Genetics News• Exam I

– Returned Monday (?)

– Our responses on web– Thanks for YOUR responses

• Weekend help– Sun 4:30 (S-111)– Sun 8:00 (E-107)

• Summary for Exam II

• Lab– Figure out dilutions

Topics for Today

Terminology Not today

Monohybrid cross Quick review

Chi2 test LOTS

Monohybrid Cross

Colorless compound Purple compoundEnzyme

P

...GGATCGAT… …CCTAGCTA...

...GGACCGAT… …CCTGGCTA...

Study Question 7Which is the dominant allele?

Homozygous PP Homozygous pp

Heterozygous Pp

then P is dominant over pIf Pp has P phenotype,...

Parental Cross

Meiosis I

Meiosis II

Study Question 8

Write down the genotypes of the two F1 parents and the gametes they produce

Parental Cross

F1 Cross

Meiosis I

Study Question 13

Restate the Law of Segregation in the

language of chromosomes

Law of Segregation: Heterozygotes produce equal numbers of the two allelesLaw of Segregation: Each of two homologous chromosomes has an equal chance of arising in a gamete through meiosis.

Are Mendel’s conclusions valid?

Mendel concluded:

Pp x Pp 3:1 purple:white

Mendel observed:

Pp x Pp 705 purple + 224 white

But 3:1 with 929 flowers is really:

~694 purple + 235 white

705 purple and 224 white is close...

Is it close enough?

Result: 705 purple 224 white = 929 plantsResult: 698 purple 231 white = 929 plantsResult: 688 purple 241 white = 929 plantsResult: 710 purple 219 white = 929 plantsResult: 695 purple 234 white = 929 plantsResult: 702 purple 227 white = 929 plants

Where does 2 come from?A million repetitions of Mendel’s experiment

Create a million universes -- purple:white on average = 3:1

200,000 repetitions

Where does 2 come from?A million repetitions of Mendel’s experiment

500,000 repetitions

Where does 2 come from?A million repetitions of Mendel’s experiment

Where does 2 come from?A million repetitions of Mendel’s experiment

1,000,000 repetitionsStudy Question 15: What’s the most likely result?

How often does it occur?Study Question 14: Why is it that the two dotted

lines are on opposite sides of the mean?

Deviation from ExpectationTwo example experiments

Study Question 17: Why is there shading on both sides of the curve?

Study Question 18:The farther away O from E, the smaller/larger the shaded area?

Steps in Performing a Chi2 Test

Determine the expected values for the experiment

Model: 3 purple : 1 white flowerTotal counted: 929Purple = 75% of 929 = 696.75White = 25% of 929 = 232.25

Calculate the squares of the deviations

Chi2 = Sum of (O - E)2 / EChi2 = (705 - 696.75)2 /696.75 + (224 - 232.25)2 /232.25 ~82 / 700 + ~82 / 230 ~0.09 ~0.3 Chi2 = approx 0.39 (actually = 0.37)

Steps in Performing a Chi2 Test

Determine the degrees of freedom

What was the experiment? - Count 929 flowers a million timesAsk: purple? (if not, then white)

Look up probability for 2 value

2 = 0.30

80% > P > 50%. Call it ~60%

Therefore ONE degree of freedom

Steps in Performing a Chi2 Test

P ~60%

Draw a conclusion

The result has a 50% chance of being correctThe hypothesis has a 50% chance of being correct60% of the time, Mendel’s result or worse would have arisen by chance if purple:white truly occurs in a 3:1ratio.

Deviation from ExpectationTwo example experiments

Study Question 20:What if Mendel had counted not 929 but 929,000 plants -- what does the curve and shading look like then? (d still = 29)

P = .50 P = ???

Study Question 25

Are there as many female as male biology majors? How many degrees of freedom?

What’s the experiment?

Study Questions 27 and 28

Does a high P value indicate the hypothesis is correct?

Does a low P value indicate the hypothesis is incorrect?

Bag of Marbles

1000’s of marbles!50% red, 50% blue

Guaranteed!

Test Claim of 50%:50%

47 marbles

53 marbles

100 marblesTOTAL

Is their claim correct?

How to tell how close is close enough?

2 Test of Claim

Chi2 = Sum of (O - E)2 / E

Chi2 = (53 - 50)2 /50 + (47 - 50)2 /50 9 / 50 + 9 / 50 18/50 0.36

P = ?P = ~60%

Study Question 26

Does dwarfism appear to be dominant or recessive?

Although dwarfism is a rare condition in humans, it is nonetheless not so uncommon to encounter a productive union between two dwarves. Suppose you consider the progeny of many such unions and find that 132 are also dwarves and 60 are of normal stature.

Can progeny be reasonably be described as in 3:1 ratio?