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Mendel and His Discoveries
Chapter 11 (pg. 207-218)
Gregor Mendel
1822-1884, Austrian monk
The “Father of Genetics”
Thought about researching mice genetics, but switched to plants
…...decided to use Pea plants…….
Advantages: - easy to grow
- fast growing
- high yield of seed each generation
- are self-fertile (self-pollinating)
- but fairly easy to cross pollinate too (by emasculating
the flowers and carrying pollen from another plant)
- lots of different varieties on the market that have
easily recognized differences
Looked at the inheritance of these 7 different traits:
Seed Color =
Seed Shape =
Flower Color =
Position of Flower =
Height =
Color of young pods =
Pod shape =
First, he made sure that each variety bred
“true” (was stable for two generations).
Then he started making crosses between plants that were
different for the same trait…..for instance, tall and short.
Observation 1 =
Parents = parental
generation
P
Tall x Tall
Short x Short
Tall
Short
First filial generation
F
Observation 2 = Mendel made a cross between a Tall
plant and a Short plant…..
X
Progeny were all Tall >
…not medium height!
Observation 3 = If these Tall-Short hybrids
were self-pollinated:
F2 generation
Offspring were:
75% Tall
25% Dwarf
He observed this same phenomenon for all 7 traits
How could such a thing happen?
Mendel concluded that each plant
carries two “particles” .....
One of these particles comes from female parent and the
other from the male parent (even if same plant).
He also concluded that one of these
particles are “Dominant”over the other.
He called the non-dominant ones “Recessive”
It turns out there is a lot of truth to this conclusion....we now
call these “particles” genes....to be specific they are alleles.
Alleles are often indicated with a single letter.
A CAPITAL letter denotes a dominant allele = T
A lowercase letter denotes a recessive allele = t
Genotype (chemical code):
TT
x
Tt
tt
Phenotype (visual):
Tall
x
Tall
Dwarf
Alleles are often indicated with a single letter.
A CAPITAL letter denotes a dominant allele = T
A lowercase letter denotes a recessive allele = t
Genotype (chemical code):
TT
x
Tt
tt
Phenotype (visual):
Tall
x
Tall
Dwarf
In the next generation (where he
self-pollinated) the offspring:
Genotype (chemical code):
Tt
x
Tt
25% TT
50% Tt
25% tt
Phenotype (visual):
Tall
x
Tall
75% Tall
25% Dwarf
…see next slide
…one parent (female)
T
t
T
TT
Tt
t
Tt
tt
..other
Parent (male)
25% TT (tall)
50% Tt (tall)
25% tt (dwarf)
P1
Yellow Seed x Green Seed
F1
All Yellow Seeds
Selfed the Yellow Seeds
F2
6,022 yellow seeds
2,001 green seeds
Monohybrid Cross
?
Mendel theorized three basic Principles:
“The Principle of Segregation”
That each plant has two “factors” (alleles) for any genetic trait.
One of these alleles comes from female parent and the
other from the male parent (even if same plant).
So the female sex cell (called egg or ovule)
and
the male sex cell (pollen or sperm)…
each contain only one of these alleles.
“The Principle of Dominance”
He also concluded that some of these particles are “Dominant”
over the other.
He called the non-dominant ones “Recessive”
So, the only way a plant will show the Recessive characteristic
is if it has none of the Dominant factors at all.
Crosses that involve one characteristic, such as
seed color, is known as a monohybrid cross
But Mendel when on to ask the questions, “what
happens if two characteristics were breed?”
F2 generation
315 smooth yellow peas
101 wrinkled yellow peas
108 smooth green peas
32 wrinkled green peas
Dihybrid cross
P
SS YY x
F1
F2
Ss Yy
ss yy
x
Ss Yy
“The Principle of
Independent Assortment”
….that each pair of traits assort independently
Incomplete Dominance
RR
rr
Rr
Red pigment gene produces the color red
White snapdragon produces no color at all
because the gene for pigment is not functional
Two alleles are needed for color production
Rr
Rr
1 red: 2 pink : 1 white
HYBRID =
“progeny (offspring) resulting from a cross between
parents that are very different from one another”
…this could be between two organisms from
completely different species
(e.g.horse x donkey results in a mule)
…it could also be between two organisms
(two “varieties” or “breeds”) within the same species
(e.g. hybrid corn, hybrid tomatoes)
….humans would not be normally considered “hybrids”…
they are too similar….
(our traits only differ quantitatively, not qualitatively)
PHENOTYPE =
“How a genetic trait outwardly expresses itself
(ie. How it influences the physical body)”
Like:
Tall or Short plants
Green or Yellow pods
Straight or Curly hair
GENOTYPE =
“The chemical makeup of that gene”
Like:
Aa
YY
zz
ALLELE =
“A single ‘factor’, ‘particle’, gene’
which ever you want to count it”
Like:
A
Y
z
HOMOZYGOUS =
“When both alleles are identical”
Like:
AA
YY
zz
HETEROZYGOUS =
“When the 2 alleles are different”
Like:
Aa
Yy
Zz
BB = Homozygous Dominant
Cc = ? Heterozygous
tt
= ? Homozygous Recessive
Ww = ? Heterozygous