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Genetics News
• Exam I
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• Weekend help
– Sun 4:30 (S-111)
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• Summary for Exam II
• Lab
– Figure out dilutions
Topics for Today
Terminology
Not today
Monohybrid cross
Quick review
Chi2 test
LOTS
Monohybrid Cross
...GGACCGAT…
…CCTGGCTA...
...GGATCGAT…
…CCTAGCTA...
Colorless compound
Purple compound
Enzyme
P
Study Question 7
Which is the dominant allele?
Heterozygous Pp
Homozygous PP
Homozygous pp
If Pp has P phenotype,...
then P is dominant over p
Parental Cross
Meiosis I
Meiosis II
Study Question 8
Parental Cross
Write down the genotypes of the two
F1 parents and the gametes they produce
F1 Cross
Meiosis I
Study Question 13
Restate the Law of
Segregation in the
language of
chromosomes
Law of Segregation: Heterozygotes
Each of two homologous
produce equal
chromosomes
numbers of
the
has two
an equal
alleles
chance of arising in a gamete through meiosis.
Are Mendel’s conclusions valid?
Mendel observed:
Pp x Pp
705 purple + 224 white
Mendel concluded:
Pp x Pp
3:1 purple:white
But 3:1 with 929 flowers is really:
~694 purple + 235 white
705 purple and 224 white is close...
Is it close enough?
Where does 2 come from?
A million repetitions of Mendel’s experiment
Create a million universes -- purple:white on average = 3:1
Result: 702
705 purple
698
688
710
695
224 white
231
241
219
234
227
= 929 plants
Where does 2 come from?
A million repetitions of Mendel’s experiment
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
1,000,000 repetitions
Study
StudyQuestion
Question15:
14:What’s
Why is the
it that
most
thelikely
two dotted
result?
lines areHow
on opposite
often does
sides
it occur?
of the mean?
Deviation from Expectation
Two 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 flower
Total counted: 929
Purple = 75% of 929 = 696.75
White = 25% of 929 = 232.25
Calculate the squares of the deviations
Chi2 = Sum of (O - E)2 / E
Chi2 = (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 times
Ask: purple? (if not, then white)
Therefore ONE degree of freedom
Look up probability for 2 value
2 = 0.30
80% > P > 50%. Call it ~60%
Steps in Performing a Chi2 Test
P ~60%
Draw a conclusion
The hypothesis
60%
result
of thehas
time,
a has
50%
Mendel’s
a 50%
chance
chance
result
of being
orofworse
being
correct
would
correct
have arisen by chance if purple:white truly occurs in
a 3:1ratio.
Deviation from Expectation
Two example experiments
P = .50
P = ???
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)
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%
TOTAL
47 marbles
Is their claim correct?
53 marbles
How to tell how close
is close enough?
100 marbles
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 = ~60%
Study Question 26
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.
Does dwarfism appear to be dominant or
recessive?
Can progeny be reasonably be described as in 3:1 ratio?