Genetics lab #3 - Chi square Test and Pipettes

Chi square test

Statistical test to determine how well observed phenotypic ratios match the expected phenotypic ratios

Hypothesized pattern of inheritance supported when?

Supported if results indicate that difference between observed and expected data is due to chance

Hypothesized pattern of inheritance rejected when?

Rejected if the difference between observed data and expected data is statistically significant

Do statistics prove hypothesis

No, they support them

Null hypothesis (H0)

Difference between observed and expected is not significant and due to chance. Pattern of inheritance is supported.

Null hypothesis interpretations

• “Fail to reject”: If calculated X² <= critical X² so there is no significant difference and chance is factor

• “Reject”: If calculated X² > critical X² so there is significant difference

Alternative hypothesis (HA)

The difference between observed and expected data is significant. (Rejecting and accepting is opposite of null)

Degrees of freedom determination

# of phenotypes - 1 = DF

P-value and meaning

0.05

We are allowing 5% error in accepting null hypothesis

95% confident in rejecting HA and failing to reject H0

Critical Value

obtained by crossing DF and p-value 

Chi square test equation and when to use

Use equation to calculate X² for each phenotype

Order of magnitude

Relative size of number referring to power of 10 that can be factored out of number

Ex: 2,500 is 2 orders of magnitude greater than 25

Sig fig rules

• #1: All non zero digits are significant

• #2: Zeros between nonzero digits are significant

• #3: Zeros to the right of first nonzero digit are significant

• #4: Zeros to the left of first nonzero digit are not significant 

Molar vs Molarity

• Molar (M): Concentration of solution in mol/L

• Molarity: Moles of solute per liter of solvent (ex. Molarity of solution is 5 moles per liter in solution)

Equation for dilutions

C1V1=C2V2

C1 = conc. of stock solution

V1 = Volume of stock solution

C2 = conc. of final solution

V2 = volume of final solution + volume of solvent

Serial dilution

series of simple dilutions which amplifies dilution factor quickly

Key terms for serial dilutions

• Aliquot: sub volume of original sample (Final volume / dilution factor)

• Dilutant: material that dilutes sample (Final volume - aliquot)

• Dilution factor: unit volumes in which material will be dissolved (Final conc./Initial conc. or Final vol./Aliquot)

Some rules to remember for using micropipettors

• Never measure higher or lower than the range

• Never turn the volume adjuster above or below range

• Never let liquid into micropipettor

• Always use appropriate tip

• Never invert or lay down when liquid is in tip

• Never let plunger snap back when filling or ejecting liquid

• Never immerse barrel in fluid

Micropipettor readings 

How to take up sample with micropipetter

• Hold micropipetter and tube at about eye level

• Press plunger down to first stop then put tip in fluid

• slowly release plunger to get fluid into tip

How to expel fluid with micropipetter

• Hold micropipetter and tube at eye level

• Touch micropipetter to tube wall to create surface tension that helps to get fluid out of tip

• slowly press plunger down to second stop to expel fluid

• Hold plunger down and remove from fluid to ensure none is taken back up

• After removing dispose of contaminated tip