a genotype of Ww, but its phenotype will still be for a full wing. However, we also have some flies with small, underdeveloped wings which we call vestigial. These flies are homozygous for the recessive allele and their genotype would be ww. If we cross a fly which is homozygous for normal wings(WW) with a fly which is homozygous for vestigial wings (ww) we might want to know if we could predict what the genotype and phenotype of the offspring's wings will be.

Bitsy: That's right, Itsy. And I remember that the Punnett square is a handy tool for predicting the probability of the outcome of this cross in an easy to read chart!

Itsy: Correcto! The Punnett square was developed by a poultry geneticist, R.C. Punnett; although why he would want to work on birds, when Arthropods are so much more interesting is one of the great mysteries of science!

Bitsy: This is very exciting, Itsy. Let's try a few Punnett squares on our flies and see what we think we're going to get! (drooling a little bit) I'm getting hungrier by the day!!

Itsy: Sounds good to me, Bitsy. Let's remember a few more terms, though, so we can keep all these flies straight. The original cross is called the parent generation or P; their offspring are called the first filial generation or F1. If you cross two members of the F1 generation, we will get offspring called the second filial or F2 generation. Crosses in which we only look at one trait is called a monohybrid cross. Crosses in which we look at two traits are called dihybrid crosses.

Bitsy: And if we look at any more traits than that we'll need more eyes!!

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