Mendel and His Discoveries

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Mendel and His Discoveries Chapter 11 (pg.
207-218)
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Gregor Mendel 1822-1884, Austrian monk The
Father of Genetics
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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
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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
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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.
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Observation 1 Tall x Tall
Short x Short Tall
Short
Parents parental generation P
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Observation 2 Mendel made a cross between a
Tall plant and a Short plant..
X
not medium height!
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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?
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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 Dominantover
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.
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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)
Phenotype (visual)
TT x tt
Tt
Tall x Dwarf
Tall
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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)
Phenotype (visual)
TT x tt
Tt
Tall x Dwarf
Tall
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In the next generation (where he
self-pollinated) the offspring
Genotype (chemical code)
Phenotype (visual)
Tt x Tt 25 TT
50 Tt 25 tt see
next slide
Tall x Tall 75
Tall 25 Dwarf
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one parent (female)
T
t
T
TT
Tt
25 TT (tall) 50 Tt (tall) 25 tt (dwarf)
t
Tt
tt
..other Parent (male)
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P1 Yellow Seed x Green Seed F1
All Yellow Seeds
Selfed the Yellow Seeds F2
6,022 yellow seeds
2,001 green seeds
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?
Monohybrid Cross
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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.
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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.
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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?
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F2 generation 315 smooth yellow peas
101 wrinkled yellow peas 108
smooth green peas 32 wrinkled green
peas
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Dihybrid cross
P SS YY x ss
yy F1 F2
Ss Yy
x Ss Yy
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The Principle of Independent Assortment .tha
t each pair of traits assort independently
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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
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Rr
Rr
1 red 2 pink 1 white
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Time for some definitions
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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)
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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
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GENOTYPE The chemical makeup of that gene
Like Aa YY
zz
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ALLELE A single factor, particle, gene
which ever you want to count it Like
A Y z
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HOMOZYGOUS When both alleles are identical
Like AA YY
zz
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HETEROZYGOUS When the 2 alleles are
different Like Aa
Yy Zz
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BB Homozygous Dominant
Cc ? tt ? Ww ?
Heterozygous
Homozygous Recessive
Heterozygous