genetics. mendelian genetics gregor mendel “father of genetics” augustinian monk at brno...
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Genetics
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Mendelian Genetics
Gregor Mendel
“Father of Genetics”
Augustinian Monk at Brno Monastery in Austria (now Czech Republic)
Not a great teacher but well trained in math, statistics, probability, physics, and interested in plants and heredity.
While assigned to teach, he was also assigned to tend the gardens and grow vegetables for the monks to eat.
Mountains with short, cool growing season meant pea (Pisum sativum) was an ideal crop plant.
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Gregor Mendel’s Work• Starting in 1856 Mendel studied peas which he grew
in a garden out side the Abbey he lived in.
• Showed that the traits he studied behaved in a precise mathematical way and disproved the theory of "blended inheritance.”
• Mendel’s work was rediscovered in 1900 by three botanists:
– Carl Correns (Germany)
– Erich von Tschermak (Austria)
– Hugo de Vries (Holland)
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Why Peas?
Mendel used peas to study inheritance because:– True breeding commercial strains were
available– Peas are easy to grow– Peas have many easy to observe traits
including:
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Consistency is Good
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Gene Expression• Each form of the particular gene is an
allele.
• Alleles can be either 1. Dominant – always show trait - T
2. Recessive – only see if dominant trait absent – t
• In order to see the trait expressed, 2 alleles must be paired together (one from mom and one from dad)
T t+ Tt
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Gene Expression
• Genes come in pairs that separate during the formation of gametes (meiosis).
• The members of these pairs can be the same (homozygous) or different (heterozygous).
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Practice:
Take out the Applying Genetics Terminology paper and
complete.
You have 2 minutes.
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Gene Expression
• When two alleles are paired we can express them by their phenotype or genotype.– Phenotype – physical characteristics
• Ex. Tall, Short, Dark, Round, Wrinkled
– Genotype – the allele combination or genes• Ex. AA, Aa, aa, Dd, Rr, rr, tt
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Using terminology:
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Results of Mendel’s monohybrid parental cross:“Mendel’s Principle of Dominance”
F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents.
Why?Smooth seeds (allele S) are completely dominant to wrinkled seeds (allele s).
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SmoothP Wrinkledx
F1All Smooth
Phenotype
Example of Mendel’s Work
Clearly Smooth is Inherited…What happened to wrinkled?
F1 x F1 = F2
F23/4 Smooth1/4 wrinkled
wrinkled is not missing…just masked as “recessive” in diploid state
1. Smooth is dominant to wrinkled
2. Use S/s rather than W/w for symbolic logic
SS ss
Ss
GenotypeHomozygous
DominantHomozygous
Recessive
Heterozygous
wrinkledss
SmoothSs
s
SmoothSs
SmoothSS
S
sSPunnett Square:
possible gametes
possible gametes NEVER use S/W or s/w
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Unknown Smooth Wrinkledx
Mendel as a Scientist
ss
SmoothSs
SmoothSs
S
SmoothSs
SmoothSs
S
ss
possible gametes
possible gametes
F1 x F1 = F2F2
wrinkledss
SmoothSs
s
SmoothSs
SmoothSS
S
sSPunnett Square:
possible gametes
possible gametes
Test Cross:
If Unknown is SS:
Wrinkledss
Wrinkledss
s
SmoothSs
SmoothSs
S
ss
possible gametes
possible gametesIf Unknown is Ss:
Test Progeny All Smooth
Test Progeny Half Smooth Half wrinkled
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“Mendel’s Principle of Segregation”:
• Recessive characters masked in the F1 progeny of two true-breeding strains, reappear in a specific proportion of the F2 progeny.
• Two members of a gene pair segregate (separate) from each other during the formation of gametes.
Segregation
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Monohybrid Crosses Yielded Consistent Results
Therefore, the Principle of Segregation indeed is a general principle of genetics.
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Probability and Genetics
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Equation for probability
NUMBER OF THINGS YOU
ARE LOOKING FOR
PROBABILITY = ----------------------------------- TOTAL
NUMBER OF THINGS
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I have quarter in my pocket. What is the probability that I get heads when
flipped?
Answer:
½
You have a total of 2 sides and 1 of them is heads.
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I have 3 pennies and 5 nickels in my pocket. If I pull out one coin what is the probability that I get a nickel?
Answer:
5/8
You have a total of 8 coins and 5 of them are nickels.
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Chi-Square Analysis – determine how close your data is to the known probability of occurrence
If I tossed a coin 100 times, how many heads would you expect to get? Tails?
50 each
What if you didn’t get 50: 50?
How would you know if the numbers you got were good enough?
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Chi-SquareLets look at the chance of flipping heads or tails
Add this column for X2
N (degree of freedom) = # of options – 1 X2 = 1.96N = 2 - 1
N = 1
Options Observed (o)
Expected (e) o – e (d) d2 d2/e
Heads 43 5043-50 =
-7
49/50 =
.98
Tails 57 5057-50 =
7
72 =
49
49/50 =
.98
72 =
49
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N = 1
X2 = 1.96
Chi-Square cont. What do I do with these numbers?
Once the Chi-square and N values are computed, look on the chart.
Degrees of Freedom
Probability Values (P)
(N) .95 .90 .80 .70 .50 .30 .20 .10 .05 .01
1 .004 .016 .064 .148 .455 1.07 1.64 2.71 3.84 6.64
2 .103 .211 .446 .713 1.39 2.41 3.22 4.61 5.99 9.21
3 .352 .584 1.00 1.42 2.37 3.66 4.64 6.25 7.82 11.34
If the probability (P) given in the table is high, it is very likely that this would occur by chance, and we have a good “fit”. If P is low, we conclude that it is not likely that the deviation observed would occur by chance alone.
N value look hereX2 values are in the shaded region
P = about 17%
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Activity: Probability and Chi-Squares
With a partner, get 2 pennies ( or any coin). Toss your coins 100 times. Make sure you record on the chart how many HH, HT and TT you got.
Using Chi-Square analysis, how “fit was your data?
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What is the probability of each landing on heads or tails?
Heads
1/2
Tails
1/2
Heads
1/2
Tails
1/2
HH
1/4
HT
1/4
TT
1/4
HT
1/4
Phenotype: 1/4 Heads/Heads:
1/2 Heads/Tails:
1/4 Tails/Tails
Punnett Squares
work in the same
manner
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Alleles: T = tall
t = short
Phenotype:
Genotype:Tt
TtWhat are the possible gametes produced by these parents? T t
t
T
½ or 25% short
¾ or 75% Tall
¼ or 25% TT
2/4 or 50% Tt
Tall Tall
Tall Short
TT Tt
Tt tt
¼ or 25% tt
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Exercise: Punnett Squares
•Do numbers 1, 2 and 3 – make sure to include phenotype and genotype ratios
•Get each problem checked by me before you move to the next
Practicing Monohybrid Punnett Squares
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Lab – Corn Genetics
With a partner, go to the back of the room and complete the lab. You will need your chi-square chart.