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Mendelian Analysis
•  branch of biology that deals with heredity and
variation.
•  explains life at the level of molecules,
organisms, and the populations.
•  relationship between genes and traits.
• This course will take us from Mendel’s
observations that led to the basic laws of
genetics, to the new science of genomics and
gene therapy.
Gregor Mendel,
Father of
Genetics
1822-1884
Traits were known to be inherited from the parents but the
results of crosses were not always predictable
X
What determines the different characteristics
that are passed on from one generation to the
next and what are the rules by which they are
passed on?
Mid-1800’s
•  Just established,
fertilized egg (zygote)
result of 1 sperm uniting
with 1 egg (gametes)
•  Didn’t know about
chromosomes yet
•  Theory of Epigenesis (1600’s): substances
present in egg and sperm that directed the
development of adult organism
Aristotle promoted the idea of the homunculus, a
preformed baby that was formed by particles in the body,
and microscopists at the time thought they could see it in
sperm, suggesting that the male would contribute everything.
Mendel was skeptical of these ideas, and was particularly
intrigued by some early observations by Kolreuter, 1840.
Crossed purple flowered plants with white flowered plants,
the progeny were all purple, but then in the next generation,
white flowered plants reappeared. How can it be that traits can be lost in the hybrid, and then
reappear in the next generation.
. Prevalent Theories
•  Blending inheritance:
–  Substances blended together
to yield unique individual with
traits from both parents
X
cross
fertilize
•  Darwin:
• Particles, called gemmules,
were collected from all
parts of body and became
concentrated in germ cells
Mendel chose to study the garden pea.
Two plant breeders paved the way for
Mendel’s Experiments
•  Knight (1799)
•  Goss (1824)
•  Showed that the edible pea, Pisum sativum, could
be used for genetics.
–  Short generation time (3 months)
–  Numerous varieties available (lots of variation
to study).
–  Ability to cross fertilize and self fertilize
Cross fertilize:
Transfer pollen
from one plant to
the ovule of the
second plant
Self fertilize:
Allow pollen of the
plant to fertilize
it’s own ovules
Parentals (P)
X
cross fertilize
1st Filial Generation
(F1)
Hybrid
self fertilize
F2
self fertilize
self fertilize
F3
As long as you self = F2, F3, F4 etc. When you cross again = F1.
Kolreuter Established:
•  Parental characteristics could
disappear for a generation and
then reappear.
•  This phenomena could only be
explained if units of heredity
were particulate in nature.
•  But this is as far as it was taken.
Who was Mendel?
What did he do
differently?
Mendel was trained in several
disciplines.
• Physics (with Christian Doppler)
• Mathematics
• Botany
Mendel returned to Brno and
set out to answer the question:
Mendel brought to Biology
methods that were standard in
Physics
•  Limited the number of variables
•  Quantitated results
•  Came up with models that could be
tested
How is genetic information transmitted
from one generation to the next?
Mendel did the following:
Mendel was aware of the
following about hybrids:
•  their uniformity of
phenotype* in F1
•  their tendency to
revert to parental
phenotypes in F2
P
X
cross
F1
self
F2
*phenotype:
1. Isolated “pure” true-breeding lines of
peas for seven different characteristics
(plants that breed the same characteristics
after selfing for at least two generations).
visible characteristics
2. Performed reciprocal crosses to determine if
character was linked to sex or if one parent
contributed more to progeny than the other
(wanted to disprove Aristotle’s idea).
Reciprocal
cross
X
White (pollen)
X
Purple (pollen)
Purple (ovule)
White (ovule)
Purple progeny
Since the hybrids were always purple, both
parents contribute equally and the flower color
trait is not sex linked.
Purple progeny
Actual results of some of Mendel’s experiments.
3. Followed all crosses to the F3 and quantitated
the phenotypes of the progeny.
X
X
Pure breeding plant 1
X
Pure breeding plant 2
F1 generation
F1 (hybrid)
yellow
round
green Basis design of his experiments
Parents
yellow
hybrid progeny F2 generation
self cross count
round X
wrinkled
self F2
yellow
6022 3:1
green
2001
round wrinkled
5474 1850
3:1
F3
1/3
Pure
2/3 3:1
all pure
The trait that was hidden in the hybrids was called “recessive”, and the
trait that appears was called “dominant”.
A
How could one explain the 3:1
ratios observed in monohybrid
crosses?
Mendel had a strong background in
probabilities and quickly developed a
model
a
Egg
Sperm
Single
determinant in
egg and sperm
that come
together in the
zygote.
Aa
Zygote
The simplest model
Mendel’s Theory
1-Hereditary determinants are of a particulate nature
2-Each adult pea has 2 determinants (which we now call
alleles) for each character
3-The gametes only have 1 determinant for each
character
4-Each determinant segregates equally into gametes
5-Union of 2 gametes occurs randomly with regard to
genetic determinants
Schematically:
Y - dominant allele*, yellow
y - recessive allele, green
Each adult has two determinants:
If both are the same, homozygous
If they are different, heterozygous
* different forms of same gene
Using this nomenclature, lets go through the cross again.
Each adult has two
P
determinants
YY x yy
genotype
phenotype
yellow
green
Y
y
Random union
of gametes
Yy
F1
F2 Selfing
F3 (heterozygote)
1/4 YY + 1/2Yy + 1/4 yy
1 : 2
: 1
All YY
1/3 of the
Yellow F2
3:1 again
2/3 of the
Yellow F2
F1 male gametes (pollen)
(homozygotes,
Pure breeding)
Each determinant
segregates equally
gametes
Equal segregation of
determinants
F2 results
All yy (pure breeding
green)
1/2 Y 1/2 y
F1
female
gametes
(egg)
1/2 Y YY
Yy
1/2 y Yy
yy
Punnett Square
Phenotypic ratio. 3/4
Genotypic ratio.
Y- (yellow)
1/4 yy (green)
1/4 YY + 1/2Yy + 1/4 yy
1 :
2
: 1
Mendel’s First Law
(coined in 1900)
•  Law of Equal
Segregation:
–  The two alleles of
each trait separate
(segregate) during
gamete formation,
then unite at
random, one from
each parent, at
fertilization.
Test Cross
Conclusions from Monohybrid (single
character segregating) crosses:
Phenotypic Classes Genotypic Classes
yellow:green
3 : 1
YY : Yy : yy
1 : 2 : 1
x
yellow
yy
green
1/2 yy
green
pollen
X
Parents
F1
RR YY
Rr Yy X
rr yy - means the allele type is unknown
Yy
1/2 Yy
Test cross allows one to observe the ratio of the alleles
in the F1 parent, because the test cross parent can only
contribute the recessive allele. This way the phenotype
of the plant tells you which allele came from the F1
parent. No need to infer the alleles in the parent from
ratios.
Dihybrid Cross
F1
round yellow X
yellow
How could one test this model?
Parents
round yellow X
wrinkled green
F1
F2
round yellow wrinkled yellow
round green wrinkled green
315 101
108
32 R- Yrr YR- yy
rr yy
9 3
3
1 F2
• Wrinkled appearance didn’t stay with green or round with yellow. Punnett
Square of
Dihybrid
Cross
Each dihybrid plant
produces 4 gamete
types equally
frequently.
eggs
For example, Y can be
with R or r in any
gamete with equal
probability.
Each trait alone = 3:1
Mendel’s Second Law:
Law of independent assortment
Segregation of
alleles of two
different
genes are
independent of
one another
Bb
Aa
Bb
aA
B
a
b
A
gametes
B
A
Test Cross.
phenotype of test cross progeny directly show the
alleles from the F1 dihybrid.
Rr Yy round yellow 31
F1
Rr Yy
X
x
round yellow
rr Yy wrinkled yellow 27
rr yy
Rr yy round green
wrinkled green
rr yy wrinkled green 26
b
a
gametes
Hybrid Rr Yy Expect 1/4 R Y
in
gametes
1/4 r Y
X
rr yy
r y
test cross progeny
round yellow 31
r y
wrinkled yellow 27
1/4 R y
r y
round green
1/4 r y
r y
wrinkled green 26
Test cross confirms independent assortment of characters
Summary
One Characteristic (two phenotypes)
•  3:1 F2 phenotypic ratio
•  1:1 Test cross phenotypic ratio
Two characteristics (four phenotypes)
•  9 : 3 : 3 : 1 F2 phenotypic ratio
•  1 : 1 : 1 : 1 Test cross phenotypic ratio
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