Worksheet 1 Colony Counting

Let’s check if the substance you tested during the last lab is a mutagen!

For this part of the worksheet, consider only the mutagen (coffee, toothpaste, shampoo, etc.) and the control. Do not count the colonies on the part with the modification!

If you have tested two mutagens (coffee and splenda) and their combination (coffee + splenda), do this worksheet twice for each mutagen (coffee versus control and then splenda versus control).

What is the null hypothesis? ______

Count the number of colonies on your control part of the plate and the part with the substance tested (coffee, mascara, shampoo, etc.):

Observed number of colonies in control / Observed number of colonies in test

What is the expected number of colonies in the control and the test, assuming that that the yeast are equally likely to grow on either side?

Note that this number will be the same for the test and the control and will be ½ of all the colonies that you’ve counted (for example, if you count 40 total colonies on your test and your control sections, the number will be ½ of 40 = 20 colonies).

Expected number of colonies in control / Expected number of colonies in test

Calculate the difference between observed (first table) and expected (second table) number of colonies:

Observed number of colonies in control - expected number of colonies in control / Observed number of colonies in test - expected number of colonies in test

Square the obtained numbers:

(Observed number of colonies in control - expected number of colonies in control)^2 / (Observed number of colonies in test - expected number of colonies in test)^2

Divide the number by the expected number of colonies:

(Observed number of colonies in control - expected number of colonies in control)^2 / expected number of colonies in control / (Observed number of colonies in test - expected number of colonies in test)^2 / expected number of colonies in test

Now add the two numbers together; this is chi square!

chi square = ______

Is the obtained number greater or less than 3.84?

**3.84 is a cutoff that tells you whether the difference you observe is significant or if it happens by chance. Chi squares greater than 3.84 are significant. Chi squares less than 3.84 are not significant (the difference happens randomly/by chance).

Can your substance cause mutations in yeast?

Worksheet 2 Colony Counting

Let’s check if the modification you made during the last lab is can make your substance more or less mutagenic!

For this part of the worksheet, consider only the modification (soy sauce + toothpaste, more shampoo, etc.) and the part without the modification (only toothpaste, one scoop of shampoo, etc).

What is the null hypothesis? ______

Count the number of colonies on your modified part of the plate and the part without the modification:

Observed number of colonies in mutagen / Observed number of colonies in modified mutagen

What is the expected number of colonies, assuming that that the yeast are equally likely to grow on either side?

Note that this number will be the same for the mutagen and the modified mutagen will be ½ of all the colonies that you’ve counted (for example, if you count 40 total colonies on your mutagenand your modified sections, the number will be ½ of 40 = 20 colonies).

Expected number of colonies in mutagen / Expected number of colonies in modified mutagen

Calculate the difference between observed (first table) and expected (second table) number of colonies:

Observed number of colonies in mutagen - expected number of colonies in mutagen / Observed number of colonies in modified mutagen - expected number of colonies in modified mutagen

Square the obtained numbers:

(Observed number of colonies in mutagen - expected number of colonies in mutagen)^2 / (Observed number of colonies in modified mutagen - expected number of colonies in modified mutagen)^2

Divide the number by the expected number of colonies:

(Observed number of colonies in mutagen - expected number of colonies in mutagen)^2 / expected number of colonies in mutagen / (Observed number of colonies in modified mutagen - expected number of colonies in modified mutagen)^2 / expected number of colonies in modified mutagen

Now add the two numbers together; this is chi square!

chi square = ______

Is the obtained number greater or less than 3.84?

**3.84 is a cutoff that tells you whether the difference you observe is significant or if it happens by chance. Chi squares greater than 3.84 are significant. Chi squares less than 3.84 are not significant (the difference happens randomly/by chance).

Did your modification make your substance more or less mutagenic?