Genetics
Mendelian Genetics
While assigned to
teach, he was also
assigned to tend
the gardens and
grow vegetables
for the monks to
eat.
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.
Gregor Mendel
“Father of Genetics”
Mountains with
short, cool growing
season meant pea
(Pisum sativum)
was an ideal crop
plant.
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)
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:
Consistency is
Good
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
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).
Practice:
Take out the Applying Genetics
Terminology paper and
complete.
You have 2 minutes.
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
Using terminology:
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).
Example of Mendel’s Work
P
Smooth
SS
x
Wrinkled
ss
Homozygous
Dominant
Homozygous
Recessive
All Smooth
Ss
F1
Heterozygous
Clearly Smooth is Inherited…
What happened to wrinkled?
1.
Smooth is dominant to wrinkled
2.
Use S/s rather than W/w for
symbolic logic
F1 x F1 = F2
Punnett Square:
F2
possible
gametes
Phenotype
Genotype
possible gametes
S
s
Smooth Smooth
S
SS
Ss
s
Smooth wrinkled
Ss
ss
NEVER use S/W or s/w
3/
Smooth
1/ wrinkled
4
4
wrinkled is not missing…just
masked as “recessive” in diploid
state
Mendel as a Scientist
Test Cross:
Unknown Smooth x
possible
gametes
If Unknown is Ss:
Punnett Square:
Wrinkled
ss
possible gametes
If Unknown is SS:
s
s
possible
gametes
possible gametes
S
s
S
Smooth Smooth
SS
Ss
s
Smooth wrinkled
Ss
ss
s
S
Smooth Smooth
Ss
Ss
S
Smooth Smooth
Ss
Ss
Test Progeny All Smooth
possible gametes
s
S Smooth
Ss
possible
gametes
F1 x F1 = F2
F2
Wrinkled
ss
s
Smooth
Ss
Wrinkled
ss
Test Progeny Half Smooth
Half wrinkled
“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
Monohybrid Crosses Yielded Consistent Results
Therefore, the Principle of Segregation indeed is a general principle of genetics.
Probability and Genetics
Equation for probability
NUMBER OF THINGS YOU
ARE LOOKING FOR
PROBABILITY = ----------------------------------TOTAL NUMBER OF THINGS
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.
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.
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?
Chi-Square Analysis –
determine how close your data is to the known probability
of occurrence
Chi-Square
Lets look at the chance of flipping heads or tails
Options
Heads
Tails
Observed (o)
43
57
Expected (e)
50
50
N (degree of freedom) = # of options – 1
N=2-1
N=1
o – e (d)
d2
d2/e
43-50 =
72 =
49/50 =
-7
49
.98
57-50 =
72 =
49/50 =
7
49
.98
X2 = 1.96
Add this column for X2
Chi-Square cont.
N=1
X2 = 1.96
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
X2 values are in the shaded region
N value look here
P = about 17%
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.
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?
What is the probability
of each landing on
heads or tails?
Heads
Tails
1/2
1/2
Heads
HH
HT
1/2
1/4
1/4
Tails
HT
TT
1/2
1/4
1/4
Phenotype:
1/4 Heads/Heads:
1/2 Heads/Tails:
1/4 Tails/Tails
Punnett
Squares
work in the
same
manner
Alleles: T = tall
What are the
possible gametes
produced by these
parents?
Tt
T
Tt
T
t
Tall
Tall
TT
Tt
t = short
Phenotype:
¾ or 75% Tall
½ or 25% short
t
Tall
Tt
Short
tt
Genotype:
¼ or 25% TT
2/4 or 50% Tt
¼ or 25% tt
Practicing Monohybrid
Punnett Squares
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
With a partner, go to the back of the room
and complete the lab. You will need your
chi-square chart.
Lab – Corn Genetics