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Mendelian Genetics: Consider this….
Today:
8 million possible
chromosome
combinations in each
egg, and each
sperm…
Mendelian
Genetics and
Problems (In-Class 6)
= >70 trillion
possibilities!
How are we able to predict ANYTHING
about inheritance??
With all these possibilities, how can we
predict anything about inheritance?
Gregor Mendel
Studies peas-
•1857- this monk
(with extensive
training in physics
and botany) begins
studying genetics
•Typically SelfFertilizing
•Current dogma is
“Blended
Inheritance”
What
Mendel
Observes:
What does
this data
suggest about
“blended
inheritance”?
Mendel’s
Technique:
•Multiple distinct
CHARACTERS, with easy
to identify TRAITS
•Several TRUEBREEDING varieties
available
Mendel consistently
observes this pattern
of 3:1 ratios, and
develops a 4 part
HYPOTHESIS:
1. Alternate versions of different genes
(alleles) account for variations in
inherited characteristics.
2. For each character, an organism inherits
two alleles, one from each parent.
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Mendel consistently
observes this pattern
of 3:1 ratios, and
develops a 4 part
HYPOTHESIS:
3. If the two alleles differ, then one, the
DOMINANT allele, controls the
phenotype; the other, the RECESSEVE
allele, has no noticeable effect on the
organism’s appearance.
4. The two alleles from each character
segregate during gamete production
(Mendel’s Law of Segregation)
For many traits, we can predict the
genotypic frequencies of the offspring of
two individuals using a PUNNETT SQUARE:
The PUNNETT
Square
constructed for
Mendel’s
experiments
predicts a 3:1
ratio.
Note that you
could also do
this
mathematically!
Simple Mendelian InheritanceA Practice Problem (Part 1 A & B)
Cystic Fibrosis is a Recessive Trait with Unusual
Gene Frequencies
Does this data
support Mendel’s
hypotheses?
2
Understanding the predicted results
of a PUNNETT SQUARE, allows for a
TESTCROSS
What’s my
phenotype?
My
genotype?
Understanding
the predicted
results of a
PUNNETT
SQUARE, allows
for a
TESTCROSS!
You Try!
Part 1,
Question 2
so
All the examples considered
far are MONOHYBRIDS
Testing the Inheritance Patterns of Multiple Characters:
Will Mendel’s principles hold for the analysis of
MULTIPLE CHARACTERS (DIHYBRID CROSSES)?
Complication #1: (Mendel was lucky!)
INCOMPLETE
DOMINANCE
Complication #1: (Mendel was lucky!)
INCOMPLETE DOMINANCE
Heterozygotes have a
unique phenotype,
between that of the
homozygous dominant
or recessive parents.
Note: This is not
blended inheritance!
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Complication
#2:
PLEIOTROPY
(multiple
effects)
Example:
Sickle-Cell
Disease
Epistasis and Lab Pups
Coat color in labradors is
determined by 2 genes, a
pigment gene (B), and a
pigment delivery gene (E).
Black is dominant to Brown, so
Heterozygotes (Bb) are black. The
delivery gene is also dominant, so EE or
Ee individuals both express their
pigments. Only ee individuals are
yellow.
Complication
#3:
EPISTASIS
Example:
The “color gene”,
C, allows pigment
to be deposited in
hair. When
lacking, a mouse
is albino,
regardless of its
genotype at the
other locus.
Bonus Question:
Epistasis and Lab Pups
If I cross a Brown Lab (bbEe)
with a Black Lab (BbEe),
can I expect any yellow
puppies?
If so, what proportion of the pups
would I expect to be yellow?
Complication #4: Codominance + Multiple Alleles
Complication #4: Codominance + Multiple Alleles
Example: Human Blood Types
Example: Human Blood Types
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Other Issues: Individuals
may display a range of
small differences in traits,
known as CONTINUOUS
VARIATION
Example: Paternity Testing
Scenario : Suppose mother is
Type A, baby is
Type B.
Consider these three putative fathers: can any
be the actual father? You Try! (Question 3)
#1 (Type A): Yes or No?
#2 (Type B): Yes or No?
#3 (Type O): Yes or No?
This usually
indicates
POLYGENIC
INHERITANCE,
where two or more
genes create a
single phenotypic
character
Other Issues: Environmental Effects on
Phenotype
Adding
Chromosomes
to the
Inheritance
Story!
Many factors,
both genetic
and
environmental,
influence the
phenotype.
Thomas Hunt Morgan’s
Contributions: Fruit Fly
Genetics
•Single mating produces
100+ offspring
•A new generation can
be bred every two weeks
•Only four pairs of
chromosomes- 3 pairs of
autosomes, 1 pair sex
chromosomes (XX and XY)
Sex
Chromosomes
& Autosomes
Unlike Mendel,
Morgan does not
have access to truebreeding strains.
He breeds flies for a
year, looking for
distinct varieties.
He discovers a male
fly with white eyes,
instead of red.
In Drosophila,
red eyes = Wild type
(the most common
phenotype in a natural
population)
white eyes = a Mutant
Phenotype.
5
Morgan’s Results:
Morgan’s Results:
First Experiment:
Morgan crosses a
red-eyed female with
a white-eyed male.
ALL the offspring
have red eyes.
Next Experiment:
Morgan crosses two
of the red-eyed F1
flies with each other.
What should he see
if Mendel is
correct??
How would Mendel
explain these results??
What would Mendel do
next??
Morgan Discovers Sex-Linked
Genes!
Morgan’s Results:
He DOES find a
3:1 ratio, but
ALL the whiteeyed flies are
male!!
Was Mendel
wrong?? What
happened?!?
The Transmission of SEX-LINKED Traits
Practice Question: Sex-Linked Traits

“A” = dominant allele carried on the X
chromosome;
“a” = the recessive allele.
Note that both males and females are
affected by sex-linked disorders!
If you see the
number 74, then you
do not have redgreen color
blindness. If you see
the number 21, you
are color blind to
some extent. A
totally color-blind
person will not be
able to see any of
the numbers.
You Try!
Question 4
&5
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More on Linked Genes:
More on Linked Genes:
We can use our
understanding of
Meiosis to think
about WHY
Mendel’s Results
were so
predictable!
So… What happens if two genes
are on the SAME chromosome?!?
So… What happens if two genes
are on the SAME chromosome?!?
So… What happens
if two genes are on
the SAME
chromosome?!?
Cross-over!
So why isn’t this ratio
what we expect for
a dihybrid cross???
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Thought
Question:
Consider a chromosome containing 5
genes, A through E.
Are genes A and B (or a and b) more or
less likely to be inherited together than
genes A and D? Explain your answer.
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