Today:

1

• Today
• Mendelian Genetics!
• Intro to Mitosis?

2
The Father of Genetics?
Gregor Mendel, 1822-1884
3
Setting the Stage for Mendel
Leading theory at the time is Blended Inheritance
What makes a good model??
Mendel will need a good model organism!
4

• Mendels Technique
• Studies peas-
• Typically Self- Fertilizing
• Multiple distinct CHARACTERS, with easy to
  identify TRAITS
• Several TRUE-BREEDING varieties available

5
What Mendel Observes, Part 1
What does this data suggest about blended
inheritance?
6
What Mendel Observes, Part 2
What does this data suggest about blended
inheritance?
7
Mendels Hypothesis- Part 1
Different genes account for the variation in
inherited characters
8
Mendels Hypothesis- Part 2
For each character, an organism inherits two
alleles, one from each parent.
9
Mendels Hypothesis- Part 3
If the alleles are different, than one will
control the organisms appearance (the dominant
allele) while the other will have no noticeable
effect (the recessive allele)
10
Mendels Hypothesis- Part 4
The two alleles are separated during gamete
production
11
Testing the Law of Segregation The Punnett
Square
12
Part 1 The Punnett Square for Mendels
Experiments
What will the F1 Generation look like? The F2
Generation?
13
vs
14
Understanding the predicted results of a PUNNETT
SQUARE, allows for a TESTCROSS
Whats my phenotype? My genotype?
15
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 dogs 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?
16
Using Simple Mendelian Genetics
Sickle Cell Disease
17
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?
18
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?
19
An Aside Unusual Gene Frequencies!?
What do you notice? What does this suggest?
20
Mendelian Genetics- Example 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?
21
Calculating Probabilities
22
Dependent Assortment?
Mendels Next Question What happens in a
dihybrid cross?
What would the outcome look like if its
dependent assortment??
23
What Mendel Sees
So is it dependent assortment??
You Try! Part 5.
24
Mendels Contributions

• Law 1 Segregation
• Law 2 Independent Assortment

25
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?
26
Complication 1 (Mendel was lucky!)
INCOMPLETE DOMINANCE
27
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

28
Another Exception Codominance
Example Tay-Sachs disease- Heterozygous
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.
29
Part 6 One Other Complication Multiple
Alleles and Codominance!

• Multiple Alleles Suppose youve 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?

30

• Three Important Points about Dominant/Recessive
  Traits
• They range from complete dominance ? incomplete
  dominance ? codominance. (can be a subtle
  distinction!)
• 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)
• Theyre not related to the abundance of an allele
  within a population!

31
Further Complications Pleiotropy

• Most genes have multiple phenotypic effects!

32
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.

33
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.
34
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.

35
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?
36
Theres more Polygenic Inheritance
This results in a broad norm of reaction
37
Other Issues Environmental Effects on Phenotype
Many factors, both genetic and environmental,
influence the phenotype.