Transcript Gregor Mendel, 1822-1884

Today:
•Mendelian
Genetics!
• Intro to Mitosis?
The “Father” of Genetics?
Gregor Mendel, 1822-1884
Setting the Stage
for Mendel
Leading theory at the time is
Blended Inheritance
Mendel will need a
good model organism!
What
makes a
good
model??
Mendel’s
Technique:
Studies peas•Typically SelfFertilizing
•Multiple distinct
CHARACTERS, with
easy to identify TRAITS
•Several TRUEBREEDING varieties
available
What Mendel Observes, Part 1:
What does
this data
suggest
about
“blended
inheritance”?
What Mendel Observes, Part 2:
What does
this data
suggest
about
“blended
inheritance”?
Mendel’s Hypothesis- Part 1
Different genes
account for
the variation in
inherited
characters
Mendel’s Hypothesis- Part 2
For each
character, an
organism
inherits two
alleles, one
from each
parent.
Mendel’s Hypothesis- Part
3
If the alleles are different,
than one will control the
organism’s appearance
(the dominant allele) while
the other will have no
noticeable effect (the
recessive allele)
Mendel’s Hypothesis- Part 4
The two alleles are
separated during
gamete production
Testing the Law of Segregation:
The Punnett Square
Part 1: The Punnett Square for
Mendel’s Experiments:
What will the F1 Generation look
like? The F2 Generation?
vs
Understanding the predicted results
of a PUNNETT SQUARE, allows for a
TESTCROSS
What’s my
phenotype?
My
genotype?
Part 2: Try a Test Cross!
In dogs, there is an hereditary deafness caused by a
recessive gene, “d.” A kennel owner has a male dog that
she wants to use for breeding purposes if possible.
The dog can hear, so the owner knows his genotype
is either DD or Dd. If the dog’s genotype is Dd, the owner
does not wish to use him for breeding so that the deafness
gene will not be passed on. This can be tested by
breeding the dog to a deaf female (dd).
Draw the Punnett squares to illustrate these two
possible crosses. In each case, what percentage/how
many of the offspring would be expected to be hearing?
deaf? How could you tell the genotype of this male dog?
Using Simple Mendelian Genetics
Sickle Cell Disease
3A: Sickle Cell Disease Questions
Two individuals who are heterozygous at
the Sickle Cell locus have four children
together. One of the children is affected
with the disorder. Based on this
information, is the sickle cell trait dominant
or recessive?
3B: Sickle Cell Disease Questions
If the affected offspring has a child with
an unaffected individual (who does not
carry the sickle allele), what is the
probability that any given child will be
unaffected? Be a carrier? Be affected?
An Aside: Unusual Gene Frequencies!?
What do you
notice?
What does
this suggest?
Mendelian GeneticsExample 4:
Cystic Fibrosis is also an
Autosomal Recessive Trait
with Unusual Gene
Frequencies
A. If two carriers of the cystic fibrosis trait have
children, what is the probability that their first
child will be affected?
B. If they eventually have three children, what
is the probability that all three will be
affected?
Calculating
Probabilities
Mendel’s Next Question: What
happens in a dihybrid cross?
Dependent Assortment?
What would the
outcome look
like if it’s
dependent
assortment??
What
Mendel
Sees:
So isTry!
it
You
dependent
Part 5.
assortment??
Mendel’s Contributions
Law #1: Segregation
Law #2: Independent
Assortment
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!
Why?
Complication #1: (Mendel was lucky!)
INCOMPLETE DOMINANCE
Another Exception:
Codominance
In codominance, both alleles affect
the phenotype in separate,
distinguishable ways.
Example:
•Human blood groups M, N, and
MN
Group MN produce both
antigens on the surface of
blood cells
Another Exception:
Codominance
Example:
Tay-Sachs diseaseHeterozygous individuals produce
both functional, and
dysfunctional enzymes.
organismal level = recessive,
biological level = codominant.
A section of the brain of a Tay
Sachs child. The empty vacuoles
are lysosomes that had been filled
with glycolipid until extracted with
alcohol in preparing the tissue.
Part 6: One Other Complication:
Multiple Alleles and Codominance!
Multiple Alleles: Suppose you’ve been asked
to help a new mother identify the biological
father of her child. She has Type A blood,
and her new baby is Type B.
Consider these three putative fathers: can any be the
biological father? Why or why not?
#1 (Type A): Yes or No?
#2 (Type B): Yes or No?
#3 (Type O): Yes or No?
Three Important Points about
Dominant/Recessive Traits:
1. They range from complete dominance 
incomplete dominance  codominance.
(can be a subtle distinction!)
2. They reflect mechanisms through which
specific alleles are expressed in the
phenotype (i.e. this is not one allele subduing
another at the DNA level)
3. They’re not related to the abundance of an
allele within a population!
Further Complications: Pleiotropy
Most genes have multiple phenotypic effects!
Further Complications: Pleiotropy
No production of melanocytes
during development causes:
1. White fur color
and
2. Inability to transmit electrical signals to
brain from hair cells in the ear.
More Complications:
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.
Part 7: 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.
Part 7: 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?
There’s
more…
Polygenic
Inheritance
This results in a
broad norm of
reaction
Other Issues: Environmental Effects on
Phenotype
Many factors,
both genetic
and
environmental,
influence the
phenotype.