Today:
Mendelian Genetics
Mendelian Genetics: Consider this….
8 million possible
chromosome combinations
in each egg, and each
sperm…
= >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
•1857- this monk
(with extensive
training in physics and
botany) begins
studying genetics
•Current dogma??
Mendel’s Technique:
Studies peas•Typically Self- Fertilizing
•Multiple distinct
CHARACTERS, with
easy to identify TRAITS
•Several TRUEBREEDING varieties
available
What
Mendel
Observes:
What does this
data suggest
about “blended
inheritance”?
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.
The data support
Mendel’s Hypothesis!
Simple Mendelian
InheritanceA Practice Problem
Cystic Fibrosis is a Recessive Trait
with Unusual Gene Frequencies
1. If two carriers of the cystic fibrosis trait marry,
what is the probability that their first child will be
affected?
2. It they eventually have three children, what is the
probability that all three will be affected?
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!
Complication
#2:
PLEIOTROPY
(multiple effects)
Example:
Sickle-Cell
Disease
Complication #3: Codominance + Multiple Alleles
Example: Human Blood Types
Example: Paternity testing
Scenario : Suppose mother is
Type A, baby is
Type B.
Consider these three putative fathers: can any be the
actual father?
#1 (Type A)
#2 (Type B)
#3 (Type O)
Complication #4:
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.
Other Issues: Individuals may
display a range of small
differences in traits, known as
CONTINUOUS VARIATION
This usually indicates
POLYGENIC
INHERITANCE,
where two or more
genes create a single
phenotypic character
Other Issues: Environmental Effects on
Phenotype
Many factors, both
genetic and
environmental,
influence the
phenotype.
A More Recent Complication…

If you see the number
74, then you do not
have red-green 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.
Human males are more likely to be
color-blind than human females.
Why???
Chromosomes!
Is this what Mendel would expect?
Is this what
Mendel
would
expect?
Practice Question: Chromosomal
Inheritance



If a color blind man marries a “wild-type”
woman, what are the chances that a daughter of
theirs will be colorblind?
What are the chances that their son will be
colorblind?
Can females be colorblind? What would the
genotype of the parents have to be?
Connecting Mendel to Lab:
Reminder: One of
the restriction
enzymes (molecular
scissors) we used
can cut the normal
hemoglobin gene,
but not the mutant
sickle version.
Sickle cell allele
normal
Connecting Mendel to Lab:
Reminder: One of
the restriction
enzymes (molecular
scissors) we used
can cut the normal
hemoglobin gene,
but not the mutant
sickle version.
Sickle cell allele
normal